linux-kernel - Re: [PATCH 1/1] CFQ: fix handling 'deep' cfqq

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Date:	Fri, 9 Sep 2011 21:41:28 +0400
From:	Maxim Patlasov <maxim.patlasov@...il.com>
To:	Shaohua Li <shli@...nel.org>
Cc:	axboe@...nel.dk, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] CFQ: fix handling 'deep' cfqq

Shaohua,

> So the key problem here is how to detect if a device is fast. Doing
> the detection
> in the dispatch stage always can't give us correct result. A fast device really
> should be requests can be finished in short time. So I have something attached.
> In my environment, a hard disk is detected slow and a ssd is detected fast, but
> I haven't run any benchmark so far. How do you think about it?

Thanks for the patch, I'll test it in several h/w configurations soon
and let you know about results.

As a first thought that comes to mind, this patch would hardly meet
needs of h/w raids. Every particular hdd may be not very fast, but
being assembled in RAID-0, it's still very beneficial to submit many
requests in a row. That's why I made estimation in the dispatch stage:
if hdd (or h/w raid) is regularly able to drain deep queue quite fast,
we should claim it as 'fast' and avoid idling if possible.

Thanks,
Maxim
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