[Bug malloc/30579] trim_threshold in realloc lead to high memory usage

["siddhesh at sourceware dot org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30579

Siddhesh Poyarekar <siddhesh at sourceware dot org> changed:

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                 CC|                            |siddhesh at sourceware dot org

--- Comment #1 from Siddhesh Poyarekar <siddhesh at sourceware dot org> ---
(In reply to Nicolas Dusart from comment #0)
> I'm not an expert but reusing trim_threshold as an heuristic to decide to
> reclaim memory or not does not seem a good fit for heap memory. 

That's the entire point of trim_threshold; to control fragmentation tolerance
for heap memory.

> By reading the comments, I would be tempted to set
> glibc.malloc.trim_threshold (via GLIBC_TUNABLES) to a value high enough for
> performance reason but now, we are somewhat forced to set it to a very low
> value to avoid consuming too much memory.

There are multiple ways to reach this kind of fragmentation I'm afraid and
setting trim threshold (and mmap_threshold for that matter) is the way to
customize the allocator to your specific needs.  There are other ways to
control trim threshold than tunables FWIW, e.g. by using mallopt() to move the
threshold around the way you need at any time in the program.

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