linux-kernel - Re: [RFC V2 PATCH 1/5] cfq-iosched: adapt slice to number of processes doing I/O

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Message-ID: <x49ws2ixkbq.fsf@segfault.boston.devel.redhat.com>
Date:	Mon, 26 Oct 2009 11:10:17 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Corrado Zoccolo <czoccolo@...il.com>
Cc:	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: [RFC V2 PATCH 1/5] cfq-iosched: adapt slice to number of  processes doing I/O

Corrado Zoccolo <czoccolo@...il.com> writes:

> Hi Jeff,
> On Wed, Oct 21, 2009 at 6:36 PM, Jeff Moyer <jmoyer@...hat.com> wrote:
>> Corrado Zoccolo <czoccolo@...il.com> writes:
>>
>> OK.  The idea was to try to make the thing a bit more digestable.
>> Honestly, I had to re-write it to figure out what it was doing.  Could
>> you rework it so the logic is more obvious to others and still correct?
>
> What about:
> /* interested queues (we consider only the ones with the same priority class) */
> unsigned iq = cfq_get_avg_queues(cfqd, cfq_class_rt(cfqq));
> unsigned sync_slice = cfqd->cfq_slice[1];
> unsigned expected_latency = sync_slice * iq;
> if (expected_latency > target_latency) {
> 	unsigned base_low_slice = 2 * cfqd->cfq_slice_idle;
> 	/* scale low_slice according to IO priority and sync vs async */
> 	unsigned low_slice = min(slice,  base_low_slice * slice / sync_slice);
> 	/* the adapted slice value is scaled to fit all iqs into the target latency */
> 	slice = max(slice * cfq_target_latency / expected_latency, low_slice);
> }

Yeah, that looks a ton better.  Thanks for doing that!

Cheers,
Jeff
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