linux-kernel - Re: Misleading OOM messages

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Date:	Fri, 22 May 2009 09:53:59 -0400 (EDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Pavel Machek <pavel@....cz>
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 Tue, 19 May 2009, Pavel Machek wrote:

> > 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'.

This is only an issue for anonymous page and is therefore load dependent.
Memory can be provided through additional swap space.

> 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?

We could add a message spitting out a warning in get_swap_page() to cover
the "out of memory" case. Would be triggered once only when we first run
out of swap space.

> And perhaps you can invent some better message for 2) case?

The something-goes-wrong with reclaim occurs for a variety of reasons
on other machines. Even on the small machine that I currently work with.
I am not in the embedded space right now so this likely means that I do
not see the out of swap -> OOM condition. The out of memory issues that
I see are misconfigurations on a varity of levels. On top right now is
running out of memory on 32 bit machines since someone put too much memory
into them. Thus ZONE_NORMAL gets exhausted.

Then they add more memory and therefore OOM occurs faster. Which leaves
them somewhat confused.

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