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Date:	Sun, 8 Nov 2009 17:16:21 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Corrado Zoccolo <czoccolo@...il.com>
Cc:	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Jeff Moyer <jmoyer@...hat.com>
Subject: Re: [PATCH] cfq-iosched: fix next_rq computation

On Sat, Nov 07 2009, Corrado Zoccolo wrote:
> Cfq has a bug in computation of next_rq, that affects transition
> between multiple sequential request streams in a single queue
> (e.g.: two sequential buffered writers of the same priority),
> causing the alternation between the two streams for a transient period.
> 
>   8,0    1    18737     0.260400660  5312  D   W 141653311 + 256
>   8,0    1    20839     0.273239461  5400  D   W 141653567 + 256
>   8,0    1    20841     0.276343885  5394  D   W 142803919 + 256
>   8,0    1    20843     0.279490878  5394  D   W 141668927 + 256
>   8,0    1    20845     0.292459993  5400  D   W 142804175 + 256
>   8,0    1    20847     0.295537247  5400  D   W 141668671 + 256
>   8,0    1    20849     0.298656337  5400  D   W 142804431 + 256
>   8,0    1    20851     0.311481148  5394  D   W 141668415 + 256
>   8,0    1    20853     0.314421305  5394  D   W 142804687 + 256
>   8,0    1    20855     0.318960112  5400  D   W 142804943 + 256
> 
> The fix makes sure that the next_rq is computed from the last
> dispatched request, and not affected by merging.
> 
>   8,0    1    37776     4.305161306     0  D   W 141738087 + 256
>   8,0    1    37778     4.308298091     0  D   W 141738343 + 256
>   8,0    1    37780     4.312885190     0  D   W 141738599 + 256
>   8,0    1    37782     4.315933291     0  D   W 141738855 + 256
>   8,0    1    37784     4.319064459     0  D   W 141739111 + 256
>   8,0    1    37786     4.331918431  5672  D   W 142803007 + 256
>   8,0    1    37788     4.334930332  5672  D   W 142803263 + 256
>   8,0    1    37790     4.337902723  5672  D   W 142803519 + 256
>   8,0    1    37792     4.342359774  5672  D   W 142803775 + 256
>   8,0    1    37794     4.345318286     0  D   W 142804031 + 256

Thanks, this is a nice patch! I will apply it to for-2.6.33. Can I added
your signed-off-by?

-- 
Jens Axboe

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