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

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Message-ID: <20090405115629.521057fc@infradead.org>
Date:	Sun, 5 Apr 2009 11:56:29 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Theodore Tso <tytso@....edu>, Jens Axboe <jens.axboe@...cle.com>,
	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 Sun, 5 Apr 2009 10:01:06 -0700 (PDT)
Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> Also, one of the issues seems to literally be that the higher-level 
> request handling doesn't care AT ALL about the priority. Allocating
> the request itself does keep reads and writes separated, but if the
> request is a SYNCIO request, and non-sync writes have filled up th
> write requests, we'll have to wait for the background writes to free
> up requests.
> 
> That is quite possibly the longest wait we have in the system.

it often is; latencytop tends to point that out.
> 
> See get_request():

our default number of requests is so low that we very regularly hit the
limit. In addition to setting kjournald to higher priority, I tend to
set the number of requests to 4096 or so to improve interactive
performance on my own systems. That way at least the elevator has a
chance to see the requests ;-)


-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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