GNU bug report logs - #45200
Wishlist: There should be a `malloc-trim' function

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From: DJ Delorie <dj <at> redhat.com>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: fweimer <at> redhat.com, carlos <at> redhat.com, rudalics <at> gmx.at, hi-angel <at> yandex.ru, eliz <at> gnu.org, 45200 <at> debbugs.gnu.org
Subject: bug#45200: [PATCH] Force Glibc to free the memory freed
Date: Wed, 03 Feb 2021 23:26:45 -0500

Stefan Monnier <monnier <at> iro.umontreal.ca> writes:
> [ I have no idea what "fastbins" are, but I'll nod along approvingly.  ]

https://sourceware.org/glibc/wiki/MallocInternals

Fastbins are one of the caches layered on the core malloc code.

> OK, so `malloc_trim` can do a more thorough job and discover that
> there's indeed 200MB free at the top which malloc/free wouldn't
> have noticed?  I guess that makes sense.

Yes.

> PS: One last thing: I must say I'm surprised to see the amount of
> effort that seems to be spent on trying to release that "top" free
> memory.  From where I stand, it seems that it will only very rarely
> allow you to release any non-trivial amount of memory.  Why bother?

We have to keep track of the top of heap anyway, in order to grow it.
Adding a check to shrink it is pretty trivial, and it's our *only* way
to return memory to the system.

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