linux-ext4 - Re: [GIT PULL] Ext3 latency fixes

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Message-ID: <20090406081244.GR5178@kernel.dk>
Date:	Mon, 6 Apr 2009 10:12:44 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Theodore Tso <tytso@....edu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [GIT PULL] Ext3 latency fixes

On Sat, Apr 04 2009, Theodore Tso wrote:
> On Sat, Apr 04, 2009 at 05:16:50PM +0200, Jens Axboe wrote:
> > Big nack on this patch. Ted, this is EXACTLY where I told you we saw big
> > write regressions (sqlite performance drops by a factor of 4-5). Do a
> > git log on fs/buffer.c and see the original patch (which does what your
> > patch does) and the later revert. 
> 
> You mean this revert, right?
> 
> commit 78f707bfc723552e8309b7c38a8d0cc51012e813
> Author: Jens Axboe <jens.axboe@...cle.com>
> Date:   Tue Feb 17 13:59:08 2009 +0100
> 
>     block: revert part of 18ce3751ccd488c78d3827e9f6bf54e6322676fb
>     
>     The above commit added WRITE_SYNC and switched various places to using
>     that for committing writes that will be waited upon immediately after
>     submission. However, this causes a performance regression with AS and CFQ
>     for ext3 at least, since sync_dirty_buffer() will submit some writes with
>     WRITE_SYNC while ext3 has sumitted others dependent writes without the sync
>     flag set. This causes excessive anticipation/idling in the IO scheduler
>     because sync and async writes get interleaved, causing a big performance
>     regression for the below test case (which is meant to simulate sqlite
>     like behaviour)....
> 
> OK, let me test things out first, but note that with the changes that
> Linus has already accepted, this may not be an issue --- since we've
> now fixed ext3 to submit those dependent writes with the SYNC flag
> now.  So I'm not sure the performance regression still applies, but
> I'll test using the test case supplied in the rest of the commit log
> and get back to you.

Yep that's the one. I'll throw some testing together here, too.

-- 
Jens Axboe

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