[OpenSIPS-Devel] [OpenSIPS/opensips] bdaaf6: F_MALLOC: Add LHS coalescing on free() operations

[Liviu Chircu]

  Branch: refs/heads/master
  Home:   https://github.com/OpenSIPS/opensips
  Commit: bdaaf60b2c42681da70f095468c2df1b8dfa8c3e
      https://github.com/OpenSIPS/opensips/commit/bdaaf60b2c42681da70f095468c2df1b8dfa8c3e
  Author: Liviu Chircu <liviu at opensips.org>
  Date:   2023-04-19 (Wed, 19 Apr 2023)

  Changed paths:
    M mem/f_malloc.c
    M mem/f_malloc.h
    M mem/f_malloc_dyn.h

  Log Message:
  -----------
  F_MALLOC: Add LHS coalescing on free() operations

This is a continuation/rework of commit fb9a3777, which intended to
address the de-fragmentation issues where performance would drop.
Unfortunately, there were still memory allocation / releasing patterns
which still lead to heavy fragmentation.  For example, an MI "dlg_list"
command, which allocates fragments from Left -> Right as the dialog list
is walked:

    Chunk-1  Chunk-2  Chunk-3 ... Chunk-N

After building the MI output, the chunks were also freed Left -> Right.
Due to the former allocator logic of RHS-only coalescing, the coalescing
would never happen!  In some cases, this could lead to fragmentation of
an indefinite amount of memory, given enough time (e.g., even 8 GB PKG).

This patch adds LHS coalescing to F_MALLOC, such that when "Chunk-2" is
freed, in the above example, the allocator tries to merge it with both
"Chunk-1" and "Chunk-3" before exiting the free() function (depending on
the state of chunks 1 and 3, this could mean 0, 1 or 2 coalescings).

This F_MALLOC patch was stress-tested using the mem/test/ testing suite.

Many thanks to @fedkis and @ankogan for helping troubleshoot the issue
and also test the current version of this patch!
Fixes #2726