Public bug reported:
I'm using Ubuntu 15.10 dev (x86_64) with libc6 2.21-0ubuntu4 and I have
noticed that the free list can store a huge amount of memory and on
analyzing this and reading manpages I have found some strange things.
First here is a testcase to reproduce this issue (compiled with "gcc
-Wall -pedantic -o test test.c"):
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#define BLOCKSIZE 4096
#define NUMBER_BLOCKS 524288
int main()
{
char *block[NUMBER_BLOCKS];
int unsigned i;
for(i = 0; i < NUMBER_BLOCKS; ++i)
memset(block[i] = malloc(BLOCKSIZE), 0, BLOCKSIZE);
for(i = 0; i < NUMBER_BLOCKS - 1; ++i)
free(block[i]);
pause();
return 0;
}
On executing this the application uses actively only 4 KiB on the heap while
RES is ~2 GiB because of glibc's caching methods. But there are some strange
things:
- Freeing up the memory by decrementing in the testcase results that RES is ~5
MiB. On reading the manpage of mallopt() I'm assuming this happens because in
the original testcase the last allocated block is at the end of the heap which
prevents trimming it. I'm thinking that this part describes it: "(By contrast,
the heap can be
trimmed only if memory is freed at the top end.)". If I'm right
maybe this could be stated in a better context as directly before it is talked
about mmap().
- Using malloc_trim() before pause() in the testcase causes that RES is ~5 MiB
too. The manpage of malloc_trim() says that releasing is done at the top of the
heap which should not cause this result if my previous assumption should be
correct.
In the end I think it is not healthy that the caching behavior from glibc can
reserve such huge amounts of memory. The potential performance penalty if this
would cause any swapping could be insane. For example I'm seeing this caching
behavior on GIMP if I'm making huge scaling operations which causes ~9 GiB to
be in the free list. Who knows how many other applications are caching moderate
amounts of memory without being as suspicious as in this case with GIMP.
** Affects: glibc (Ubuntu)
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/1462853
Title:
Free list can store enormous amounts of memory
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