linux-kernel - Re: Bug in kernel 2.6.31, Slow wb

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Date:	Fri, 31 Jul 2009 00:43:08 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Martin Bligh <mbligh@...gle.com>
Cc:	Chad Talbott <ctalbott@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, wfg@...l.ustc.edu.cn,
	Michael Rubin <mrubin@...gle.com>,
	Andrew Morton <akpm@...gle.com>, sandeen@...hat.com
Subject: Re: Bug in kernel 2.6.31, Slow wb_kupdate writeout

On Thu, Jul 30 2009, Martin Bligh wrote:
> > The test case above on a 4G machine is only generating 1G of dirty data.
> > I ran the same test case on the 16G, resulting in only background
> > writeout. The relevant bit here being that the background writeout
> > finished quickly, writing at disk speed.
> >
> > I re-ran the same test, but using 300 100MB files instead. While the
> > dd's are running, we are going at ~80MB/sec (this is disk speed, it's an
> > x25-m). When the dd's are done, it continues doing 80MB/sec for 10
> > seconds or so. Then the remainder (about 2G) is written in bursts at
> > disk speeds, but with some time in between.
> 
> OK, I think the test case is sensitive to how many files you have - if
> we punt them to the back of the list, and yet we still have 299 other
> ones, it may well be able to keep the disk spinning despite the bug
> I outlined.Try using 30 1GB files?

If this disk starts spinning, then we have bigger bugs :-)
> 
> Though it doesn't seem to happen with just one dd streamer, and
> I don't see why the bug doesn't trigger in that case either.
> 
> I believe the bugfix is correct independent of any bdi changes?

Yeah I think so too, I'll run some more tests on this tomorrow and
verify it there as well.

-- 
Jens Axboe

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