linux-kernel - Re: [RFC PATCH 0/3] Avoid the use of congestion

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Message-ID: <20100318174220.GP12388@csn.ul.ie>
Date:	Thu, 18 Mar 2010 17:42:21 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>,
	linux-mm@...ck.org, Nick Piggin <npiggin@...e.de>,
	Chris Mason <chris.mason@...cle.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	linux-kernel@...r.kernel.org, gregkh@...ell.com
Subject: Re: [RFC PATCH 0/3] Avoid the use of congestion_wait under zone
	pressure

On Mon, Mar 15, 2010 at 01:09:35PM -0700, Andrew Morton wrote:
> On Mon, 15 Mar 2010 13:34:50 +0100
> Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com> wrote:
> 
> > c) If direct reclaim did reasonable progress in try_to_free but did not
> > get a page, AND there is no write in flight at all then let it try again
> > to free up something.
> > This could be extended by some kind of max retry to avoid some weird
> > looping cases as well.
> > 
> > d) Another way might be as easy as letting congestion_wait return
> > immediately if there are no outstanding writes - this would keep the 
> > behavior for cases with write and avoid the "running always in full 
> > timeout" issue without writes.
> 
> They're pretty much equivalent and would work.  But there are two
> things I still don't understand:
> 
> 1: Why is direct reclaim calling congestion_wait() at all?  If no
> writes are going on there's lots of clean pagecache around so reclaim
> should trivially succeed.  What's preventing it from doing so?
> 
> 2: This is, I think, new behaviour.  A regression.  What caused it?
> 

I looked at this a bit closer using an iozone test very similar to
Christian's. Despite buying a number of disks, I still can't reproduce his
problem but I instrumented congestion_wait counts and times similar to
what he did.

2.6.29-instrument:congestion_waittime 990
2.6.30-instrument:congestion_waittime 2823
2.6.31-instrument:congestion_waittime 193169
2.6.32-instrument:congestion_waittime 228890
2.6.33-instrument:congestion_waittime 785529
2.6.34-rc1-instrument:congestion_waittime 797178

So in the problem window, there was *definite* increases in the time spent
in congestion_wait and the number of times it was called. I'll look
closer at this tomorrow and Monday and see can I pin down what is
happening.

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