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Currently I analyze a C++ application and its memory consumption. Checking the memory consumption of the process before and after a certain function call is possible. However, it seems that, for technical reasons or for better efficiency the OS (Linux) assigns not only the required number of bytes but always a few more which can be consumed later by the application. This makes it hard to analyze the memory behavior of the application.

Is there a workaround? Can one switch Linux to a mode where it assigns just the required number of bytes/pages?

leftangle
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  • What memory use do you measure (e.g. heap, or stack), and how? Did you use `valgrind` ? – Basile Starynkevitch Aug 26 '13 at 14:02
  • I measure the virtual size of the process. Valgrind has a limited set of number rounding modes thus my application won't run under Valgrind. – leftangle Aug 26 '13 at 14:16
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    See http://stackoverflow.com/questions/10540845/linux-heap-structure-and-the-behaviour-with-malloc-and-free the accepted answer. – ott-- Aug 26 '13 at 14:17
  • Show some source code and explain much more about your application. `valgrind` is a very trustful tool, so if your application can't run with it, it is very likely your fault.... – Basile Starynkevitch Aug 26 '13 at 14:54
  • @Basile Starynkevitch: It's a limitation of Valgrind. My application uses multiple precision arithmetic. Operations require a rounding mode that exists on real CPUs but is not implemented in Valgrind. – leftangle Aug 26 '13 at 15:18

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if you use malloc/new, the allocator will always alloc a little more bytes than you requested , as it needs some room to do its housekeeping, also it may need to align the bytes on pages boundaries. The amount of supplementary bytes allocated is implementation dependent. you can consider to use tools such as gperftools (google) to monitor the memory used.

Louis Hugues
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I wanted to check a process for memory leeks some years ago.

What I did was the following: I wrote a very small debugger (it is easier than it sounds) that simply set breakpoints to malloc(), free(), mmap(), ... and similar functions (I did that under Windows but under Linux it is simpler - I did it in Linux for another purpose!).

Whenever a breakpoint was reached I logged the function arguments and continued program execution...

By processing the logfile (semi-automated) I could find memory leaks.

Disadvantage: It is not possible to debug the program using another debugger in parallel.

Martin Rosenau
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