massif reported heap usage much less than VmRss, what could be wrong?

Question

massif output:

time=3220706
mem_heap_B=393242041
mem_heap_extra_B=73912175
mem_stacks_B=93616
heap_tree=peak

process shows 1.2GB in VmRss, so the huge difference comes from where? (I saw Rss grows up continuously).

Answer 1

Valgrind can use significant memory for its own internal house keeping. So, it is normal to have massif reporting memory significantly less than the process size, as the process size includes the 'client/guest' memory + valgrind's own memory.

You can use the valgrind option --stats=yes to have more information about the memory used by the client versus the memory used by valgrind.

Answer 2

Per http://cs.swan.ac.uk/~csoliver/ok-sat-library/internet_html/doc/doc/Valgrind/3.8.1/html/ms-manual.html

Heap allocation functions such as malloc are built on top of these system calls. For example, when needed, an allocator will typically call mmap to allocate a large chunk of memory, and then hand over pieces of that memory chunk to the client program in response to calls to malloc et al. Massif directly measures only these higher-level malloc et al calls, not the lower-level system calls.

There is no way to guarantee RSS size based on massif output. With --pages-as-heap=yes option you maybe able to estimate VIRT size, but that is including everything that was mapped into memory, not necessary residing in RAM.

You may want to play with alloc-fn option, which may bring you closer to estimating real memory usage by manually specifying all "custom" memory allocation functions.