linux-kernel - Re: 2.6.20-rc2+: CFQ halving disk throughput.

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Message-ID: <20070102084447.GS2483@kernel.dk>
Date:	Tue, 2 Jan 2007 09:44:47 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	Rene Herman <rene.herman@...il.com>, Tejun Heo <htejun@...il.com>,
	Jens Axboe <axboe@...e.de>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	linux-kernel@...r.kernel.org, Linus Torvalds <torvalds@...l.org>
Subject: Re: 2.6.20-rc2+: CFQ halving disk throughput.

On Mon, Jan 01 2007, Andrew Morton wrote:
> On Mon, 01 Jan 2007 23:46:58 +0100
> Rene Herman <rene.herman@...il.com> wrote:
> 
> > > Everything seems fine in the dmesg.  Performance degradation is
> > > probably some other issue in -rc kernel.  I'm suspecting recently
> > > fixed block layer bug.  If it's still the same in the next -rc,
> > > please report.
> > 
> > In fact, it's CFQ. The PATA thing was a red herring. 2.6.20-rc2 and 3 
> > give me ~ 24 MB/s from "hdparm t /dev/hda" while 2.6.20-rc1 and below 
> > give me ~ 50 MB/s.
> > 
> > Jens: this is due to "[PATCH] cfq-iosched: tighten allow merge 
> > criteria", 719d34027e1a186e46a3952e8a24bf91ecc33837:
> > 
> > http://www2.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=719d34027e1a186e46a3952e8a24bf91ecc33837
> > 
> > If I revert that one, I have my 50 M/s back. config and dmesg attached 
> > in case they're useful.
> 
> The patch would appear to need this fix:
> 
> --- a/block/cfq-iosched.c~a
> +++ a/block/cfq-iosched.c
> @@ -592,7 +592,7 @@ static int cfq_allow_merge(request_queue
>  	if (cfqq == RQ_CFQQ(rq))
>  		return 1;
>  
> -	return 1;
> +	return 0;
>  }
>  
>  static inline void
> _
> 
> But that might not fix things...

Yeah it is, but I don't think it'll fix it (if anything, it'll be more
conservative).

-- 
Jens Axboe

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