linux-kernel - Re: [PATCH 2/3] cfq-iosched: Implement a tunable group

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Message-ID: <x49r5iwblp0.fsf@segfault.boston.devel.redhat.com>
Date:	Wed, 21 Jul 2010 16:54:03 -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 2/3] cfq-iosched: Implement a tunable group_idle

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

> On Wed, Jul 21, 2010 at 03:40:44PM -0400, Jeff Moyer wrote:
>> Vivek Goyal <vgoyal@...hat.com> writes:
>> 
>> > o Implement a new tunable group_idle, which allows idling on the group
>> >   instead of a cfq queue. Hence one can set slice_idle = 0 and not idle
>> >   on the individual queues but idle on the group. This way on fast storage
>> >   we can get fairness between groups at the same time overall throughput
>> >   improves.
>> >
>> > Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
>> > ---
>> [snip]
>> > @@ -1929,13 +1941,21 @@ static void cfq_arm_slice_timer(struct cfq_data *cfqd)
>> >  		return;
>> >  	}
>> >  
>> > +	/* There are other queues in the group, don't do group idle */
>> > +	if (group_idle && cfqq->cfqg->nr_cfqq > 1)
>> > +		return;
>> > +
>> >  	cfq_mark_cfqq_wait_request(cfqq);
>> >  
>> > -	sl = cfqd->cfq_slice_idle;
>> > +	if (group_idle)
>> > +		sl = cfqd->cfq_group_idle;
>> > +	else
>> > +		sl = cfqd->cfq_slice_idle;
>> 
>> What happens when both group_idle and slice_idle are set?
>
> slice_idle prevails. Notice that "group_idle" is a local variable which
> is set to 1 only if we decide not to idle on the cfq queue.

Ah, silly me.

Cheers,
Jeff
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