linux-kernel - Re: [patch] Give kjournald a IOPRIO_CLASS

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Message-ID: <20071114185526.GB4943@elte.hu>
Date:	Wed, 14 Nov 2007 19:55:26 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Alan D. Brunelle" <Alan.Brunelle@...com>,
	Rik van Riel <riel@...hat.com>, jens.axboe@...cle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] Give kjournald a IOPRIO_CLASS_RT io priority


* Arjan van de Ven <arjan@...radead.org> wrote:

> > > >     * The read-a-different-kernel-tree task, average over 10 runs:
> > > >           o 2.6.24 - 46.8145945549 seconds
> > > >           o 2.6.24 + Arjan's patch - 39.6430601119 seconds
> > > > 
> > > >     * The large-linear-read task (on an 8GiB file), average over
> > > > 10 runs: o 2.6.24 - 290.32522 seconds
> > > >           o 2.6.24 + Arjan's patch - 386.34860 seconds
> > > 
> > > These are *large* differences, making this a very signifcant
> > > patch. Much care is needed now.
> > 
> > and the numbers suggest it's mostly a severe performance regression. 
> > That's not what i have expected - ho hum. Apologies for my earlier 
> > "please merge it already!" whining.
> 
> that's.. not automatic; it depends on what the right thing is :-( What 
> for sure changes is that who gets to do IO changes. Some of the tests 
> we ran internally (we didn't publish yet because we saw REALLY large 
> variations for most of them even without any patch) show for example 
> that "dbench" got slower. But.. dbench gets slower when things get 
> more fair, and faster when things get unfair. What conclusion you draw 
> out of that is a whole different matter and depends on exactly what 
> the test is doing, and what is the right thing for the OS to do in 
> terms of who gets to do the IO.

yeah, i'd agree to not too much faith into dbench results.

	Ingo
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