linux-kernel - Re: Misleading OOM messages

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Message-ID: <20090519203944.GD1362@ucw.cz>
Date:	Tue, 19 May 2009 22:39:45 +0200
From:	Pavel Machek <pavel@....cz>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>,
	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Nick Piggin <npiggin@...e.de>, Mel Gorman <mel@....ul.ie>,
	Peter Ziljstra <a.p.ziljstra@...llo.nl>,
	San Mehat <san@...roid.com>, Arve Hj?nnev?g <arve@...roid.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Misleading OOM messages

On Fri 2009-05-15 17:15:54, Christoph Lameter wrote:
> On Fri, 15 May 2009, Pavel Machek wrote:
> 
> > > Nope. If you have too little memory for your app then the kernel pages
> > > portions of the app out to disk. Thats is why you have a VM (VIRTUAL
> > > machine). The app is not running with physical memory.
> >
> > Try running your machine with mem=8M, then tell me how virtual memory
> > works.
> 
> Well that is of course not enough memory.

Ok, so in the end, there are two reasons for OOM:

1) Out of virtual memory.

   there's simply not enough ram+swap to fit the data. You go OOM.
   This seems to be common on small machines. 8M is pushing it, but
   64M ram + 64M swap + todays gnome would probably do that.

   And maybe the way to hint people would be printing 'out of
   _virtual_ memory'.

2) Something goes very wrong with reclaim

   this seems to be common on very  big machines you have experience
   with.

Perhaps 1 and 2 can be told appart by zero swap free in the 1) case?
And perhaps you can invent some better message for 2) case?
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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