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List:       linux-man
Subject:    Re: [PATCH] malloc_trim.3: Update trimming information.
From:       "Michael Kerrisk (man-opages)" <mtk.manpages () gmail ! com>
Date:       2019-04-12 14:29:41
Message-ID: b9b85583-cd4a-1dcf-e6b0-2d726822f481 () gmail ! com
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Hi Carlos,

On 3/28/19 7:01 PM, Carlos O'Donell wrote:
> 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>

Thank you! Patch applied.

Cheers,

Michael

> ---
> 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),


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