Re: [PATCH] cfq-iosched: fix next_rq computation
From: Jens Axboe
Date: Sun Nov 08 2009 - 11:16:26 EST
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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