linux-kernel - Re: ext3 IO latency measurements (was: Linux 2.6.29)

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Message-ID: <alpine.LFD.2.00.0903261534490.3032@localhost.localdomain>
Date:	Thu, 26 Mar 2009 15:39:53 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jan Kara <jack@...e.cz>
cc:	Ingo Molnar <mingo@...e.hu>, Theodore Tso <tytso@....edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <npiggin@...e.de>,
	Jens Axboe <jens.axboe@...cle.com>,
	David Rees <drees76@...il.com>, Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Roland McGrath <roland@...hat.com>
Subject: Re: ext3 IO latency measurements (was: Linux 2.6.29)

On Thu, 26 Mar 2009, Jan Kara wrote:
>
>   Reads are measurably better with the patch - the test with cat you
> describe below took ~0.5s per file without the patch and always less than
> 0.02s with the patch. So it seems to help something.

That would seem to be a _huge_ improvement.

Reads are the biggest issue for starting a new process (eg starting 
firefox while under load), and if cat'ing that small file improved by that 
much, then I bet there's a huge practical implication for a lot of desktop 
uses.

The fundamental fsync() latency problem we sadly can't help much with, the 
way ext3 seems to work. But I do suspect that the whole "don't synchronize 
with the journal for normal write-outs" may end up helping even fsync just 
a bit, if only because I suspect it will improve writeout throughput too 
and thus avoid one particular bottleneck.

			Linus
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