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Dimitri Shvorob may not appear at first glance to be a prolific contributor on Matlab Central, but from the little he has posted over the years I regard him to be a Matlab power-user. So when Dimitri reports something, I take it seriously. Such was the case several months ago, when he contacted me regarding very odd behavior that he saw in his code: the * for* loop worked well, but the

The core issue is that if we have a class object that is used within a * for* loop, Matlab can access the object directly in memory. But with a

In other words, when we use data objects in a * parfor* loop, the data object is not necessarily sent “as-is”: additional processing may be involved under the hood that modify the data in a way that may be invisible to the user (or the loop code), resulting in different processing results of the parallel (

For additional aspects of Matlab serialization/deserialization, see my article from 2 years ago (and its interesting feedback comments).

*The following section was contributed by guest blogger Lior Perlmuter-Shoshany, head algorithmician at a private equity fund.*

In my work, I had to work with matrixes in the order of 10^{9} cells. To reduce the memory footprint (and hopefully also improve performance), I decided to work with data of type `single`

instead of Matlab’s default `double`

. Furthermore, in order to speed up the calculation I use * parfor* rather than

What I discovered to my surprise is that the results from the ** parfor** and

The following simplified code snippet illustrate the problem by calculating a simple standard-deviation (* std*) over the same data, in both

`single`

– and `double`

-precision. Note that the loops are ran with only a single iteration, to illustrate the fact that the problem is with the parallelization mechanism (probably the serialization/deserialization parts once again), not with the distribution of iterations among the workers.clear rng('shuffle','twister'); % Prepare the data in both double and single precision arr_double = rand(1,100000000); arr_single = single(arr_double); % No loop - direct computation std_single0 = std(arr_single); std_double0 = std(arr_double); % Loop #1 - serial for loop std_single = 0; std_double = 0; for i=1 std_single(i) = std(arr_single); std_double(i) = std(arr_double); end % Loop #2 - parallel parfor loop par_std_single = 0; par_std_double = 0; parfor i=1 par_std_single(i) = std(arr_single); par_std_double(i) = std(arr_double); end % Compare results of for loop vs. non-looped computation isForSingleOk = isequal(std_single, std_single0) isForDoubleOk = isequal(std_double, std_double0) % Compare results of single-precision data (for vs. parfor) isParforSingleOk = isequal(std_single, par_std_single) parforSingleAccuracy = std_single / par_std_single % Compare results of double-precision data (for vs. parfor) isParforDoubleOk = isequal(std_double, par_std_double) parforDoubleAccuracy = std_double / par_std_double

Output example :

isForSingleOk = 1 % <= true (of course!) isForDoubleOk = 1 % <= true (of course!) isParforSingleOk = 0 % <= false (odd!) parforSingleAccuracy = 0.73895227413361 % <= single-precision results are radically different in parfor vs. for isParforDoubleOk = 0 % <= false (odd!) parforDoubleAccuracy = 1.00000000000021 % <= double-precision results are almost [but not exactly] the same in parfor vs. for

From my testing, the larger the data array, the bigger the difference is between the results of `single`

-precision data when running in * for* vs.

In other words, my experience has been that if you have a huge data matrix, it’s better to parallelize it in `double`

-precision if you wish to get [nearly] accurate results. But even so, I find it deeply disconcerting that the results are not exactly identical (at least on R2015a-R2016b on which I tested) even for the native `double`

-precision .

Hmmm… bug?

I will shortly be traveling to clients in Zürich and Geneva, Switzerland. If you are in the area and wish to meet me to discuss how I could bring value to your work with some advanced Matlab consulting or training, then please email me (altmany at gmail):

**Zürich**: January 15-17**Geneva**: January 18-21

Happy new year everybody!

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Well, sometimes it so happens that we really need to know the size of the Java object, or the size difference between two objects (to help resolve memory leaks, for example). There are several resources online that explain how to do this in Matlab (examples 1, 2, 3). Today I will show two alternatives that I found useful within the context of Matlab:

A full decade ago, Vladimir Roubtsov posted a very detailed article in the JavaWorld magazine explaining how to profile and report Java object sizes. The article contained a downloadable Java archive that we can easily use in Matlab. After downloading the zip file, extract the contained *objectprofiler.jar* file, add it to the Java classpath and start using it, as follows:

>> javaaddpath 'C:\path\to\where\you\placed\your\copy\of\objectprofiler.jar' >> com.vladium.utils.ObjectProfiler.sizeof(java.awt.Color.red) ans = 28

Note that the reported sizes may be different on different Matlab releases (=JVM versions) and platforms. Also note that the reported size (28 bytes for the Java Color object) is much smaller than the size required to serialize the object into a byte stream (408 bytes in this case), as I’ve shown in last week’s article.

Running the *sizeof* method on deeply referenced objects could quickly exhaust Matlab’s memory:

>> jDesktop = com.mathworks.mde.desk.MLDesktop.getInstance; % on R2012a: takes a long time and finally reports >> com.vladium.utils.ObjectProfiler.sizeof(jDesktop) ans = 72011200 % on R2014a: takes a long time and finally croaks % (which is not surprising, considering the Desktop's new toolstrip) >> com.vladium.utils.ObjectProfiler.sizeof(jDesktop) Java exception occurred: java.lang.OutOfMemoryError: Java heap space at java.util.IdentityHashMap.resize(Unknown Source) at java.util.IdentityHashMap.put(Unknown Source) at com.vladium.utils.ObjectProfiler.computeSizeof(ObjectProfiler.java:329) at com.vladium.utils.ObjectProfiler.sizeof(ObjectProfiler.java:85)

`ObjectProfiler`

has a very handy feature of enabling a visual display of the object’s reference tree. For example:

>> jObject = java.util.Hashtable jObject = {} >> com.vladium.utils.ObjectProfiler.sizeof(jObject) ans = 105 >> com.vladium.utils.ObjectProfiler.profile(jObject).dump ans = 105 -> <INPUT> : Hashtable 60 (57.1%) -> Hashtable#table : Hashtable$Entry[] 60 (57.1%) -> <shell: Hashtable$Entry[], length=11> 45 (42.9%) -> <shell: 6 prim/4 ref fields> >> jObject.put('key1',1.23); >> com.vladium.utils.ObjectProfiler.sizeof(jObject) ans = 189 >> com.vladium.utils.ObjectProfiler.profile(jObject).dump ans = 189 -> <INPUT> : Hashtable 144 (76.2%) -> Hashtable#table : Hashtable$Entry[] 84 (44.4%) -> Hashtable#table[4] : Hashtable$Entry 44 (23.3%) -> Hashtable$Entry#key : String 24 (12.7%) -> String#value : char[] 24 (12.7%) -> <shell: char[], length=4> 20 (10.6%) -> <shell: 2 prim/1 ref fields> 24 (12.7%) -> <shell: 1 prim/3 ref fields> 16 (8.5%) -> Hashtable$Entry#value : Double 16 (8.5%) -> <shell: 1 prim/0 ref fields> 60 (31.7%) -> <shell: Hashtable$Entry[], length=11> 45 (23.8%) -> <shell: 6 prim/4 ref fields>

As we can see, adding the `'key1'`

key to the hashtable object actually added 2 new references: a 44-byte `String`

and a 16-byte `Double`

, plus 24 additional overhead bytes, for a total addition of 84 bytes.

`ObjectProfiler`

has a convenience method *sizedelta(jObject1,jObject2)* which returns the size delta in bytes between the two specified objects. There are a few additional methods for `ObjectProfiler`

and the `ObjectProfiler.profile()`

object – interested readers are referred to the original article and to the source code (which is included within the zip file that we downloaded).

The `Classmexer`

utility works a bit differently but is also very easy to use once the initial setup is done. First we need to download the zip file, then extract the *classmexer.jar* and place it in your Matlab’s startup folder. In that same folder, edit (create if necessary) a *java.opts* file with the following line:

-javaagent:classmexer.jar

After restarting Matlab, we can use `Classmexer`

as follows:

>> com.javamex.classmexer.MemoryUtil.deepMemoryUsageOf(java.awt.Color.red) ans = 32 >> jObject = java.util.Hashtable; >> com.javamex.classmexer.MemoryUtil.deepMemoryUsageOf(jObject) ans = 120 >> jObject.put('key1',1.23); jObject jObject = {key1=1.23} >> com.javamex.classmexer.MemoryUtil.deepMemoryUsageOf(jObject) ans = 264

Note how the values reported by `Classmexer`

differ from those of `ObjectProfiler`

. To tell the truth, I’m not sure which of them to believe: `ObjectProfiler`

seems more detailed, but `Classmexer`

uses Java’s preferred mechanism of using an instrumentation agent.

We can also use `java.lang.Runtime.getRuntime`

‘s methods (*maxMemory()*, *freeMemory()* and *totalMemory()*) to monitor overall Java memory (note a MathWorks blog article on this). Note that this reports the total memory values, and fluctuates (sometimes dramatically) from second to second, as Matlab’s desktop and other Java-heavy tools create Java objects, which the JVM garbage-collects.

Jeff Gullet has suggested to monitor these values and programmatically activate a synchronous Java garbage-collection when the memory appears too “crowded” (I fixed Jeff’s posted idea in the snippet below):

r = java.lang.Runtime.getRuntime; if (r.freeMemory/r.totalMemory) < 0.1 r.gc(); end

A MathWorks technical article provided some assistance on using the `JConsole`

utility to profile Java memory in Matlab. We can also use the `JMap`

and `JHat`

utilities. All these utilities are part of the free Java Development Kit (JDK) that can be downloaded online, just ensure you’re using the same Java version as reported by Matlab:

>> version -java ans = Java 1.7.0_11-b21 % i.e., Java 7 update 11

In addition to the JDK tools, I find the open-source `JVisualVM`

utility informative and easy to use. We can also use JMP (R2007a and earlier), TIJMP (R2007b and later), or other 3rd-party tools. A list of Java-centric resources is available in the Java SE Troubleshooting guide.

To complete the picture, a couple of years ago I posted an article on profiling Matlab’s memory usage, which included a section on Java memory. You may also find useful another article I wrote, on finding and fixing a Java memory leak in Matlab.

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`.^`

operator. Trying to improve on Mike’s results, I used my finding that `zeros(n,m)+scalar`

is often faster than `ones(n,m)*scalar`

(see my article on pre-allocation performance). Applying this to Mike’s powers-vector example, `zeros(n,m)+scalar`

only gave me a 25% performance boost (i.e., 1-1/1.25 or 20% faster), rather than the x5 speedup that I received in my original article.Naturally, the difference could be due to different conditions: a different running platform, OS, Matlab release, and allocation size. But the difference felt intriguing enough to warrant a small investigation. I came up with some interesting new findings, that I cannot fully explain:

function t=perfTest % Run tests multiple time, for multiple allocation sizes n=100; tidx = 1; iters = 100; while n < 1e8 t(tidx,1) = n; clear y; tic, for idx=1:iters, clear y; y=ones(n,1); end, t(tidx,2)=toc; % clear; ones() clear y; tic, for idx=1:iters, clear y; y=zeros(n,1); end, t(tidx,3)=toc; % clear; zeros() clear y; tic, for idx=1:iters, y=ones(n,1); end, t(tidx,4)=toc; % only ones() clear y; tic, for idx=1:iters, y=zeros(n,1); end, t(tidx,5)=toc; % only zeros() n = n * 2; tidx = tidx + 1; end % Normalize result on a per-element basis t2 = bsxfun(@rdivide, t(:,2:end), t(:,1)); % Display the results h = loglog(t(:,1), t(:,2:end)); % overall durations %h = loglog(t(:,1), t2); % normalized durations set(h, 'LineSmoothing','on'); % see https://undocumentedmatlab.com/blog/plot-linesmoothing-property/ set(h(2), 'LineStyle','--', 'Marker','+', 'MarkerSize',5, 'Color',[0,.5,0]); set(h(3), 'LineStyle',':', 'Marker','o', 'MarkerSize',5); set(h(4), 'LineStyle','-.', 'Marker','*', 'MarkerSize',5); legend(h, 'clear; ones', 'clear; zeros', 'ones', 'zeros', 'Location','NorthWest'); xlabel('# allocated elements'); ylabel('duration [secs]'); box off end

The full results were (R2013a, Win 7 64b, 8MB):

n clear,ones clear,zeros only ones only zeros ======== ========== =========== ========= ========== 100 0.000442 0.000384 0.000129 0.000124 200 0.000390 0.000378 0.000150 0.000121 400 0.000404 0.000424 0.000161 0.000151 800 0.000422 0.000438 0.000165 0.000176 1600 0.000583 0.000516 0.000211 0.000206 3200 0.000656 0.000606 0.000325 0.000296 6400 0.000863 0.000724 0.000587 0.000396 12800 0.001289 0.000976 0.000975 0.000659 25600 0.002184 0.001574 0.001874 0.001360 51200 0.004189 0.002776 0.003649 0.002320 102400 0.010900 0.005870 0.010778 0.005487 204800 0.051658 0.000966 0.049570 0.000466 409600 0.095736 0.000901 0.095183 0.000463 819200 0.213949 0.000984 0.219887 0.000817 1638400 0.421103 0.001023 0.429692 0.000610 3276800 0.886328 0.000936 0.877006 0.000609 6553600 1.749774 0.000972 1.740359 0.000526 13107200 3.499982 0.001108 3.550072 0.000649 26214400 7.094449 0.001144 7.006229 0.000712 52428800 14.039551 0.001853 14.396687 0.000822

(Note: all numbers should be divided by the number of loop iterations `iters=100`

)

As can be seen from the data and resulting plots (log-log scale), the more elements we allocate, the longer this takes. It is not surprising that in all cases the allocation duration is roughly linear, since when twice as many elements need to be allocated, this roughly takes twice as long. It is also not surprising to see that the allocation has some small overhead, which is apparent when allocating a small number of elements.

A potentially surprising result, namely that allocating 200-400 elements is in some cases a bit faster than allocating only 100 elements, can actually be attributed to measurement inaccuracies and JIT warm-up time.

Another potentially surprising result, that * zeros* is consistently faster than

`bzero`

) for clearing memory, than `memset`

a value.A somewhat more surprising effect is that of the * clear* command: As can be seen in the code, calling

`y`

is being overridden with new values. However, we clearly see that the overhead of calling Another aspect that is surprising is the fact that * zeros* (with or without

The performance degradation at 100K elements has been around in Matlab for ages – I see it as far back as R12 (Matlab 6.0), for both * zeros* and

Another aspect that is strange here is that the speedup for * zeros* at 200K elements is ~12 – much higher than the expected optimal speedup of 4 on my quad-core system. The higher speedup may perhaps be explained by hyper-threading or SIMD at the CPU level.

In any case, going back to the original reason I started this investigation, the reason for getting such wide disparity in speedups between using * zeros* and

Some take-away lessons:

- Using
is always preferable to**zeros**, especially for more than 100K elements on Matlab 7.7 (R2008b) and newer.**ones** `zeros(n,m)+scalar`

is consistently faster than`ones(n,m)*scalar`

, especially for more than 100K elements, and for the same reason.- In some cases, it may be worth to use a built-in function with more elements than actually necessary, just to benefit from its internal multi-threading.
- Never take performance assumptions for granted. Always test on your specific system using a representative data-set.

p.s. – readers who are interested in the historical evolution of the * zeros* function are referred to Loren Shure’s latest post, only a few days ago (a fortunate coincidence indeed). Unfortunately, Loren’s post does not illuminate the mysteries above.

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Basically, the * import* function enables Matlab users to declare that a specific class name belongs to a particular Java namespace, without having to specifically state the full namespace in each use. In this regard, Matlab’s

`import`

, and not surprisingly also has similar syntax:% Alternative 1 - using explicit namespaces jFrame = javax.swing.JFrame; jDim = java.awt.Dimension(50,120); jPanel.add(jButton, java.awt.GridBagConstraints(0, 0, 1, 1, 1.0, 1.0, ... java.awt.GridBagConstraints.NORTHWEST, ... java.awt.GridBagConstraints.NONE, ... java.awt.Insets(6, 12, 6, 6), 1, 1)); % Alternative 2 - using import import javax.swing.* import java.awt.* jFrame = JFrame; jDim = Dimension(50,120); jPanel.add(jButton, GridBagConstraints(0, 0, 1, 1, 1.0, 1.0, ... GridBagConstraints.NORTHWEST, ... GridBagConstraints.NONE, ... Insets(6, 12, 6, 6), 1, 1));

Note how much cleaner Alternative #2 looks compared to Alternative #1. However, as with Java’s `import`

, there is a tradeoff here: by removing the namespaces from the code, it could become confusing as to which namespace a particular object belongs. For example, by specifying `java.awt.Insets`

, we immediately know that it’s an AWT insets object, rather than, say, a book’s insets. There is no clear-cut answer to this dilemma, and in fact there are many Java developers who prefer one way or the other. As in Java, the choice is yours to make also in Matlab.

Perhaps a good compromise, one which I often use, is to stay away from the `import something.*`

format and directly specify the imported classes. In the example above, I would have written:

% Alternative 3 - using explicit import import javax.swing.JFrame; import java.awt.Dimension; import java.awt.GridBagConstraints; import java.awt.Insets; jFrame = JFrame; jDim = Dimension(50,120); jPanel.add(jButton, GridBagConstraints(0, 0, 1, 1, 1.0, 1.0, ... GridBagConstraints.NORTHWEST, ... GridBagConstraints.NONE, ... Insets(6, 12, 6, 6), 1, 1));

This alternative has the benefit that it is immediately clear that Insets belongs to the AWT package, without having to explicitly use the `java.awt`

prefix everywhere. Obviously, if the list of imported classes becomes too large, we could always revert to the `import java.awt.*`

format.

Interestingly, we can also use the functional form of * import*:

% Alternative #1 import java.lang.String; % Alternative #2 import('java.lang.String'); % Alternative #3 classname = 'java.lang.String'; import(classname);

Using the third alternative format, that of dynamic import, enables us to decide ** in run-time**(!) whether to use a class C from package PA or PB. This is a cool feature but must be used with care, since it could lead to very difficult-to-diagnose errors. For example, if the code later invokes a method that exists only in PA.C but not in PB.C. The correct way to do this would probably be to define a class hierarchy where PA.C and PB.C both inherit from the same superclass. But in some cases this is simply not feasible (for example, when you have 2 JARs from different vendors, which use the same classname) and dynamic importing can help.

It is possible to specify multiple input parameters to * import* in the same directive. However, note that Matlab 7.5 R2007b and older releases crash (at least on WinXP) when one of the imported parameters is any MathWorks-derived (

`com.mathworks...`

) package/class. This bug was fixed in Matlab 7.6 R2008a, but to support earlier releases simply separate such imports into different lines:% This crashes Matlab 7.5 R2007b and earlier; OK on Matlab 7.6 R2008a and later import javax.swing.* com.mathworks.mwswing.* % This is ok in all Matlab releases import javax.swing.* import com.mathworks.mwswing.*

* import* does NOT load the Java class into memory – it just declares its namespace for the JVM. This mechanism is sometimes called

A few additional notes:

- Although not strictly mandatory, it is good practice to place all the
directives at the top of the function, for visibility and code maintainability reasons**import** - There is no need to end the
declaration with a semicolon (;). It’s really a matter of style consistency. I usually omit it because I find that it is a bit intrusive when placed after a ***import** by itself, without any input arguments (class/package names) returns the current list of imported classes/packages**import**- Imported classes and packages can be un-imported using the
directive from the Command Window**clear import** - It has been reported that in some cases using
in deployed (compiled) application fails – the solution is to use the FQCN in such cases**import**

**Note: This topic is covered and extended in Chapter 1 of my Matlab-Java programming book**

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The specific mechanisms I’ll describe today are * Copy on Write* (aka

Matlab implements an automatic copy-on-write (sometimes called *copy-on-update* or *lazy copying*) mechanism, which transparently allocates a temporary copy of the data only when it sees that the input data is modified. This improves run-time performance by delaying actual memory block allocation until absolutely necessary. COW has two variants: during regular variable copy operations, and when passing data as input parameters into a function:

When a variable is copied, as long as the data is not modified, both variables actually use the same shared memory block. The data is only copied onto a newly-allocated memory block when one of the variables is modified. The modified variable is assigned the newly-allocated block of memory, which is initialized with the values in the shared memory block before being updated:

data1 = magic(5000); % 5Kx5K elements = 191 MB data2 = data1; % data1 & data2 share memory; no allocation done data2(1,1) = 0; % data2 allocated, copied and only then modified

If we profile our code using any of Matlab’s memory-profiling options, we will see that the copy operation `data2=data1`

takes negligible time to run and allocates no memory. On the other hand, the simple update operation `data2(1,1)=0`

, which we could otherwise have assumed to take minimal time and memory, actually takes a relatively long time and allocates 191 MB of memory.

We first see a memory spike (used during the computation of the magic square data), closely followed by a leveling off at 190.7MB above the baseline (this is due to allocation of data1). Copying `data2=data1`

has no discernible effect on either CPU or memory. Only when we set `data2(1,1)=0`

does the CPU return, in order to allocate the extra 190MB for data2. When we exit the test function, data1 and data2 are both deallocated, returning the Matlab process memory to its baseline level.

There are several lessons that we can draw from this simple example:

Firstly, creating copies of data does not necessarily or immediately impact memory and performance. Rather, it is the update of these copies which may be problematic. If we can modify our code to use more read-only data and less updated data copies, then we would improve performance. The Profiler report will show us exactly where in our code we have memory and CPU hotspots – these are the places we should consider optimizing.

Secondly, when we see such odd behavior in our Profiler reports (i.e., memory and/or CPU spikes that occur on seemingly innocent code lines), we should be aware of the copy-on-write mechanism, which could be the cause for the behavior.

The copy-on-write mechanism behaves similarly for input parameters in functions: whenever a function is invoked (called) with input data, the memory allocated for this data is used up until the point that one of its copies is modified. At that point, the copies diverge: a new memory block is allocated, populated with data from the shared memory block, and assigned to the modified variable. Only then is the update done on the new memory block.

data1 = magic(5000); % 5Kx5K elements = 191 MB data2 = perfTest(data1); function outData = perfTest(inData) outData = inData; % inData & outData share memory; no allocation outData2(1,1) = 0; % outData allocated, copied and then modified end

One lesson that can be drawn from this is that whenever possible we should attempt to use functions that do not modify their input data. This is particularly true if the modified input data is very large. Read-only functions will be faster than functions that do even the simplest of data updates.

Another lesson is that perhaps counter intuitively, it does not make a difference from a performance standpoint to pass read-only data to functions as input parameters. We might think that passing large data objects around as function parameters will involve multiple memory allocations and deallocations of the data. In fact, it is only the data’s reference (or more precisely, its mxArray structure) which is being passed around and placed on the function’s call stack. Since this reference/structure is quite small in size, there are no real performance penalties. In fact, this only benefits code clarity and maintainability.

The only case where we may wish to use other means of passing data to functions is when a large data object needs to be updated. In such cases, the updated copy will be allocated to a new memory block with an associated performance cost.

Matlab’s interpreter, at least in recent releases, has a very sophisticated algorithm for using in-place data manipulation (report). Modifying data in-place means that the original data block is modified, rather than creating a new block with the modified data, thus saving any memory allocations and deallocations.

For example, let us manipulate a simple 4Kx4K (122MB) numeric array:

>> m = magic(4000); % 4Kx4K = 122MB >> memory Maximum possible array: 1022 MB (1.072e+09 bytes) Memory available for all arrays: 1218 MB (1.278e+09 bytes) Memory used by MATLAB: 709 MB (7.434e+08 bytes) Physical Memory (RAM): 3002 MB (3.148e+09 bytes) % In-place array data manipulation: no memory allocated >> m = m * 0.5; >> memory Maximum possible array: 1022 MB (1.072e+09 bytes) Memory available for all arrays: 1214 MB (1.273e+09 bytes) Memory used by MATLAB: 709 MB (7.434e+08 bytes) Physical Memory (RAM): 3002 MB (3.148e+09 bytes) % New variable allocated, taking an extra 122MB of memory >> m2 = m * 0.5; >> memory Maximum possible array: 1022 MB (1.072e+09 bytes) Memory available for all arrays: 1092 MB (1.145e+09 bytes) Memory used by MATLAB: 831 MB (8.714e+08 bytes) Physical Memory (RAM): 3002 MB (3.148e+09 bytes)

The extra memory allocation of the not-in-place manipulation naturally translates into a performance loss:

% In-place data manipulation, no memory allocation >> tic, m = m * 0.5; toc Elapsed time is 0.056464 seconds. % Regular data manipulation (122MB allocation) – 50% slower >> clear m2; tic, m2 = m * 0.5; toc; Elapsed time is 0.084770 seconds.

The difference may not seem large, but placed in a loop it could become significant indeed, and might be much more important if virtual memory swapping comes into play, or when Matlab’s memory space is exhausted (out-of-memory error).

Similarly, when returning data from a function, we should try to update the original data variable whenever possible, avoiding the need for allocation of a new variable:

% In-place data manipulation, no memory allocation >> d=0:1e-7:1; tic, d = sin(d); toc Elapsed time is 0.083397 seconds. % Regular data manipulation (76MB allocation) – 50% slower >> clear d2, d=0:1e-7:1; tic, d2 = sin(d); toc Elapsed time is 0.121415 seconds.

Within the function itself we should ensure that we return the modified input variable, and not assign the output to a new variable, so that in-place optimization can also be applied within the function. The in-place optimization mechanism is smart enough to override Matlab’s default copy-on-write mechanism, which automatically allocates a new copy of the data when it sees that the input data is modified:

% Suggested practice: use in-place optimization within functions function x = function1(x) x = someOperationOn(x); % temporary variable x is NOT allocated end % Standard practice: prevents future use of in-place optimizations function y = function2(x) y = someOperationOn(x); % new temporary variable y is allocated end

In order to benefit from in-place optimizations of function results, we must both use the same variable in the caller workspace (x = function1(x)) and also ensure that the called function is optimizable (e.g., function x = function1(x)) – if any of these two requirements is not met then in-place function-call optimization is not performed.

Also, for the in-place optimization to be active, we need to call the in-place function from within another function, not from a script or the Matlab Command Window.

A related performance trick is to use masks on the original data rather than temporary data copies. For example, suppose we wish to get the result of a function that acts on only a portion of some large data. If we create a temporary variable that holds the data subset and then process it, it would create an unnecessary copy of the original data:

% Original data data = 0 : 1e-7 : 1; % 10^7 elements, 76MB allocated % Unnecessary copy of data into data2 (extra 8MB allocated) data2 = data(data>0.1); % 10^6 elements, 7.6MB allocated results = sin(data2); % another 10^6 elements, 7.6MB allocated % Use of data masks obviates the need for temporary variable data2: results = sin(data(data>0.1)); % no need for the data2 allocation

A note of caution: we should not invest undue efforts to use in-place data manipulation if the overall benefits would be negligible. It would only help if we have a real memory limitation issue and the data matrix is very large.

Matlab in-place optimization is a topic of continuous development. Code which is not in-place optimized today (for example, in-place manipulation on class object properties) may possibly be optimized in next year’s release. For this reason, it is important to write the code in a way that would facilitate the future optimization (for example, obj.x=2*obj.x rather than y=2*obj.x).

Some in-place optimizations were added to the JIT Accelerator as early as Matlab 6.5 R13, but Matlab 7.3 R2006b saw a major boost. As Matlab’s JIT Accelerator improves from release to release, we should expect in-place data manipulations to be automatically applied in an increasingly larger number of code cases.

In some older Matlab releases, and in some complex data manipulations where the JIT Accelerator cannot implement in-place processing, a temporary storage is allocated that is assigned to the original variable when the computation is done. To implement in-place data manipulations in such cases we could develop an external function (e.g., using Mex) that directly works on the original data block. Note that the officially supported mex update method is to always create deep-copies of the data using *mxDuplicateArray()* and then modify the new array rather than the original; modifying the original data directly is both discouraged and not officially supported. Doing it incorrectly can easily crash Matlab. If you do directly overwrite the original input data, at least ensure that you unshare any variables that share the same data memory block, thus mimicking the copy-on-write mechanism.

Using Matlab’s internal in-place data manipulation is very useful, especially since it is done automatically without need for any major code changes on our part. But sometimes we need certainty of actually processing the original data variable without having to guess or check whether the automated in-place mechanism will be activated or not. This can be achieved using several alternatives:

- Using global or persistent variable
- Using a parent-scope variable within a nested function
- Modifying a reference (handle class) object’s internal properties

Related posts:

- Internal Matlab memory optimizations – Copy-on-write and in-place data manipulations are very useful Matlab performance improvement techniques. ...
- Allocation performance take 2 – The clear function has some non-trivial effects on Matlab performance. ...
- Quirks with parfor vs. for – Parallelizing loops with Matlab's parfor might generate unexpected results. Users beware! ...

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Unfortunately, such simple preallocation is not always possible. Apparently, all is not lost. There are still a few things we can do to mitigate the performance pain. As in last week, there is much more here than meets the eye at first sight.

The interesting newsgroup thread from 2005 about this issue that I mentioned last week contains two main solutions to this problem. The effects of these solutions is negligible for small data sizes and/or loop iterations (i.e., number of memory reallocations), but could be dramatic for large data arrays and/or a large number of memory reallocations. The difference could well mean the difference between a usable and an unusable (“hang”) program:

The idea is to dynamically grow the array by a certain percentage factor each time. When the array first needs to grow by a single element, we would in fact grow it by a larger chunk (say 40% of the current array size, for example by using the * repmat* function, or by concatenating a specified number of

This method has a theoretical cost of N·log(N), which is nearly linear in N for most practical purposes. It is similar to preallocation in the sense that we are preparing a chunk of memory for future array use in advance. You might say that this is on-the-fly preallocation.

The idea here is to use cell arrays to store and grow the data, then use cell2mat to convert the resulting cell array to a regular numeric array. Cell elements are implemented as references to distinct memory blocks, so concatenating an object to a cell array merely concatenates its reference; when a cell array is reallocated, only its internal references (not the referenced data) are moved. Note that this relies on the internal implementation of cell arrays in Matlab, and may possibly change in some future release.

Like factor growth, using cell arrays is faster than quadratic behavior (although not quite as fast enough as we would have liked, of course). Different situations may favor using either the cell arrays method or the factor growth mechanism.

John D’Errico has posted a well-researched utility called * growdata* that optimizes dynamic array growth for maximal performance. It is based in part on ideas mentioned in the aforementioned 2005 newsgroup thread, where

As an interesting side-note, John D’Errico also recently posted an extremely fast implementation of the Fibonacci function. The source code may seem complex, but the resulting performance gain is well worth the extra complexity. I believe that readers who will read this utility’s source code and understand its underlying logic will gain insight into several performance tricks that could be very useful in general.

The introduction of JIT Acceleration in Matlab 6.5 (R13) caused a dramatic boost in performance (there is an internal distinction between the Accelerator and JIT: JIT is apparently only part of the Matlab Accelerator, but this distinction appears to have no practical impact on the discussion here).

Over the years, MathWorks has consistently improved the efficiency of its computational engine and the JIT Accelerator in particular. JIT was consistently improved since that release, giving a small improvement with each new Matlab release. In Matlab 7.11 (R2010b), the short Fibonacci snippet used in last week’s article showed executed about 30% faster compared to Matlab 7.1 R14SP3. The behavior was still quadratic in nature, and so in these releases, using any of the above-mentioned solutions could prove very beneficial.

In Matlab 7.12 (R2011a), a major improvement was done in the Matlab engine (JIT?). The execution run-times improved significantly, and in addition have become linear in nature. This means that multiplying the array size by N only degrades performance by N, not N^{2} – an impressive achievement:

% This is ran on Matlab 7.14 (R2012a): clear f, tic, f=[0,1]; for idx=3:10000, f(idx)=f(idx-1)+f(idx-2); end, toc => Elapsed time is 0.004924 seconds. % baseline loop size, & exec time clear f, tic, f=[0,1]; for idx=3:20000, f(idx)=f(idx-1)+f(idx-2); end, toc => Elapsed time is 0.009971 seconds. % x2 loop size, x2 execution time clear f, tic, f=[0,1]; for idx=3:40000, f(idx)=f(idx-1)+f(idx-2); end, toc => Elapsed time is 0.019282 seconds. % x4 loop size, x4 execution time

In fact, it turns out that using either the cell arrays method or the factor growth mechanism is much slower in R2011a than using the naïve dynamic growth!

This teaches us a very important lesson: It is not wise to program against a specific implementation of the engine, at least not in the long run. While this may yield performance benefits on some Matlab releases, the situation may well be reversed on some future release. This might force us to retest, reprofile and potentially rewrite significant portions of code for each new release. Obviously this is not a maintainable solution. In practice, most code that is written on some old Matlab release would likely we carried over with minimal changes to the newer releases. If this code has release-specific tuning, we could be shooting ourselves in the leg in the long run.

MathWorks strongly advises (and again, and once again), and I concur, to program in a natural manner, rather than in a way that is tailored to a particular Matlab release (unless of course we can be certain that we shall only be using that release and none other). This will improve development time, maintainability and in the long run also performance.

*(and of course you could say that a corollary lesson is to hurry up and get the latest Matlab release…)*

If we are left with using a naïve dynamic resize, there are several equivalent alternatives for doing this, having significantly different performances:

% This is ran on Matlab 7.12 (R2011a): % Variant #1: direct assignment into a specific out-of-bounds index data=[]; tic, for idx=1:100000; data(idx)=1; end, toc => Elapsed time is 0.075440 seconds. % Variant #2: direct assignment into an index just outside the bounds data=[]; tic, for idx=1:100000; data(end+1)=1; end, toc => Elapsed time is 0.241466 seconds. % 3 times slower % Variant #3: concatenating a new value to the array data=[]; tic, for idx=1:100000; data=[data,1]; end, toc => Elapsed time is 22.897688 seconds. % 300 times slower!!!

As can be seen, it is much faster to directly index an out-of-bounds element as a means to force Matlab to enlarge a data array, rather than using the end+1 notation, which needs to recalculate the value of end each time.

In any case, try to avoid using the concatenation variant, which is significantly slower than either of the other two alternatives (300 times slower in the above example!). In this respect, there is no discernible difference between using the [] operator or the * cat()* function for the concatenation.

Apparently, the JIT performance boost gained in Matlab R2011a does not work for concatenation. Future JIT improvements may possibly also improve the performance of concatenations, but in the meantime it is better to use direct indexing instead.

The effect of the JIT performance boost is easily seen when we run the same variants on pre-R2011a Matlab releases. The corresponding values are 30.9, 34.8 and 34.3 seconds. Using direct indexing is still the fastest approach, but concatenation is now only 10% slower, not 300 times slower.

When we need to append a non-scalar element (for example, a 2D matrix) to the end of an array, we might think that we have no choice but to use the slow concatenation method. This assumption is incorrect: we can still use the much faster direct-indexing method, as shown below (notice the non-linear growth in execution time for the concatenation variant):

% This is ran on Matlab 7.12 (R2011a): matrix = magic(3); % Variant #1: direct assignment – fast and linear cost data=[]; tic, for idx=1:10000; data(:,(idx*3-2):(idx*3))=matrix; end, toc => Elapsed time is 0.969262 seconds. data=[]; tic, for idx=1:100000; data(:,(idx*3-2):(idx*3))=matrix; end, toc => Elapsed time is 9.558555 seconds. % Variant #2: concatenation – much slower, quadratic cost data=[]; tic, for idx=1:10000; data=[data,matrix]; end, toc => Elapsed time is 2.666223 seconds. data=[]; tic, for idx=1:100000; data=[data,matrix]; end, toc => Elapsed time is 356.567582 seconds.

As the size of the array enlargement element (in this case, a 3×3 matrix) increases, the computer needs to allocate more memory space more frequently, thereby increasing execution time and the importance of preallocation. Even if the system has an internal memory-management mechanism that enables it to expand into adjacent (contiguous) empty memory space, as the size of the enlargement grows the empty space will run out sooner and a new larger memory block will need to be allocated more frequently than in the case of small incremental enlargements of a single 8-byte double.

If preallocation is not possible, JIT is not very helpful, vectorization is out of the question, and rewriting the problem so that it doesn’t need dynamic array growth is impossible – if all these are not an option, then consider using one of the following alternatives for array growth (read again the interesting newsgroup thread from 2005 about this issue):

- Dynamically grow the array by a certain percentage factor each time the array runs out of space (on-the-fly preallocation)
- Use John D’Errico’s
utility**growdata** - Use cell arrays to store and grow the data, then use cell2mat to convert the resulting cell array to a regular numeric array
- Reuse an existing data array that has the necessary storage space
- Wrap the data in a referential object (a class object that inherits from handle), then append the reference handle rather than the original data (ref). Note that if your class object does not inherit from handle, it is not a referential object but rather a value object, and as such it will be appended in its entirety to the array data, losing any performance benefits. Of course, it may not always be possible to wrap our class objects as a handle.
References have a much small memory footprint than the objects that they reference. The objects themselves will remain somewhere in memory and will not need to be moved whenever the data array is enlarged and reallocated – only the small-footprint reference will be moved, which is much faster. This is also the reason that cell concatenation is faster than array concatenations for large objects.

Related posts:

- Internal Matlab memory optimizations – Copy-on-write and in-place data manipulations are very useful Matlab performance improvement techniques. ...
- Allocation performance take 2 – The clear function has some non-trivial effects on Matlab performance. ...
- Quirks with parfor vs. for – Parallelizing loops with Matlab's parfor might generate unexpected results. Users beware! ...

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A note of caution: in the examples that follow, don’t take any speedup as an expected actual value – the actual value may well be different on your system. Your mileage may vary. I only mean to display the relative differences between different alternatives.

Memory management has a direct influence on performance. I have already shown some examples of this in past articles here.

Preallocation solves a basic problem in simple program loops, where an array is iteratively enlarged with new data (dynamic array growth). Unlike other programming languages (such as C, C++, C# or Java) that use static typing, Matlab uses dynamic typing. This means that it is natural and easy to modify array size dynamically during program execution. For example:

fibonacci = [0, 1]; for idx = 3 : 100 fibonacci(idx) = fibonacci(idx-1) + fibonacci(idx-2); end

While this may be simple to program, it is not wise with regards to performance. The reason is that whenever an array is resized (typically enlarged), Matlab allocates an entirely new contiguous block of memory for the array, copying the old values from the previous block to the new, then releasing the old block for potential reuse. This operation takes time to execute. In some cases, this reallocation might require accessing virtual memory and page swaps, which would take an even longer time to execute. If the operation is done in a loop, then performance could quickly drop off a cliff.

The cost of such naïve array growth is theoretically quadratic. This means that multiplying the number of elements by N multiplies the execution time by about N^{2}. The reason for this is that Matlab needs to reallocate N times more than before, and each time takes N times longer due to the larger allocation size (the average block size multiplies by N), and N times more data elements to copy from the old to the new memory blocks.

A very interesting discussion of this phenomenon and various solutions can be found in a newsgroup thread from 2005. Three main solutions were presented: preallocation, selective dynamic growth (*allocating headroom*) and using cell arrays. The best solution among these in terms of ease of use and performance is preallocation.

The basic idea of preallocation is to create a data array in the final expected size before actually starting the processing loop. This saves any reallocations within the loop, since all the data array elements are already available and can be accessed. This solution is useful when the final size is known in advance, as the following snippet illustrates:

% Regular dynamic array growth: tic fibonacci = [0,1]; for idx = 3 : 40000 fibonacci(idx) = fibonacci(idx-1) + fibonacci(idx-2); end toc => Elapsed time is 0.019954 seconds. % Now use preallocation – 5 times faster than dynamic array growth: tic fibonacci = zeros(40000,1); fibonacci(1)=0; fibonacci(2)=1; for idx = 3 : 40000, fibonacci(idx) = fibonacci(idx-1) + fibonacci(idx-2); end toc => Elapsed time is 0.004132 seconds.

On pre-R2011a releases the effect of preallocation is even more pronounced: I got a 35-times speedup on the same machine using Matlab 7.1 (R14 SP3). R2011a (Matlab 7.12) had a dramatic performance boost for such cases in the internal accelerator, so newer releases are much faster in dynamic allocations, but preallocation is still 5 times faster even on R2011a.

Because the effect of preallocation is so dramatic on all Matlab releases, it makes sense to utilize it even in cases where the data array’s final size is not known in advance. We can do this by estimating an upper bound to the array’s size, preallocate this large size, and when we’re done remove any excess elements:

% The final array size is unknown – assume 1Kx3K upper bound (~23MB) data = zeros(1000,3000); % estimated maximal size numRows = 0; numCols = 0; while (someCondition) colIdx = someValue1; numCols = max(numCols,colIdx); rowIdx = someValue2; numRows = max(numRows,rowIdx); data(rowIdx,colIdx) = someOtherValue; end % Now remove any excess elements data(:,numCols+1:end) = []; % remove excess columns data(numRows+1:end,:) = []; % remove excess rows

It turns out that MathWorks’ official suggestion for preallocation, namely using the * zeros* function, is not the most efficient:

% MathWorks suggested variant clear data1, tic, data1 = zeros(1000,3000); toc => Elapsed time is 0.016907 seconds. % A much faster alternative - 500 times faster! clear data1, tic, data1(1000,3000) = 0; toc => Elapsed time is 0.000034 seconds.

The reason for the second variant being so much faster is because it only allocates the memory, without worrying about the internal values (they get a default of 0, *false* or ”, in case you wondered). On the other hand, * zeros* has to place a value in each of the allocated locations, which takes precious time.

In most cases the differences are immaterial since the preallocation code would only run once in the program, and an extra 17ms isn’t such a big deal. But in some cases we may have a need to periodically refresh our data, where the extra run-time could quickly accumulate.

** Update (October 27, 2015)**: As Marshall notes below, this behavior changed in R2015b when the new LXE (Matlab’s new execution engine) replaced the previous engine. In R2015b, the

When we need to preallocate a specific value into every data array element, we cannot use Variant #2. The reason is that Variant #2 only sets the very last data element, and all other array elements get assigned the default value (0, ‘’ or false, depending on the array’s data type). In this case, we can use one of the following alternatives (with their associated timings for a 1000×3000 data array):

scalar = pi; % for example... data = scalar(ones(1000,3000)); % Variant A: 87.680 msecs data(1:1000,1:3000) = scalar; % Variant B: 28.646 msecs data = repmat(scalar,1000,3000); % Variant C: 17.250 msecs data = scalar + zeros(1000,3000); % Variant D: 17.168 msecs data(1000,3000) = 0; data = data+scalar; % Variant E: 16.334 msecs

As can be seen, Variants C-E are about twice as fast as Variant B, and 5 times faster than Variant A.

7.4.5 Preallocating non-double data

When preallocating an array of a type that is not * double*, we should be careful to create it using the desired type, to prevent memory and/or performance inefficiencies. For example, if we need to process a large array of small integers (

% Bad idea: allocates 8MB double array, then converts to 1MB int8 array data = int8(zeros(1000,1000)); % 1M elements => Elapsed time is 0.008170 seconds. % Better: directly allocate the array as a 1MB int8 array – x80 faster data = zeros(1000,1000,'int8'); => Elapsed time is 0.000095 seconds.

To preallocate a cell-array we can use the cell function (explicit preallocation), or the maximal cell index (implicit preallocation). Explicit preallocation is faster than implicit preallocation, but functionally equivalent (Note: this is contrary to the experience with allocation of numeric arrays and other arrays):

% Variant #1: Explicit preallocation of a 1Kx3K cell array data = cell(1000,3000); => Elapsed time is 0.004637 seconds. % Variant #2: Implicit preallocation – x3 slower than explicit clear('data'), data{1000,3000} = []; => Elapsed time is 0.012873 seconds.

To preallocate an array of structs or class objects, we can use the * repmat* function to replicate copies of a single data element (explicit preallocation), or just use the maximal data index (implicit preallocation). In this case, unlike the case of cell arrays, implicit preallocation is much faster than explicit preallocation, since the single element does not actually need to be copied multiple times (ref):

% Variant #1: Explicit preallocation of a 100x300 struct array element = struct('field1',magic(2), 'field2',{[]}); data = repmat(element, 100, 300); => Elapsed time is 0.002804 seconds. % Variant #2: Implicit preallocation – x7 faster than explicit element = struct('field1',magic(2), 'field2',{[]}); clear('data'), data(100,300) = element; => Elapsed time is 0.000429 seconds.

When preallocating structs, we can also use a third variant, using the built-in struct feature of replicating the struct when the * struct* function is passed a cell array. For example,

`struct('field1',cell(100,1), 'field2',5)`

will create 100 structs, each of them having the empty field When preallocating in general, ensure that you are using the maximal expected array size. There is no point in preallocating an empty array or an array having a smaller size than the expected maximum, since dynamic memory reallocation will automatically kick-in within the processing-loop. For this reason, do not use the *empty()* method of class objects to preallocate, but rather * repmat* as explained above.

When using * repmat* to replicate class objects, always be careful to note whether you are replicating the object itself (this happens if your class does NOT derive from

Next week: what if we can’t avoid dynamic array resizing? – apparently, all is not lost. Stay tuned…

Do you have any similar allocation-related tricks you’re using? or unexpected differences such as the ones shown above? If so, then please do post a comment.

Related posts:

- Matlab mex in-place editing – Editing Matlab arrays in-place can be an important technique for optimizing calculations. This article shows how to do it using Mex. ...
- Parsing mlint (Code Analyzer) output – The Matlab Code Analyzer (mlint) has a lot of undocumented functionality just waiting to be used. ...
- Allocation performance take 2 – The clear function has some non-trivial effects on Matlab performance. ...

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*Disclaimer: The information in this article is provided for informational purposes only. Be aware that poking into Matlab’s internals is not condoned or supported by MathWorks, and is not recommended for any regular usage. Poking into memory has the potential to crash your computer so save your data! Moreover, be advised (as the text below will show) that the information is highly prone to change without any advance notice in future Matlab releases, which could lead to very adverse effects on any program that relies on it. On the scale of undocumented Matlab topics, this practically breaks the scale, so be EXTREMELY careful when using this.*

A few weeks ago I discussed Matlab’s copy-on-write mechanism as part of my discussion of editing Matlab arrays in-place. Today I want to explore some behind-the-scenes details of how the copy-on-write mechanism is implemented. In the process we will learn a little about Matlab’s internal array representation. I will also introduce some simple tools you can use to explore more of Matlab’s internals. I will only cover basic information, so there are plenty more details left to be filled in by others who are interested.

Copy-on-write is Matlab’s mechanism for avoiding unnecessary duplication of data in memory. To implement this, Matlab needs to keep track internally of which sets of variables are copies of each other. As described in MathWorks’s article, “*the Matlab language works with a single object type: the Matlab array. All Matlab variables (including scalars, vectors, matrices, strings, cell arrays, structures, and objects) are stored as Matlab arrays. In C/C++, the Matlab array is declared to be of type mxArray*“. This means that

`mxArray`

defines how Matlab lays out all the information about an array (its Matlab data type, its size, its data, etc.) in memory. So understanding Matlab’s internal array representation basically boils down to understanding `mxArray`

.Unfortunately, MathWorks also tells us that “* mxArray is a C language opaque type*“. This means that MathWorks does not expose the organization of

`mxArray`

to users (i.e. Matlab or Mex programmers). Instead, MathWorks defines `mxArray`

internally, and allows users to interact with it only through an API, a set of functions that know how to handle `mxArray`

in their back end. So, for example, a Mex programmer does not get the dimensions of an `mxArray`

by directly accessing the relevant field in memory. Instead, the Mex programmer only has a pointer to the `mxArray`

, and passes this pointer into an API function that knows where in memory to find the requested information and then passes the result back to the programmer.This is generally a good thing: the API provides an abstraction layer between the programmer and the memory structures so that if MathWorks needs to change the back end organization (to add a new feature for example), we programmers do not need to modify our code; instead MathWorks just updates the API to reflect the new internal organization. On the other hand, being able to look into the internal structure of `mxArray`

on occasion can help us understand how Matlab works, and can help us write more efficient code if we are careful as in the example of editing arrays in-place.

So how do we get a glimpse inside `mxArray`

? The first step is simply to find the region of memory where the `mxArray`

lives: its beginning and end. Finding where in memory the `mxArray`

begins is pretty easy: it is given by its pointer value. Here is a simple Mex function that takes a Matlab array as input and prints its memory address:

/* printaddr.cpp */ #include "mex.h" void mexFunction( int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) { if (nrhs < 1) mexErrMsgTxt("One input required."); printf("%p\n", prhs[0]); }

This function is nice as it prints the address in a standard hexadecimal format. The same information can also be received directly in Matlab (i.e., without needing * printaddr*), using the undocumented

>> format debug >> A = 1:10 A = Structure address = 7fc3b8869ae0 m = 1 n = 10 pr = 7fc44922c890 pi = 0 1 2 3 4 5 6 7 8 9 10 >> printaddr(A) 7fc3b8869ae0

To play with this further from within Matlab however, it’s nice to have the address returned to us as a 64-bit unsigned integer; here’s a Mex function that does that:

/* getaddr.cpp */ #include "mex.h" void mexFunction( int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) { if (nrhs < 1) mexErrMsgTxt("One input required."); plhs[0] = mxCreateNumericMatrix(1, 1, mxUINT64_CLASS, mxREAL); unsigned long *out = static_cast<unsigned long *>(mxGetData(plhs[0])); out[0] = (unsigned long) prhs[0]; }

Here’s * getaddr* in action:

>> getaddr(A) ans = 139870853618400 % And using pure Matlab: >> hex2dec('7f36388b5ae0') % output of printaddr or format debug ans = 139870853618400

So now we know where to find our array in memory. With this information we can already learn a lot. To make our exploration a little cleaner though, it would be nice to know where the array ends in memory too, in other words we would like to know the size of the `mxArray`

.

The first thing to understand is that the amount of memory taken by an `mxArray`

does not have anything to do with the dimensions of the array in Matlab. So a 1×1 Matlab array and a 100×100 Matlab array have the same size `mxArray`

representation in memory. As you will know if you have experience programming in Mex, this is simply because the Matlab array’s data contents are not stored directly within `mxArray`

. Instead, `mxArray`

only stores a pointer to another memory location where the actual data reside. This is fine; the internal information we want to poke into is all still in `mxArray`

, and it is easy to get the pointer to the array’s data contents using the API functions *mxGetData* or *mxGetPr*.

So we are still left with trying to figure out the size of `mxArray`

. There are a couple paths forward. First I want to talk about a historical tool that used to make a lot of this internal information easily available. This was a function called *headerdump*, by Peter Boetcher (described here and here). *headerdump* was created for exactly the goal we are currently working towards: to understand Matlab’s copy-on-write mechanism. Unfortunately, as Matlab has evolved, newer versions have incrementally broken this useful tool. So our goal here is to create a replacement. Still, we can learn a lot from the earlier work.

One of the things that helped people figure out Matlab’s internals in the past is that in older versions of Matlab `mxArray`

is not a completely opaque type. Even in recent versions up through at least R2010a, if you look into $MATLAB/extern/include/matrix.h you can find a definition of `mxArray_tag`

that looks something like this:

/* R2010a */ struct mxArray_tag { void *reserved; int reserved1[2]; void *reserved2; size_t number_of_dims; unsigned int reserved3; struct { unsigned int flag0 : 1; unsigned int flag1 : 1; unsigned int flag2 : 1; unsigned int flag3 : 1; unsigned int flag4 : 1; unsigned int flag5 : 1; unsigned int flag6 : 1; unsigned int flag7 : 1; unsigned int flag7a : 1; unsigned int flag8 : 1; unsigned int flag9 : 1; unsigned int flag10 : 1; unsigned int flag11 : 4; unsigned int flag12 : 8; unsigned int flag13 : 8; } flags; size_t reserved4[2]; union { struct { void *pdata; void *pimag_data; void *reserved5; size_t reserved6[3]; } number_array; } data; };

This is what you could call murky or obfuscated, but not completely opaque. The fields mostly have unhelpful names like “reserved”, but on the other hand we at least have a sense for what fields there are and their layout.

A more informative (yet unofficial) definition was provided by James Tursa and Peter Boetcher:

#include "mex.h" /* Definition of structure mxArray_tag for debugging purposes. Might not be fully correct * for Matlab 2006b or 2007a, but the important things are. Thanks to Peter Boettcher. */ struct mxArray_tag { const char *name; mxClassID class_id; int vartype; mxArray *crosslink; int number_of_dims; int refcount; struct { unsigned int scalar_flag : 1; unsigned int flag1 : 1; unsigned int flag2 : 1; unsigned int flag3 : 1; unsigned int flag4 : 1; unsigned int flag5 : 1; unsigned int flag6 : 1; unsigned int flag7 : 1; unsigned int private_data_flag : 1; unsigned int flag8 : 1; unsigned int flag9 : 1; unsigned int flag10 : 1; unsigned int flag11 : 4; unsigned int flag12 : 8; unsigned int flag13 : 8; } flags; int rowdim; int coldim; union { struct { double *pdata; // original: void* double *pimag_data; // original: void* void *irptr; void *jcptr; int nelements; int nfields; } number_array; struct { mxArray **pdata; char *field_names; void *dummy1; void *dummy2; int dummy3; int nfields; } struct_array; struct { void *pdata; /*mxGetInfo*/ char *field_names; char *name; int checksum; int nelements; int reserved; } object_array; } data; };

For comparison, here is another definition from an earlier version of Matlab.

/* R11 aka Matlab 5.0 (1999) */ struct mxArray_tag { char name[mxMAXNAM]; int class_id; int vartype; mxArray *crosslink; int number_of_dims; int nelements_allocated; int dataflags; int rowdim; int coldim; union { struct { void *pdata; void *pimag_data; void *irptr; void *jcptr; int reserved; int nfields; } number_array; } data; };

I took this R11 definition from the source code to *headerdump* (specifically, from *mxinternals.h*, which also has `mxArray_tag`

definitions for R12 (Matlab 6.0) and R13 (Matlab 6.5)), and you can see that it is much more informative, because many fields have been given useful names thanks to the work of Peter Boetcher and others. Note also that the definition from this old version of Matlab is quite different from the version from R2010a.

At this point, if you are running a much earlier version of Matlab like R11 or R13, you can break off from the current article and start playing around with *headerdump* directly to try to understand Matlab’s internals. For more recent versions of Matlab, we have more work to do. Getting back to our original goal, if we take the `mxArray_tag`

definition from R2010a and run *sizeof*, we get an answer for the amount of memory taken up by an `mxArray`

in R2010a: **104 bytes**.

It was nice to derive the size of `mxArray`

from actual MathWorks code, but unfortunately this information is no longer available as of R2011a. Somewhere between R2010a and R2011a, MathWorks stepped up their efforts to make `mxArray`

completely opaque. So we should find another way to get the size of `mxArray`

for current and future Matlab versions.

One ugly trick that works is to create many new arrays quickly and see where their starting points end up in memory:

>> A = num2cell(1:100)'; >> addrs = sort(cellfun(@getaddr, A));

What we did here is create 100 new arrays, and then get all their memory addresses in sorted order. Now we can take a look at how far apart these new arrays ended up in memory:

>> semilogy(diff(addrs));

The resulting plot will look different each time you run this; it is not really predictable where Matlab will put new arrays into memory. Here is an example from my system:

Your results may look different, and you might have to increase the number of new arrays from 100 to 1000 to get the qualitative result, but the important feature of this plot is that there is a minimum distance between new arrays of about 10^{2}. In fact, if we just go straight for this minimum distance:

>> min(diff(addrs)) ans = 104

we find that although `mxArray`

has gone completely opaque from R2010a to R2011a, the full size of `mxArray`

in memory has stayed the same: 104 bytes.

We now have all the information we need to start looking into Matlab’s array representation. There are many tools available that allow you to browse memory locations or dump memory contents to disk. For our purposes though, it is nice to be able to do everything from within Matlab. Therefore I introduce a simple tool that prints memory locations into the Matlab console:

/* printmem.cpp */ #include "mex.h" void mexFunction( int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) { if (nrhs < 1 || !mxIsUint64(prhs[0]) || mxIsEmpty(prhs[0])) mexErrMsgTxt("First argument must be a uint64 memory address"); unsigned long *addr = static_cast<unsigned long *>(mxGetData(prhs[0])); unsigned char *mem = (unsigned char *) addr[0]; if (nrhs < 2 || !mxIsDouble(prhs[1]) || mxIsEmpty(prhs[1])) mexErrMsgTxt("Second argument must be a double-type integer byte size."); unsigned int nbytes = static_cast<unsigned int>(mxGetScalar(prhs[1])); for (int i = 0; i < nbytes; i++) { printf("%.2x ", mem[i]); if ((i+1) % 16 == 0) printf("\n"); } printf("\n"); }

Here is how you use it in Matlab:

>> A = 0; >> printmem(getaddr(A), 104) 00 00 00 00 00 00 00 00 06 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 01 02 00 00 01 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 70 fa 33 df 6f 7f 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

And there you have it: the inner guts of `mxArray`

laid bare. I have printed each byte as a two character hexadecimal value, as is standard, so there are 16 bytes printed per row.

So now we have 104 bytes of Matlab internals to dig into. We can start playing with this with a few simple examples:

>> A = 0; B = 1; >> printmem(getaddr(A), 104) 00 00 00 00 00 00 00 00 06 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 01 02 00 00 01 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 c0 b0 27 df 6f 7f 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 >> printmem(getaddr(B), 104) 00 00 00 00 00 00 00 00 06 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 01 02 00 00 01 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 70 fa 33 df 6f 7f 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

In this and subsequent examples, I will highlight bytes that are different or that are of interest. What we can see from this example is that although arrays A and B have different content, almost nothing is different between their `mxArray`

representations. What is different, is the memory address stored in the highlighted bytes. This confirms our earlier assertion that `mxArray`

does not store the array contents, but only a pointer to the content location.

Now let us try to figure out some of the other fields:

>> A = 1:3; B = 1:10; C = (1:10)'; >> printmem(getaddr(A), 64) 00 00 00 00 00 00 00 00 06 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 00 02 00 00 01 00 00 00 00 00 00 00 03 00 00 00 00 00 00 00 60 80 22 df 6f 7f 00 00 >> printmem(getaddr(B), 64) 00 00 00 00 00 00 00 00 06 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 00 02 00 00 01 00 00 00 00 00 00 00 0a 00 00 00 00 00 00 00 80 83 29 df 6f 7f 00 00 >> printmem(getaddr(C), 64) 00 00 00 00 00 00 00 00 06 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 00 02 00 00 0a 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00 80 83 29 df 6f 7f 00 00

(Note that this time I only printed the first four lines of each array as this is where the interesting differences are for this example.)

In red I highlighted the bytes in each array that give its number of rows and columns (note that hexadecimal 0a is 10 in decimal). In blue I highlighted areas that store the value “02”, which could be the location for storing the number of dimensions. Let us look into this more:

>> A = rand([3 3 3]); >> printmem(getaddr(A), 64) 00 00 00 00 00 00 00 00 06 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 03 00 00 00 00 00 00 00 00 00 00 00 00 02 00 00 30 4a 3f df 6f 7f 00 00 09 00 00 00 00 00 00 00 b0 d3 24 df 6f 7f 00 00

Two interesting results here: The first highlighted region changed from 02 to 03, so this must be the place where `mxArray`

indicates a 3-dimensional array rather than 2D. Another important thing also changed though: we can see in the second highlighted region that there is a new memory address stored where we used to find the number of rows. And in the third highlighted region we now have the number 09 instead of the number of columns.

Clearly, Matlab has a different way of representing a 2D matrix versus arrays of higher dimension such as 3D. In the 2D case, `mxArray`

simply holds the nrows and ncols directly, but for a higher dimension case we hold only the number of dimensions (03), the total number of elements (09), and a pointer to another memory location (0x7f6fdf3f4a30) which holds the array of sizes for each dimension.

Finally, we are in a position to understand how Matlab internally implements copy-on-write:

>> A = 1:10; >> printmem(getaddr(A), 64); 00 00 00 00 00 00 00 00 06 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 02 00 00 00 00 00 00 00 00 00 00 00 00 02 00 00 01 00 00 00 00 00 00 00 0a 00 00 00 00 00 00 00 90 f3 24 df 6f 7f 00 00 >> B = A; >> printaddr(B); 0x7f6f4c7b6810 >> printmem(getaddr(A), 64); 10 68 7b 4c 6f 7f 00 00 06 00 00 00 00 00 00 00 10 68 7b 4c 6f 7f 00 00 02 00 00 00 00 00 00 00 00 00 00 00 00 02 00 00 01 00 00 00 00 00 00 00 0a 00 00 00 00 00 00 00 90 f3 24 df 6f 7f 00 00

What we see is that by setting B = A, we change the internal representation of A itself. Two new memory address pointers are added to the `mxArray`

for A. As it turns out, both of these point to the address for array B, which makes sense; this is how Matlab internally keeps track of arrays that are copies of each other. Note that because byte order is little-endian, the memory addresses from * printmem* are byte-wise, i.e. every two characters, reversed relative to the address from

We can also look into array B:

>> printmem(getaddr(B), 64); f0 41 7a 4c 6f 7f 00 00 06 00 00 00 00 00 00 00 f0 41 7a 4c 6f 7f 00 00 02 00 00 00 00 00 00 00 00 00 00 00 00 02 00 00 01 00 00 00 00 00 00 00 0a 00 00 00 00 00 00 00 90 f3 24 df 6f 7f 00 00 >> printaddr(A); 0x7f6f4c7a41f0

As I have highlighted, there are two interesting points here. First the red highlights show that array B has pointers back to array A. Second the blue highlight shows that the Matlab data for array B actually just points back to the same memory as the data for array A (the values 1:10).

Finally, we would like to understand why there are two pointers added. Let us see what happens if we add a third linked variable:

>> C = B; >> printaddr(A); printaddr(B); printaddr(C); 0x7f6f4c7a41f0 0x7f6f4c7b6810 0x7f6f4c7b69b0 >> printmem(getaddr(A), 32) b0 69 7b 4c 6f 7f 00 00 06 00 00 00 00 00 00 00 10 68 7b 4c 6f 7f 00 00 02 00 00 00 00 00 00 00 >> printmem(getaddr(B), 32) f0 41 7a 4c 6f 7f 00 00 06 00 00 00 00 00 00 00 b0 69 7b 4c 6f 7f 00 00 02 00 00 00 00 00 00 00 >> printmem(getaddr(C), 32) 10 68 7b 4c 6f 7f 00 00 06 00 00 00 00 00 00 00 f0 41 7a 4c 6f 7f 00 00 02 00 00 00 00 00 00 00

So it turns out that Matlab keeps track of a set of linked variables with a kind of cyclical, doubly-linked list structure; array A is linked to B in the forward direction and is also linked to C in the reverse direction by looping back around, etc. The cyclical nature of this makes sense, as we need to be able to start from any of A, B, or C and find all the linked arrays. But it is still not entirely clear why the list needs to be cyclical AND linked in both directions. In fact, in earlier versions of Matlab this cyclical list was only singly-linked.

Obviously there is a lot more to `mxArray`

and Matlab internals than what we have delved into here. Still, with this basic introduction I hope to have whet your appetite for understanding more about Matlab internals, and provided some simple tools to help you explore. I want to reiterate that in general MathWorks’s approach of an opaque `mxArray`

type with access abstracted through an API layer is a good policy. The last thing you would want to do is take the information here and write a bunch of code that relies on the structure of `mxArray`

to work; next time MathWorks needs to add a new feature and change `mxArray`

, all your code will break. So in general we are all better off playing within the API that MathWorks provides. And remember: poking into memory can crash your computer, so save your data!

On the other hand, occasionally there are cases, like in-place editing, where it is useful to push the capabilities of Matlab a little beyond what MathWorks envisioned. In these cases, having an understanding of Matlab’s internals can be critical, for example in understanding how to avoid conflicting with copy-on-write. Therefore I hope the information presented here will prove useful. Ideally, someone will be motivated to take this starting point and repair some of the tools like *headerdump* that made Matlab’s internal workings more transparent in the past. I believe that having more of this information out in the Matlab community is good for the community as a whole.

Related posts:

- Undocumented profiler options – The Matlab profiler has some undocumented options that facilitate debugging memory bottlenecks and JIT (Just-In-Time Java compilation) problems....
- Datenum performance – The performance of the built-in Matlab function datenum can be significantly improved by using an undocumented internal help function...
- Undocumented Profiler options part 3 – An undocumented feature of the Matlab Profiler can report call history timeline - part 3 of series. ...
- Undocumented Profiler options part 4 – Several undocumented features of the Matlab Profiler can make it much more useful - part 4 of series. ...

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In Matlab releases of the past few years, this has been addressed by expanding the information reported by the built-in * memory* function. In addition, an undocumented feature was added to the Matlab Profiler that enables monitoring memory usage.

In Matlab release R2008a (but not on newer releases) we could also use a nifty parameter of the undocumented * feature* function:

>> feature mtic; a=ones(100); feature mtoc ans = TotalAllocated: 84216 TotalFreed: 2584 LargestAllocated: 80000 NumAllocs: 56 NumFrees: 43 Peak: 81640

As can easily be seen in this example, allocating 100^{2} doubles requires 80000 bytes of allocation, plus some 4KB others that were allocated (and 2KB freed) within the function * ones*. Running the same code line again gives a very similar result, but now there are 80000 more bytes freed when the matrix

`a`

is overwritten:>> feature mtic; a=ones(100); feature mtoc ans = TotalAllocated: 84120 TotalFreed: 82760 LargestAllocated: 80000 NumAllocs: 54 NumFrees: 49 Peak: 81328

This is pretty informative and very handy for debugging memory bottlenecks. Unfortunately, starting in R2008b, features mtic and mtoc are no longer supported *“under the current memory manager“*. Sometime around 2010 the mtic and mtoc features were completely removed. Users of R2008b and newer releases therefore need to use the internal structs returned by the * memory* function, and/or use the profiler’s memory-monitoring feature. If you ask me, using mtic/mtoc was much simpler and easier. I for one miss these features.

In a related matter, if we wish to monitor Java’s memory used within Matlab, we are in a bind, because there are no built-in tools to help us. there are several JVM switches that can be turned on in the *java.opts* file: -Xrunhprof[:help]|[:option=value,…], -Xprof, -Xrunprof, -XX:+PrintClassHistogram and so on. There are several memory-monitoring (so-called “heap-walking”) tools: the standard JDK jconsole, jmap, jhat and jvisualvm (with its useful plugins) provide good basic coverage. MathWorks has posted a tutorial on using jconsole with Matlab. There are a number of other third-party tools such as JMP (for JVMs 1.5 and earlier) or TIJMP (for JVM 1.6). Within Matlab, we can use utilities such as Classmexer to estimate a particular object’s size (both shallow and deep referencing), or use `java.lang.Runtime.getRuntime()`

‘s methods (*maxMemory(), freeMemory()* and *totalMemory()*) to monitor overall Java memory (sample usage).

We can monitor the Java memory (which is part of the overall Matlab process memory) usage using Java’s built-in `Runtime`

class:

>> r=java.lang.Runtime.getRuntime r = java.lang.Runtime@5fb3b54 >> r.freeMemory ans = 86147768 >> r.totalMemory ans = 268304384 >> usedMemory = r.totalMemory - r.freeMemory;

Specifically in R2011b (but in no other release), we can also use a built-in Java memory monitor. Unfortunately, this simple and yet useful memory monitor was removed in R2012a (or maybe it was just moved to another package and I haven’t found out where… *yet*…):

com.mathworks.xwidgets.JavaMemoryMonitor.invoke

As I have already noted quite often, using undocumented Matlab features and functions carries the risk that they will not be supported in some future Matlab release. Today’s article is a case in point.

]]>Related posts:

- Matlab’s internal memory representation – Matlab's internal memory structure is explored and discussed. ...

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Editing Matlab arrays in-place can be an important technique for optimizing calculations, especially when handling data that use large blocks of memory. The Matlab language itself has some limited support for in-place editing, but when we are really concerned with speed we often turn to writing C/C++ extensions using the Mex interface. Unfortunately, editing arrays in-place from Mex extensions is not officially supported in Matlab, and doing it incorrectly can cause data inconsistencies or can even cause Matlab to crash. In this article, I will introduce the problem and show a simple solution that exhibit the basic implementation details of Matlab’s internal copy-on-write mechanism.

To demonstrate the techniques in this article, I use the *fast_median* function, which is part of my nth_element package on Matlab’s File Exchange. You can download the package and play with the code if you want. The examples are fairly self-explanatory, so if you do not want to try the code you should be okay just following along.

Let us try a few function calls to see how editing in-place can save time and memory:

>> A = rand(100000000, 1); >> tic; median(A); toc Elapsed time is 4.122654 seconds. >> tic; fast_median(A); toc Elapsed time is 1.646448 seconds. >> tic; fast_median_ip(A); toc Elapsed time is 0.927898 seconds.

If you try running this, be careful not to make A too large; tune the example according to the memory available on your system. In terms of the execution time for the different functions, your mileage may vary depending on factors such as: your system, what Matlab version you are running, and whether your test data is arranged in a single vector or a multicolumn array.

This example illustrates a few general points: first, *fast_median* is significantly faster than Matlab’s * native* median function. This is because

The second point is that the in-place version, *fast_median_ip*, yields an additional speed improvement. What is the difference? Let us look behind the scenes; here are the CPU and memory traces from my system monitor after running the above:

You can see four spikes in CPU use, and some associated changes in memory allocation:

The first spike in CPU is when we created the test data vector; memory use also steps up at that time.

The second CPU spike is the largest; that is Matlab’s median function. You can see that over that period memory use stepped up again, and then stepped back down; the median function makes a copy of the entire input data, and then throws its copy away when it is finished; this is expensive in terms of time and resources.

The *fast_median* function is the next CPU spike; it has a similar step up and down in memory use, but it is much faster.

Finally, in the case of *fast_median_ip* we see something different; there is a spike in CPU use, but memory use stays flat; the in-place version is faster and more memory efficient because it does not make a copy of the input data.

There is another important difference with the in-place version; it modifies its input array. This can be demonstrated simply:

>> A = randi(100, [10 1]); >> A' ans = 39 42 98 25 64 75 6 56 71 89 >> median(A) ans = 60 >> fast_median(A) ans = 60 >> A' ans = 39 42 98 25 64 75 6 56 71 89 >> fast_median_ip(A) ans = 60 >> A' ans = 39 6 25 42 56 64 75 71 98 89

As you can see, all three methods get the same answer, but * median* and

Modifying a function’s input is common in many languages, but in Matlab there are only a few special conditions under which this is officially sanctioned. This is not necessarily a bad thing; many people feel that modifying input data is bad programming practice and makes code harder to maintain. But as we have shown, it can be an important capability to have if speed and memory use are critical to an application.

Given that in-place editing is not officially supported in Matlab Mex extensions, what do we have to do to make it work? Let us look at the normal, input-copying *fast_median* function as a starting point:

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) { mxArray *incopy = mxDuplicateArray(prhs[0]); plhs[0] = run_fast_median(incopy); }

This is a pretty simple function (I have taken out a few lines of boiler plate input checking to keep things clean). It relies on helper function *run_fast_median* to do the actual calculation, so the only real logic here is copying the input array `prhs[0]`

. Importantly, *run_fast_median* edits its inputs in-place, so the call to *mxDuplicateArray* ensures that the Mex function is overall well behaved, i.e. that it does not change its inputs.

Who wants to be well behaved though? Can we save time and memory just by taking out the input duplication step? Let us try it:

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) { plhs[0] = run_fast_median(const_cast<mxarray *>(prhs[0])); // </mxarray> }

Very bad behavior; note that we cast the original `const mxArray*`

input to a `mxArray*`

so that the compiler will let us mess with it; naughty.

But does this accomplish edit in-place for *fast_median*? Be sure to save any work you have open and then try it:

>> mex fast_median_tweaked.cpp >> A = randi(100,[10 1]); >> fast_median_tweaked(A) ans = 43

Hmm, it looks like this worked fine. But in fact there are subtle problems:

>> A = randi(100,[10 1]); >> A' ans = 65 92 14 26 41 2 45 85 53 2 >> B = A; >> B' ans = 65 92 14 26 41 2 45 85 53 2 >> fast_median_tweaked(A) ans = 43 >> A' ans = 2 2 41 26 14 45 65 85 53 92 >> B' ans = 2 2 41 26 14 45 65 85 53 92

Uhoh, spooky; we expected that running *fast_median_tweaked* would change input A, but somehow it has also changed B, even though B is supposed to be an independent copy. Not good. In fact, under some conditions this kind of uncontrolled editing in-place can crash the entire Matlab environment with a segfault. What is going on?

The answer is that our simple attempt to edit in-place conflicts with Matlab’s internal copy-on-write mechanism. Copy-on-write is an optimization built into Matlab to help avoid expensive copying of variables in memory (actually similar to what we are trying to accomplish with edit in-place). We can see copy-on-write in action with some simple tests:

% Test #1: update, then copy >> tic; A = zeros(100000000, 1); toc Elapsed time is 0.588937 seconds. >> tic; A(1) = 0; toc Elapsed time is 0.000008 seconds. >> tic; B = A; toc Elapsed time is 0.000020 seconds. % Test #2: copy, then update >> clear >> tic; A = zeros(100000000, 1); toc Elapsed time is 0.588937 seconds. >> tic; B = A; toc Elapsed time is 0.000020 seconds. >> tic; A(1) = 0; toc Elapsed time is 0.678160 seconds.

In the first set of operations, time and memory are used to create A, but updating A and “copying” A into B take no memory and essentially no time. This may come as a surprise since supposedly we have made an independent copy of A in B; why does creating B take no time or memory when A is clearly a large, expensive block?

The second set of operations makes things more clear. In this case, we again create A and then copy it to B; again this operation is fast and cheap. But assigning into A at this point takes time and consumes a new block of memory, even though we are only assigning into a single index of A. This is copy-on-write: Matlab tries to save you from copying large blocks of memory unless you need to. So when you first assign B to equal A, nothing is copied; the variable B is simply set to point to the same memory location already used by A. Only after you try to change A (or B), does Matlab decide that you really need to have two copies of the large array.

There are some additional tricks Matlab does with copy-on-write. Here is another example:

>> clear >> tic; A{1} = zeros(100000000, 1); toc Elapsed time is 0.573240 seconds. >> tic; A{2} = zeros(100000000, 1); toc Elapsed time is 0.560369 seconds. >> tic; B = A; toc Elapsed time is 0.000016 seconds. >> tic; A{1}(1) = 0; toc Elapsed time is 0.690690 seconds. >> tic; A{2}(1) = 0; toc Elapsed time is 0.695758 seconds. >> tic; A{1}(1) = 0; toc Elapsed time is 0.000011 seconds. >> tic; A{2}(1) = 0; toc Elapsed time is 0.000004 seconds.

This shows that for the purposes of copy-on-write, different elements of cell array A are treated independently. When we assign B equal to A, nothing is copied. Then when we change any part of A{1}, that whole element must be copied over. When we subsequently change A{2}, that whole element must also be copied over; it was not copied earlier. At this point, A and B are truly independent of each other, as both elements have experienced copy-on-write, so further assignments into either A or B are fast and require no additional memory.

Try playing with some struct arrays and you will find that copy-on-write also works independently for the elements of structs.

Now it is clear why we cannot simply edit arrays in-place from Mex functions; not only is it naughty, it fundamentally conflicts with copy-on-write. Naively changing an array in-place can inadvertently change other variables that are still waiting for a copy-on-write, as we saw above when *fast_median_tweaked* inadvertently changed B in the workspace. This is, in the best case, an unmaintainable mess. Under more aggressive in-place editing, it can cause Matlab to crash with a segfault.

Luckily, there is a simple solution, although it requires calling internal, undocumented Matlab functions.

Essentially what we need is a Mex function that can be run on a Matlab array that will do the following:

- Check whether the current array is sharing data with any other arrays that are waiting for copy-on-write.
- If the array is shared, it must be unshared; the underlying memory must be copied and all the relevant pointers need to be fixed so that the array we want to work on is no longer accessible by anyone else.
- If the array is not currently shared, simply proceed; the whole point is to avoid copying memory if we do not need to, so that we can benefit from the efficiencies of edit in-place.

If you think about it, this is exactly the operation that Matlab needs to run internally when it is deciding whether an assignment operation requires a copy-on-write. So it should come as no surprise that such a Mex function already exists in the form of a Matlab internal called *mxUnshareArray*. Here is how you use it:

extern "C" bool mxUnshareArray(mxArray *array_ptr, bool noDeepCopy); void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) { mxUnshareArray(const_cast<mxarray *>(prhs[0]), true); //</mxarray> plhs[0] = run_fast_median(const_cast<mxarray *>(prhs[0])); //</mxarray> }

This is the method actually used by *fast_median_ip* to efficiently edit in-place without risking conflicts with copy-on-write. Of course, if the array turns out to need to be unshared, then you do not get the benefit of edit in-place because the memory ends up getting copied. But at least things are safe and you get the in-place benefit as long as the input array is not being shared.

The method shown here should allow you to edit normal Matlab numerical or character arrays in-place from Mex functions safely. For a Mex function in C rather than C++, omit the “C” in the `extern`

declaration and of course you will have to use C-style casting rather than `const_cast`

. If you need to edit cell or struct arrays in-place, or especially if you need to edit subsections of shared cell or struct arrays safely and efficiently while leaving the rest of the data shared, then you will need a few more tricks. A good place to get started is this article by Benjamin Schubert.

Unfortunately, over the last few years Mathworks seems to have decided to make it more difficult for users to access these kinds of internal methods to make our code more efficient. So be aware of the risk that in some future version of Matlab this method will no longer work in its current form.

Ultimately much of what is known about *mxUnshareArray* as well as the internal implementation details of how Matlab keeps track of which arrays are shared goes back to the work of Peter Boettcher, particularly his headerdump.c utility. Unfortunately, it appears that HeaderDump fails with Matlab releases >=R2010a, as Mathworks have changed some of the internal memory formats – perhaps some smart reader can pick up the work and adapt HeaderDump to the new memory format.

In a future article, I hope to discuss headerdump.c and its relevance for copy-on-write and edit in-place, and some other related tools for the latest Matlab releases that do not support HeaderDump.

]]>Related posts:

- File deletion memory leaks, performance – Matlab's delete function leaks memory and is also slower than the equivalent Java function. ...
- Undocumented XML functionality – Matlab's built-in XML-processing functions have several undocumented features that can be used by Java-savvy users...

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A client of one of my Matlab programs recently complained that Matlab crashes after several hours of extensive use of the program. The problem looked like something that is memory related (messages such as Matlab’s out-of-memory error or Java’s heap-space error). Apparently this happens even on 64-bit systems having lots of memory, where memory should never be a problem.

Well, we know that this is only in theory, but in practice Matlab’s internal memory management has problems that occasionally lead to such crashes. This is one of the reasons, by the way, that recent Matlab releases have added the preference option of increasing the default Java heap space (the previous way to do this was a bit complex). Still, even with a high Java heap space setting and lots of RAM, Matlab crashed after using my program for several hours.

Not pleasant at all, even a bit of an embarrassment for me. I’m used to crashing Matlab, but only as a result of my playing around with the internals – I would hate it to happen to my clients.

While we can do little with Matlab’s internal memory manager, I started searching for the exact location of the memory leak and then to find a way to overcome it. I’ll save readers the description about the grueling task of finding out exactly where the memory leak occurred in a program that has thousands of lines of code and where events get fired asynchronously on a constant basis. Matlab Profiler’s undocumented memory profiling option helped me quite a bit, as well as lots of intuition and trial-and-error. Detecting memory leaks is never easy, and I consider myself somewhat lucky this time to have both detected the leak source and a workaround.

It turned out that the leakage happens in a callback that gets invoked multiple times per second by a Java object (see related articles here and here). Each time the Matlab callback function is invoked, it reads the event information from the supplied Java event-data (the callback’s second input parameter). Apparently, about 1KB of memory gets leaked whenever this event-data is being read. This may appear a very small leak, but multiply this by some 50-100K callback invocations per hour and you get a leakage of 50-100MB/hour. Not a small leak at all; more of a flood you could say…

The leakage culprit turned out to be the following code snippet:

% 160 uSecs per call, with memory leak eventData = get(hEventData,{'EventName','ParamNames','EventData'}); eventName = eventData{1}; paramNames = eventData{2}; paramData = eventData{3}.cell;

In this innocent-looking code, `hEventData`

is a Java object that contains the **EventName, ParamNames, EventData** properties: **EventName** is a Java `String`

, that is automatically converted by Matlab’s * get()* function into a Matlab string (

`String`

s, that gets automatically converted into a Matlab cell-array of string; and `Object`

s that needs to be converted into a Matlab cell array using the built-in The code is indeed innocent, works really well and is actually extremely fast: each invocation of this code segment takes less than 0.2 millisecs. Unfortunately, because of the memory leak I needed to find a better alternative.

The first idea was to use the built-in * handle()* function, under the assumption that it would solve the memory leak, as reported here. In fact, MathWorks specifically advises to use

Do not use the set function on Java objects as it will cause a memory leak.

It stands to reason then that a similar memory leak happens with * get* and that a similar use of

% 300 uSecs per call, with memory leak s = get(handle(hEventData)); eventName = s.EventName; paramNames = s.ParamNames; paramData = cell(s.EventData);

Unfortunately, this variant, although working correctly, still leaks memory, and also performs almost twice as worse than the original version, taking some 0.3 milliseconds to execute per invocation. Looks like this is a dead end.

The next attempt was to use the Java object’s internal accessor methods for the requested properties. These are `public`

methods of the form *getXXX(), isXXX(), setXXX()* that enable Matlab to treat XXX as a property by its * get* and

% 380 uSecs per call, no memory leak eventName = char(hEventData.getEventName); paramNames = cell(hEventData.getParamNames); paramData = cell(hEventData.getEventData);

Here, the method *getEventName()* returns a Java `String`

, that we convert into a Matlab string using the * char* function. In our previous two variants, the

`String[]`

array into a Matlab cell array.This version at last doesn’t leak any memory. Unfortunately, it has an even worse performance: each invocation takes almost 0.4 milliseconds. The difference may seem insignificant. However, recall that this callback gets called dozens of times each second, so the total adds up quickly. It would be nice if there were a faster alternative that does not leak any memory.

Luckily, I found just such an alternative. At 0.24 millisecs per invocation, it is almost as fast as the leaky best-performance original * get* version. Best of all, it leaks no memory, at least none that I could detect.

The mechanism relies on the little-known fact that public fields of Java objects can be retrieved in Matlab using the built-in * struct* function. For example:

>> fields = struct(java.awt.Rectangle) fields = x: 0 y: 0 width: 0 height: 0 OUT_LEFT: 1 OUT_TOP: 2 OUT_RIGHT: 4 OUT_BOTTOM: 8 >> fields = struct(java.awt.Dimension) fields = width: 0 height: 0

Note that this useful mechanism is not mentioned in the main documentation page for accessing Java object fields, although it is indeed mentioned in another doc-page – I guess this is a documentation oversight.

In any case, I converted my Java object to use public (rather than private) fields, so that I could use this * struct* mechanism (Matlab can only access public fields). Yes I know that using private fields is a better programming practice and all that (I’ve programmed OOP for some 15 years…), but sometimes we need to do ugly things in the interest of performance. The latest version now looks like this:

% 240 uSecs per call, no memory leak s = struct(hEventData); eventName = char(s.eventName); paramNames = cell(s.paramNames); paramData = cell(s.eventData);

This solved the memory leakage issue for my client. I felt fortunate that I was not only able to detect Matlab’s memory leak but also find a working workaround without sacrificing performance or functionality.

In this particular case, I was lucky to have full control over my Java object, to be able to convert its fields to become public. Unfortunately, we do not always have similar control over the object that we use, because they were coded by a third party.

By the way, Matlab itself uses this * struct* mechanism in its code-base. For example, Matlab timers are implemented using Java objects (

`com.mathworks.timer.TimerTask`

). The timer callback in Matlab code converts the Java timer event data into a Matlab struct using the As an interesting corollary, this same * struct* mechanism can be used to detect internal properties of Matlab class objects. For example, in the timers again, we can get the underlying timer’s Java object as follows (note the highlighted warning, which I find a bit ironic given the context):

>> timerObj = timerfind Timer Object: timer-1 Timer Settings ExecutionMode: singleShot Period: 1 BusyMode: drop Running: off Callbacks TimerFcn: @myTimerFcn ErrorFcn: '' StartFcn: '' StopFcn: '' >> timerFields = struct(timerObj) Warning: Calling STRUCT on an object prevents the object from hiding its implementation details and should thus be avoided.Use DISP or DISPLAY to see the visible public details of an object. See 'help struct' for more information.(Type "warning off MATLAB:structOnObject" to suppress this warning.)timerFields = ud: {} jobject: [1x1 javahandle.com.mathworks.timer.TimerTask]

Related posts:

- Advanced JIDE Property Grids – JIDE property grids can use complex cell renderer and editor components and can signal property change events asynchronously to Matlab callbacks...
- Tri-state checkbox – Matlab checkboxes can easily be made to support tri-state functionality....
- propertiesGUI – propertiesGUI is a utility that presents property-value structs in a convenient table format, useful in Matlab GUIs. ...
- Date selection components – The JIDE package, pre-bundled in Matlab, contains several GUI controls for selecting dates - this article explains how they can be used...

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We often wish to edit properties of heterogeneous objects using a common interface. Matlab’s property inspector, invoked with the built-in * inspect* function, answers this need. The inspector is based on a two-column table of property names and values. Properties and their values are populated automatically, and the user can edit values in-place. The inspector enables property categorization, sub-categorization and sorting, which help users find and modify properties easily. For each property, the inspector displays a matching edit control: editbox/combobox/checkbox etc. This simplifies property value editing and prevents illegal value entry. Matlab’s GUI builder, GUIDE, uses the inspector to let users edit GUI properties such as position, color etc. It is also used by other tools such as the Plot Editor.

The Matlab inspector can be embedded, with not-too-much effort, within Matlab GUI applications. Examples of this can be found in the FindJObj and UIInspect utilities.

Unfortunately, Matlab’s property inspector is limited to Handle Graphics, Java and COM objects, and cannot be used for structures or user-defined Matlab classes. We shall see below how to set up our own property grid, populate it with data, and subscribe to property change events. This is a rather procedural approach. It is usually more convenient to use a declarative approach in which a structure or Matlab class is passed to a function that automatically discovers its properties and their meta-information. The Property Grid utility at Matlab File Exchange provides these services.

Matlab’s property inspector is based on a property grid control by JIDE Software. JIDE Grids is a collection of components that extend the standard Java Swing JTable component, and is included in each Matlab installation (/java/jarext/jide/jide-grids.jar under the Matlab root). In particular, JIDE Grids includes the *PropertyTable* class, which is a fully customizable property grid component. You can find further details on JIDE Grids in the Developer Guide and the Javadoc documentation.

There are several related classes associated with the *PropertyTable* class. First, a *PropertyTableModel* encapsulates all properties that are visualized in the property grid. Each property derives from the *Property* abstract class, which features some common actions to properties, most notably to get and set property value. *DefaultProperty* is a default concrete subclass of *Property*. Finally, *PropertyPane* decorates a property grid with icons for changing category view to alphabetically sorted view as well as expanding and collapsing categories, and a description text box at the bottom that can be shown or hidden.

Here are the *DefaultProperty* fields and their respective roles:

Field | Role |
---|---|

Name | Interal property name, not necessarily displayed, used as a key to identify the property. |

DisplayName | A short property name shown in the left column of the property grid. |

Description | A concise description of the property, shown at the bottom of the property pane, below the grid. |

Type | The Java type associated with the property, used to invoke the appropriate renderer or editor. |

EditorContext | An editor context object. If set, both the type and the context are used to look up the renderer or editor to use. This lets, for instance, one flag value to display as a true/false label, while another as a checkbox. |

Category | A string specifying the property’s category, for grouping purposes. |

Editable | Specifies whether the property value is modifiable or read-only. |

Value | The current property value, as a Java object. |

Just as with any Java object, these fields may either be accessed with the Java get/set semantics (e.g. *getName()* or *setName(name)*), or the Matlab * get/set* semantics (e.g.

To use a property grid in Matlab, first construct a set of *DefaultProperty* objects. For each object, set at least the name, type and initial value. Next, add the properties to a table model. Finally, construct a property grid with the given table model and encapsulate in a property pane:

% Initialize JIDE's usage within Matlab com.mathworks.mwswing.MJUtilities.initJIDE; % Prepare the properties list list = java.util.ArrayList(); prop1 = com.jidesoft.grid.DefaultProperty(); prop1.setName('stringProp'); prop1.setType(javaclass('char',1)); prop1.setValue('initial value'); prop1.setCategory('My Category'); prop1.setDisplayName('Editable string property'); prop1.setDescription('A concise description for my property.'); prop1.setEditable(true); list.add(prop1); prop2 = com.jidesoft.grid.DefaultProperty(); prop2.setName('flagProp'); prop2.setType(javaclass('logical')); prop2.setValue(true); prop2.setCategory('My Category'); prop2.setDisplayName('Read-only flag property'); prop2.setEditable(false); list.add(prop2); % Prepare a properties table containing the list model = com.jidesoft.grid.PropertyTableModel(list); model.expandAll(); grid = com.jidesoft.grid.PropertyTable(model); pane = com.jidesoft.grid.PropertyPane(grid); % Display the properties pane onscreen hFig = figure; panel = uipanel(hFig); javacomponent(pane, [0 0 200 200], panel); % Wait for figure window to close & display the prop value uiwait(hFig); disp(prop1.getValue());

Here, *com.mathworks.mwswing.MJUtilities.initJIDE* is called to initialize JIDE’s usage within Matlab. Without this call, we may see a JIDE warning message in some Matlab releases. We only need to *initJIDE* once per Matlab session, although there is no harm in repeated calls.

* javaclass* is a function (included in the Property Grid utility, or directly downloadable from here) that returns a Java class for the corresponding Matlab type with the given dimension:

* javacomponent* is the undocumented built-in Matlab function that adds Java Swing components to a Matlab figure, using the given dimensions and parent handle. When the user closes the figure, prop.

Next week’s article will show how to add more complex renderers and editors (display the flag value as a checkbox for example), define nested properties, and subscribe to property value change events. So stay tuned…

]]>Related posts:

- Profiling Matlab memory usage – mtic and mtoc were a couple of undocumented features that enabled users of past Matlab releases to easily profile memory usage. ...
- Undocumented Profiler options part 2 – Several undocumented features of the Matlab Profiler can make it much more useful - part 2 of series. ...
- Undocumented Profiler options part 3 – An undocumented feature of the Matlab Profiler can report call history timeline - part 3 of series. ...
- Undocumented Profiler options part 4 – Several undocumented features of the Matlab Profiler can make it much more useful - part 4 of series. ...

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To turn on memory stats in the profile report, run this (only once is necessary – will be remembered for future profiling runs):

profile -memory on; profile('-memory','on'); % an alternative

To turn on JIT information, run this (again, only once is necessary, prior to profile report):

`setpref('profiler','showJitLines',1);`

You will then see additional JIT and memory (allocated, freed and peak) information displayed in the profile report, as well as the options to sort by allocated, freed and peak memory:

For those interested, the references to these two options appear within the code of profview.m (line 1199 on R2007b), for the JIT option:

`showJitLines = getpref('profiler','showJitLines',false);`

…and profile.m (lines 163-165 on R2007b), for the memory option:

if memory ~= -1 callstats('memory', memory); end

Note that there appears to be two undocumented additional memory-related options in profile.m (lines 311-312):

options = {'detail', 'timer', 'history', 'nohistory', 'historysize', ... 'timestamp', 'memory', 'callmemory', 'nomemory' };

However, ‘-nomemory’ appears to simply turn the memory stats collection off, and ‘-callmemory’ is not recognized because of a bug in line 349, which looks for ‘call**no**memory’…:

case 'callnomemory' % should be 'callmemory' memory = 2;

When this bug is fixed, we see that we get only partial memory information, so the ‘-callmemory’ option is really not useful – use ‘-memory’ instead.

__Addendum (Jan 31, 2011):__ JIT information has been removed in Matlab 7.12 (R2011a). I assume that this was done so that programmers will not attempt to depend on JITC functionality in their code (see Michelle Hirsch’s comment below). It’s a good thing that the memory options remain, since these are quite useful in profiling memory-related bottlenecks.