Memory Leaks in Software

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Memory Leaks
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Figure 912 As you can see from Figure 913, both the working set size and the virtual memory size have increased Not a good sign Increasing the number of threads and the number of iterations per thread yields the result in Table 92
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9 RESOURCE LEAKS
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9
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Resource Leaks
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Table 92
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Threads Iterations Memory (Kb) Virtual Memory (Kb)
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10 10 10 20 20 20
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200 200 200 300 300 300
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948 944 944 956 964 956
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292 288 288 300 308 300
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Judging from Table 92, the theory of a potential memory leaks is now realized In addition, the memory leak is not constant with the same number of thread and iterations per thread This is similar in nature to the handle leak scenario shown earlier Rather than going to step 3, we assume that the memory leak is expensive to track down through code reviews, so we dive into step 4: use leak detection tools Tracking down handle leaks proved to be much easier using the incredibly valuable !htrace extension command Is there something similar for memory leaks Absolutely! The tool that will save the day is called UMDH
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Working Set Size Adjustments
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The working set size for any process is constantly adjusted by Windows The adjustments occur because of changes in system load and process priorities When running the previous memory leak scenario, you might find that the memory consumption reported is slightly different from what we have shown This is indeed expected The memory leak is sporadic and (more than likely) doesn t yield the same leak twice Even though you should see small differences in memory consumption, you should definitely not see large ones If you do, it might be due to minimizing the command window when looking at the resource consumption When you minimize a command window, Windows automatically assumes that the window should be put in the background (that is, not being used), and as such trims the working set of any command-line application currently running in the context of that command window By reducing the amount of physical memory the command shell is using, it can give that memory to other applications that might now be in need of it
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Step 4: Using Leak Detection Tools
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Several tools are available to help efficiently track down memory leaks In the following sections, we discuss several of the most commonly used tools
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UMDH
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UMDH is a tool that comes as part of the Debugging Tools for Windows installation The basic idea behind the tool is very similar to the !htrace extension command We begin by simply telling the operating system to store away stack traces for all calls resulting in memory allocations We take a snapshot of the memory usage before the application begins executing, and when the reproduction is finished, we take another snapshot and compare the results This yields all stack traces that have not yet been freed, and we can take a more tactical approach to our code review to find the culprit First, we need to enable stack traces for memory allocations To accomplish this, we use the gflags tool and enable Create user mode stack trace database for 09basicmleakexe For mode details on how to enable instrumentation using gflags, see 1, Introduction to the Tools When you have enabled gflags for the 09basicmleakexe application, run 09basicmleakexe with the following command line:
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C:\AWDBIN\WinXPx86chk\09basicmleakexe /t:10 /i:200 /s:0 Press any key to start stress application
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Before starting the actual reproduction, we need to run UMDH to take the initial snapshot UMDH can be run in three modes:
Mode 1: Creates a dump of the heap allocations grouped by stack traces This mode tells UMDH to create a dump of all heap allocations Several options exist for this mode, and most are self-explanatory The following options are of most interest:
p tells UMDH which process ID to record stack traces for l prints file and line number information as part of stack traces