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

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Message-ID: <33307c790907301534v64c08f59o66fbdfbd3174ff5f@mail.gmail.com>
Date:	Thu, 30 Jul 2009 15:34:12 -0700
From:	Martin Bligh <mbligh@...gle.com>
To:	Jens Axboe <jens.axboe@...cle.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

> 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?

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?

M.
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