Re: [PATCH] [v2] malloc: Consistently apply trim_threshold to all heaps

[Siddhesh Poyarekar <siddhesh at redhat dot com>]

On Tue, Feb 10, 2015 at 08:27:03PM +0000, Mel Gorman wrote:
> Trimming heaps is a balance between saving memory and the system overhead
> required to update page tables and discard allocated pages. The malloc
> option M_TRIM_THRESHOLD is a tunable that users are meant to use to decide
> where this balance point is but it is only applied to the main arena.
> 
> For scalability reasons, glibc malloc has per-thread heaps but these are
> shrunk with madvise() if there is one page free at the top of the heap.
> In some circumstances this can lead to high system overhead if a thread
> has a control flow like
> 
>     while (data_to_process) {
>         buf = malloc(large_size);
>         do_stuff();
>         free(buf);
>     }
> 
> For a large size, the free() will call madvise (pagetable teardown, page
> free and TLB flush) every time followed immediately by a malloc (fault,
> kernel page alloc, zeroing and charge accounting). The kernel overhead
> can dominate such a workload.
> 
> This patch allows the user to tune when madvise gets called by applying
> the trim threshold to the per-thread heaps.

Thanks for fixing this.  I consider this a bug, since the
complementary tunable MALLOC_MMAP_THRESHOLD_ does get applied to
non-main arenas.

> diff --git a/ChangeLog b/ChangeLog
> index dc1ed1ba1249..b860b2fe1850 100644
> --- a/ChangeLog
> +++ b/ChangeLog
> @@ -1,3 +1,8 @@
> +2015-02-10  Mel Gorman  <mgorman@suse.de>
> +
> +	* malloc/arena.c (free): Apply trim threshold to per-thread heaps
> +	as well as the main arena.
> +
>  2015-02-06  Carlos O'Donell  <carlos@systemhalted.org>

This should not be in a diff since it won't always apply to the
current tree cleanly.  Please file a bug for this (since it is an
unintended inconsistency) and I'll push it for you.

Thanks,
Siddhesh

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