linux-kernel - Re: [PATCH 3/3] vmscan: Force kswapd to take notice faster when high-order watermarks are being hit

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Message-ID: <20091104154853.GM22046@csn.ul.ie>
Date:	Wed, 4 Nov 2009 15:48:53 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Frans Pop <elendil@...net.nl>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, stable@...nel.org,
	linux-kernel@...r.kernel.org,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Jiri Kosina <jkosina@...e.cz>,
	Sven Geggus <lists@...hsschwanzdomain.de>,
	Karol Lewandowski <karol.k.lewandowski@...il.com>,
	Tobias Oetiker <tobi@...iker.ch>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Rik van Riel <riel@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Stephan von Krawczynski <skraw@...net.com>,
	Kernel Testers List <kernel-testers@...r.kernel.org>
Subject: Re: [PATCH 3/3] vmscan: Force kswapd to take notice faster when
	high-order watermarks are being hit

On Wed, Nov 04, 2009 at 03:05:55AM +0100, Frans Pop wrote:
> On Wednesday 04 November 2009, Mel Gorman wrote:
> > > If you'd like me to test with the congestion_wait() revert on top of
> > > this for comparison, please let me know.
> >
> > No, there is resistance to rolling back the congestion_wait() changes
> 
> I've never promoted the revert as a solution. It just shows the cause of a 
> regression.
> 

Yeah, I still haven't managed to figure out what exactly is wrong in there
other than "something changed with timing" and writeback behaves differently. I
still don't know the why of it because I haven't digged into that area in
depth in the past and failed at reproducing this. "My desktop is fine" :/

> > from what I gather because they were introduced for sane reasons. The
> > consequence is just that the reliability of high-order atomics are
> > impacted because more processes are making forward progress where
> > previously they would have waited until kswapd had done work. Your
> > driver has already been fixed in this regard and maybe it's a case that
> > the other atomic users simply have to be fixed to "not do that".
> 
> The problem is that although my driver has been fixed so that it no longer 
> causes the SKB allocation errors, the also rather serious behavior change 
> where due to swapping my 3rd gitk takes up to twice as long to load with 
> desktop freezes of up 45 seconds or so is still there.
> 
> Although that's somewhat separate from the issue that started this whole 
> investigation, I still feel that should be sorted out as well.
> 

You're right. That behaviour sucks.

> The congestion_wait() change, even if theoretically valid, introduced a 
> very real regression IMO. Such long desktop freezes during swapping should 
> be avoided; .30 and earlier simply behaved a whole lot better in the same 
> situation.
> 

Agreed. I'll start from scratch again trying to reproduce what you're seeing
locally. I'll try breaking my network card so that it's making high-order
atomics and see where I get. Machines that were previously tied up are now
free so I might have a better chance.

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