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List:       linux-man
Subject:    [PATCH] malloc_trim.3: Update trimming information.
From:       Carlos O'Donell <codonell () redhat ! com>
Date:       2019-03-28 18:01:28
Message-ID: d846256f-7710-a22c-d2dd-c2b2fbcb7287 () redhat ! com
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Michael,

I came across this issue while reading:
https://www.joyfulbikeshedding.com/blog/2019-03-14-what-causes-ruby-memory-bloat.html#a-magic-trick-trimming


The following patch is against master, please apply.

8< --- 8< --- 8<

Since glibc 2.8, commit 68631c8eb92, the malloc_trim function has
iterated over all arenas and free'd back to the OS all page runs that
were free.  This allows an application to call malloc_trim to
consolidate fragmented chunks and free back any pages it can to
potentially reduce RSS usage.

This correctness of the man page was recently brought to light by an
article where Ruby developers discovered that malloc_trim did not behave
as the man page indicated.

This change makes it clear that the intent of malloc_trim is to trim all
space that is no longer needed, and any restrictions are implementation
details. In the notes we highlight the change in behaviour for post
glibc 2.8 and pre glibc 2.8.

Signed-off-by: Carlos O'Donell <carlos@redhat.com>
---
  man3/malloc_trim.3 | 24 ++++++++++++++++--------
  1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/man3/malloc_trim.3 b/man3/malloc_trim.3
index 3ff8bd0ee..dd73fe631 100644
--- a/man3/malloc_trim.3
+++ b/man3/malloc_trim.3
@@ -25,7 +25,7 @@
  .\"
  .TH MALLOC_TRIM 3  2017-09-15 "Linux" "Linux Programmer's Manual"
  .SH NAME
-malloc_trim \- release free memory from the top of the heap
+malloc_trim \- release free memory from the heap
  .SH SYNOPSIS
  .B #include <malloc.h>
  .PP
@@ -33,10 +33,12 @@ malloc_trim \- release free memory from the top of the heap
  .SH DESCRIPTION
  The
  .BR malloc_trim ()
-function attempts to release free memory at the top of the heap
+function attempts to release free memory from the heap
  (by calling
  .BR sbrk (2)
-with a suitable argument).
+or
+.BR madvise (2)
+with suitable arguments).
  .PP
  The
  .I pad
@@ -82,12 +84,18 @@ and
  in
  .BR mallopt (3).
  .PP
-This function cannot release free memory located at places
-other than the top of the heap.
+Only the main heap (using
+.BR sbrk (2)
+) honors the pad argument; thread heaps do not.
+.PP
+Since glibc 2.8 this function frees memory in all arenas and in all
+chunks with whole free pages.
+.\" See commit 68631c8eb92ff38d9da1ae34f6aa048539b199cc
+.\" (dated 2007-12-16) which adds iteration over all
+.\" arenas and frees all pages in chunks which are free.
  .PP
-This function releases only memory in the main arena.
-.\" malloc/malloc.c::mTRIm():
-.\"     return result | (av == &main_arena ? sYSTRIm (pad, av) : 0);
+Before glibc 2.8 this function only freed memory at the
+top of the heap in the main arena.
  .SH SEE ALSO
  .BR sbrk (2),
  .BR malloc (3),
-- 
2.20.1


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