linux-kernel - Re: high iowait problem(Bug 12309 on bugzilla.kernel.org)

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Date:	Thu, 15 Apr 2010 00:58:05 -0500
From:	Steven B <damentz@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Frank Ren <frankrq2009@....com>, mingo <mingo@...e.hu>,
	"a.p.zijlstra" <a.p.zijlstra@...llo.nl>,
	"jens.axboe" <jens.axboe@...cle.com>, linux-kernel@...r.kernel.org,
	stable@...nel.org, dm-devel@...hat.com,
	Alasdair G Kergon <agk@...hat.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>
Subject: Re: high iowait problem(Bug 12309 on bugzilla.kernel.org)

Hi Andrew,

> So something between 2.6.32.3 and 2.6.32.8 inclusive rebroke the
> kernel.  That's useful info.
>
> Looking at the changelogs, the only thing I can immediately see in the
> block-core area is
>
> : commit cb723ba5d03bf719dbc7409b4d67572d4472ef8b
> : Author: Dmitry Monakhov <dmonakhov@...nvz.org>
> : Date:   Wed Jan 27 22:44:36 2010 +0300
> :
> :     block: fix bio_add_page for non trivial merge_bvec_fn case
>
> which seems unlikely.
>
> I'm seeing just one device-mapper change whcih went in over that
> timeframe:
>
> : commit e0f5cfa7c18b411634e73923841eccd3a4c0ce7f
> : Author: Martin K. Petersen <martin.petersen@...cle.com>
> : Date:   Mon Jan 11 03:21:50 2010 -0500
> :
> :     DM: Fix device mapper topology stacking
>
>

Although I can't provide any big details, I went through the commits between
2.6.32.2 and 2.6.32.8 and as many things that could affect IO and reverted
them from my 2.6.33.2 kernel.

Here's my list of commits I reverted that seemed to improve performance locally
running a 2.6.33.2 kernel.

cb723ba5d03bf719dbc7409b4d67572d4472ef8b (this one you suspected)
block: fix bio_add_page for non trivial merge_bvec_fn case

b2ea8cb9c8f1937cb80b9beb50548a05bfc37819
vmscan: do not evict inactive pages when skipping an active list scan

e4dd8ca5be771aa706b0797b3284328bb595bfa1
fasync: split 'fasync_helper()' into separate add/remove functions

Though, I was hoping you guys could identify which one of these is
the real culprit.


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