linux-kernel - Re: [sample] mem_notify v6: usage example

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Date:	Tue, 12 Feb 2008 03:37:10 +0900
From:	"KOSAKI Motohiro" <kosaki.motohiro@...fujitsu.com>
To:	"Andreas Dilger" <adilger@....com>
Cc:	"Jon Masters" <jonathan@...masters.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Marcelo Tosatti" <marcelo@...ck.org>,
	"Daniel Spang" <daniel.spang@...il.com>,
	"Rik van Riel" <riel@...hat.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Alan Cox" <alan@...rguk.ukuu.org.uk>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"Pavel Machek" <pavel@....cz>, "Al Boldi" <a1426z@...ab.com>,
	"Zan Lynx" <zlynx@....org>
Subject: Re: [sample] mem_notify v6: usage example

Hi Andreas,

Thank you very good comment.

> Having such notification handled by glibc to free up unused malloc (or
> any heap allocations) would be very useful, because even if a program
> does "free" there is no guarantee the memory is returned to the kernel.

Yes, no guarantee.
but current glibc-malloc very frequently return memory to kernel.

glibc default behavior

1. over 1M memory: return memory just free(3)  called.
    (you can change threshold by MALLOC_MMAP_MAX_ environment)
2. more lower:         return memory when exist continuous 128k at heap tail.
    (you can change threashold by MALLOC_TRIM_THRESHOLD_ environment)

if you know very memory consumption by already freed memory situation,
please tell me situation detail and consumption memory size.

> I think that having a generic reservation framework is too complex, but
> hiding the details of /dev/mem_notify from applications is desirable.
> A simple wrapper (possibly part of glibc) to return the poll fd, or set
> up the signal is enough.

Agreed.
if large consumption situation exist, I'm behind you.


Thanks!
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