File deletion memory leaks, performance

Last week I wrote about Matlab’s built-in pause function, that not only leaks memory but also appears to be less accurate than the equivalent Java function. Today I write about a very similar case. Apparently, using Matlab’s delete function not only leaks memory but is also slower than the equivalent Java function.

Memory leak

The memory leak in delete was (to the best of my knowledge) originally reported in the CSSM newsgroup and on this blog a few weeks ago. The reporter mentioned that after deleting 760K files using delete, he got a Java Heap Space out-of-memory error. The reported solution was to use the Java equivalent, java.io.File(filename).delete(), which does not leak anything.

I was able to recreate the report on my WinXP R2012a system, and discovered what appears to be a memory leak of ~150 bytes per file. This appears to be a very small number, but multiply by 760K (=111MB) and you can understand the problem. Of course, you can always increase the size of the Java heap used by Matlab (here’s how), but this should only be used as a last resort and certainly not when the solution is so simple.

For those interested, here’s the short test harness that I’ve used to test the memory leak:

function perfTest()
    rt = java.lang.Runtime.getRuntime;
    rt.gc();
    java.lang.Thread.sleep(1000);  % wait 1 sec to let the GC time to finish
    orig = rt.freeMemory;  % in bytes
 
    testSize = 50000;
    for idx = 1 : testSize
        % Create a temp file
        tn = [tempname '.tmp'];
        fid = fopen(tn,'wt');
        fclose(fid);
 
        % Delete the temp file
        delete(tn);
        %java.io.File(tn).delete();
    end
 
    rt.gc();
    java.lang.Thread.sleep(1000);  % wait 1 sec to let the GC time to finish
    free = rt.freeMemory;
    totalLeak = orig - free;
    leakPerCall = totalLeak / testSize
end

I placed it in a function to remove command-prompt-generated fluctuations, but it must still be run several times to smooth the data. The main reason for the changes across runs is the fact that the Java heap is constantly growing and shrinking in a seesaw manner, and explicitly calling the garbage collector as I have done does not guarantee that it actually gets performed immediately or fully. By running a large-enough loop, and rerunning the test several times, the results become consistent due to the law of large numbers.

Running the test above with the delete line commented and the java.io.File line uncommented, shows no discernible memory leak.

To monitor Matlab’s Java heap space size in runtime, see my article from several months ago, or use Elmar Tarajan’s memory-monitor utility from the File Exchange.

Note: there are numerous online resources about Java’s garbage collector. Here’s one interesting article that I have recently come across.

Performance

When running the test function using java.io.File, we notice a significant speedup compared to running using delete. The reason is that (at least on my system, YMMV) delete takes 1.5-2 milliseconds to run while java.io.File only takes 0.4-0.5 ms. Again, this doesn’t seem like much, but multiply by thousands of files and it starts to be appreciable. For our 50K test harness, the difference translates into ~50 seconds, or 40% of the overall time.

Since we’re dealing with file I/O, it is important to run the testing multiple times and within a function (not the Matlab Command Prompt), to get rid of spurious measurement artifacts.

Have you encountered any other Matlab function, where the equivalent in Java is better? If so, please add a comment below.

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