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

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Message-ID: <20090406081616.GT5178@kernel.dk>
Date:	Mon, 6 Apr 2009 10:16:16 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Theodore Tso <tytso@....edu>,
	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, Arjan van de Ven wrote:
> On Sat, 4 Apr 2009 19:22:22 -0400
> Theodore Tso <tytso@....edu> wrote:
> > 
> > However, the full latency fixes all the writes are synchronous, so it
> > must be the case that the delays are caused by the fact that queue is
> > getting implicitly unplugged after the synchronous write, and the
> > problem is no longer the mixing of WRITE and WRITE_SYNC requests as
> > posted in the commit log for 78f707bf.  If we remove the automatic
> > unplug for WRITE_SYNC requests, and add an explicit unplug where it is
> > needed, that should fix the performance regression for this particular
> > sqlite test case. 
> 
> removing the unplug is bound to be bad; after all we're waiting on the
> IO. But maybe it should be "make the unplug a REALLY short time".
> At least for rotating storage. For non-rotating .. I'd never wait.

Well, either you are submitting a single piece of IO (in which case you
just want to unplug or directly submit as part of the submit_bio()), or
you are submitting several IOS (in which case you just want to unplug at
the end of the IO submission, before waiting).

-- 
Jens Axboe

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