Re: [PATCH] release all idle heaps in the non-main-arena

[Siddhesh Poyarekar <siddhesh@gotplt.org>]

On 6/11/21 9:38 AM, liudongyun wrote:
> liu.dongyun@h3c.com report this question in follow:
> The system has a total of 32G memory and 12 core cpu.
> After frequent malloc and free memory by multiple threads, we found that many
> physical memory that should have been released have not been released.We observe that
> the maximum free memory can reach 15G.
> 
> deng.hongjie@h3c.com recommend reducing the heap size of the non-main-arena, but it
> doesn't work. liu.dongyun@h3c.com found When the last heap in the user process into
> the non-main-area has memory in use, the previous heap will not be released, even if the
> previous heap is already free. This mechanism resulting in the memory cache reaching 15G.

Does that cause any actual performance degradation or any other issues 
though?  heap_shrink() calls madvise (MADV_DONTNEED) on the free parts 
of the heap to tell the kernel that the blocks are not needed, so a 
fully freed heap ought to not have any performance impact.  The RSS 
footprint will depend on when the kernel reclaims those pages, which may 
not be immediate since madvise is just a hint.

Alternatively, if you disable memory overcommit by setting 
/proc/sys/vm/overcommit_memory to 2, heap_shrink() will drop all 
permissions on the heap with mprotect(PROT_NONE) instead of doing 
madvise.  This has a more immediate effect on RSS usage unlike madvise.

Siddhesh