linux-kernel - Re: Memory management woes - order 1 allocation failures

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Message-ID: <20100302172606.GA11355@csn.ul.ie>
Date:	Tue, 2 Mar 2010 17:26:06 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Pekka Enberg <penberg@...helsinki.fi>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Frans Pop <elendil@...net.nl>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Mel Gorman <mel@...net.ie>
Subject: Re: Memory management woes - order 1 allocation failures

On Mon, Mar 01, 2010 at 10:42:50AM +0900, KOSAKI Motohiro wrote:
> > AFAICT, even in the worst case, the latter call-site is well below 4K.
> > I have no idea of the tty one.
> 
> afaik, tty_buffer_request_room() try to expand its buffer size for efficiency. but Its failure
> doesn't cause any user visible failure. probably we can mark it as NOWARN.
> 
> In worst case, maximum tty buffer size is 64K, it can make allocation failure easily.
>
> Alan, Can you please tell us your mention?
> 

(Added Greg as current tty maintainer)

For reasons that are not particularly clear to me, tty_buffer_alloc() is
called far more frequently in 2.6.33 than in 2.6.24. I instrumented the
function to print out the size of the buffers allocated, booted under
qemu and would just "cat /bin/ls" to see what buffers were allocated.
2.6.33 allocates loads, including high-order allocations. 2.6.24
appeared to allocate once and keep silent.

While there have been snags recently with respect to high-order
allocation failures in recent kernels, this might be one of the cases
where it's due to subsystems requesting high-order allocations more.

Anyone familiar with tty that might make a guess as to why it allocates
more aggressively?

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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