linux-kernel - Re: [PATCH 1/3] cfq-iosched: Implment IOPS mode

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Message-ID: <x497hkod18f.fsf@segfault.boston.devel.redhat.com>
Date:	Wed, 21 Jul 2010 16:33:04 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	linux-kernel@...r.kernel.org, axboe@...nel.dk, nauman@...gle.com,
	dpshah@...gle.com, guijianfeng@...fujitsu.com, czoccolo@...il.com
Subject: Re: [PATCH 1/3] cfq-iosched: Implment IOPS mode

Vivek Goyal <vgoyal@...hat.com> writes:

> o Implement another CFQ mode where we charge queue/group in terms of number
>   of requests dispatched instead of measuring the time. Measuring in terms
>   of time is not possible when we are driving deeper queue depths and there
>   are requests from multiple cfq queues in the request queue.
>
> o This mode currently gets activated if one sets slice_idle=0 and associated
>   disk supports NCQ. Again the idea is that on an NCQ disk with idling disabled
>   most of the queues will dispatch 1 or more requests and then cfq queue
>   expiry happens and we don't have a way to measure time. So start providing
>   fairness in terms of IOPS.
>
> o Currently this primarily is beneficial with cfq group scheduling where one
>   can disable slice idling so that we don't idle on queue and drive deeper
>   request queue deptsh (achieving better throughput), at the same time group
>   idle is enabled so one should get service differentiation among groups.

I like that this is more isolated now.  I'm slowly warming up to it.  I
have one question--just a curiosity, really.  What do you see now for
the reported sl_used in blktrace when slice_idle is zero and the
hardware supports command queueing?

Cheers,
Jeff
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