linux-kernel - Re: Memory overcommit

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Date:	Fri, 30 Oct 2009 15:41:12 +0100
From:	Vedran Furač <vedran.furac@...il.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
CC:	David Rientjes <rientjes@...gle.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	minchan.kim@...il.com, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: Memory overcommit

Andrea Arcangeli wrote:

> On Fri, Oct 30, 2009 at 02:53:33PM +0100, Vedran Furač wrote:
>> % free -m
>>           total       used       free     shared    buffers     cached
>> Mem:      3458        3429         29          0        102       1119
>> -/+ buffers/cache:    2207       1251
>>
>> There's plenty of memory available. Shouldn't cache be automatically
>> dropped (this question was in my original mail, hence the subject)?
> 
> This is not about cache, cache amount is physical, this about
> virtual amount that can only go in ram or swap (at any later time,
> current time is irrelevant) vs "ram + swap".

Oh... so this is because apps "reserve" (Committed_AS?) more then they
currently need.

> In short add more swap if
> you don't like overcommit and check grep Commit /proc/meminfo in case
> this is accounting bug...

A the time of "malloc: Cannot allocate memory":

CommitLimit:     3364440 kB
Committed_AS:    3240200 kB

So probably everything is ok (and free is misleading). Overcommit is
unfortunately necessary if I want to be able to use all my memory.

Btw. http://www.redhat.com/advice/tips/meminfo.html says Committed_AS is
a (gu)estimate. Hope it is a good (not to high) guesstimate. :)

Regards,

Vedran

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