linux-kernel - Re: [PATCH 3/5] page allocator: Wait on both sync and async congestion after direct reclaim

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Date:	Fri, 13 Nov 2009 12:28:21 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Frans Pop <elendil@...net.nl>, Jiri Kosina <jkosina@...e.cz>,
	Sven Geggus <lists@...hsschwanzdomain.de>,
	Karol Lewandowski <karol.k.lewandowski@...il.com>,
	Tobias Oetiker <tobi@...iker.ch>, linux-kernel@...r.kernel.org,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Rik van Riel <riel@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Stephan von Krawczynski <skraw@...net.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Kernel Testers List <kernel-testers@...r.kernel.org>
Subject: Re: [PATCH 3/5] page allocator: Wait on both sync and async
	congestion after direct reclaim

On Fri, Nov 13, 2009 at 12:55:58PM +0100, Jens Axboe wrote:
> On Fri, Nov 13 2009, KOSAKI Motohiro wrote:
> > (cc to Jens)
> > 
> > > Testing by Frans Pop indicated that in the 2.6.30..2.6.31 window at least
> > > that the commits 373c0a7e 8aa7e847 dramatically increased the number of
> > > GFP_ATOMIC failures that were occuring within a wireless driver. Reverting
> > > this patch seemed to help a lot even though it was pointed out that the
> > > congestion changes were very far away from high-order atomic allocations.
> > > 
> > > The key to why the revert makes such a big difference is down to timing and
> > > how long direct reclaimers wait versus kswapd. With the patch reverted,
> > > the congestion_wait() is on the SYNC queue instead of the ASYNC. As a
> > > significant part of the workload involved reads, it makes sense that the
> > > SYNC list is what was truely congested and with the revert processes were
> > > waiting on congestion as expected. Hence, direct reclaimers stalled
> > > properly and kswapd was able to do its job with fewer stalls.
> > > 
> > > This patch aims to fix the congestion_wait() behaviour for SYNC and ASYNC
> > > for direct reclaimers. Instead of making the congestion_wait() on the SYNC
> > > queue which would only fix a particular type of workload, this patch adds a
> > > third type of congestion_wait - BLK_RW_BOTH which first waits on the ASYNC
> > > and then the SYNC queue if the timeout has not been reached.  In tests, this
> > > counter-intuitively results in kswapd stalling less and freeing up pages
> > > resulting in fewer allocation failures and fewer direct-reclaim-orientated
> > > stalls.
> > 
> > Honestly, I don't like this patch. page allocator is not related to
> > sync block queue. vmscan doesn't make read operation.
> > This patch makes nearly same effect of s/congestion_wait/io_schedule_timeout/.
> > 
> > Please don't make mysterious heuristic code.
> > 
> > 
> > Sidenode: I doubt this regression was caused from page allocator.

Probably not. As noted, the major change is really in how long callers
are waiting on congestion_wait. The tarball includes graphs from an
instrumented kernel that shows how long callers are waiting due to
congestion_wait(). This has changed significantly.

I'll queue up tests over the weekend that test without dm-crypt being involved.

> > Probably we need to confirm caller change....
> 
> See the email from Chris from yesterday, he nicely explains why this
> change made a difference with dm-crypt.

Indeed.

But bear in mind that it also possible that direct reclaimers are also
congesting the queue due to swap-in.

> dm-crypt needs fixing, not a hack like this added.
> 

As noted by Chris in the same mail, dm-crypt has not changed. What has
changed is how long callers wait in congestion_wait.


> The vm needs to drop congestion hints and usage, not increase it. The
> above changelog is mostly hand-wavy nonsense, imho.
> 

Suggest an alternative that brings congestion_wait() more in line with
2.6.30 behaviour then.

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