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From: Jeff Moyer on 21 Jul 2010 15:50
Vivek Goyal <vgoyal(a)redhat.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(a)redhat.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? Is that a
sane thing to do from a user's perspective? If not, please protect
against it in the configuration code. If so, then explain why we prefer
group_idle here, but slice_idle in completed request for the extend_sl:

> @@ -3425,7 +3458,10 @@ static void cfq_completed_request(struct request_queue *q, struct request *rq)
> * the queue.
> */
> if (cfq_should_wait_busy(cfqd, cfqq)) {
> - cfqq->slice_end = jiffies + cfqd->cfq_slice_idle;
> + unsigned long extend_sl = cfqd->cfq_slice_idle;
> + if (!cfqd->cfq_slice_idle)
> + extend_sl = cfqd->cfq_group_idle;
> + cfqq->slice_end = jiffies + extend_sl;

Also, you'll need to add documentation for this new tunable.

Cheers,
Jeff
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From: Vivek Goyal on 21 Jul 2010 16:20
On Wed, Jul 21, 2010 at 03:40:44PM -0400, Jeff Moyer wrote:
> Vivek Goyal <vgoyal(a)redhat.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(a)redhat.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.


> Is that a
> sane thing to do from a user's perspective?

In fact by default both slice_idle=8 and group_idle=8. Just that in this
mode group_idle never kicks in as slice_idle logic kicks in always before
group_idle logic gets any chance.

> If not, please protect
> against it in the configuration code. If so, then explain why we prefer
> group_idle here, but slice_idle in completed request for the extend_sl:
>

In both the places we first prefer slice_idle. Just noticed the value of
"group_idle" in the beginning of arm_time() function and notice in what
circumstances do we set group_idle=1

Thanks
Vivek

> > @@ -3425,7 +3458,10 @@ static void cfq_completed_request(struct request_queue *q, struct request *rq)
> > * the queue.
> > */
> > if (cfq_should_wait_busy(cfqd, cfqq)) {
> > - cfqq->slice_end = jiffies + cfqd->cfq_slice_idle;
> > + unsigned long extend_sl = cfqd->cfq_slice_idle;
> > + if (!cfqd->cfq_slice_idle)
> > + extend_sl = cfqd->cfq_group_idle;
> > + cfqq->slice_end = jiffies + extend_sl;
>
> Also, you'll need to add documentation for this new tunable.
>
> Cheers,
> Jeff
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From: Jeff Moyer on 21 Jul 2010 17:00
Vivek Goyal <vgoyal(a)redhat.com> writes:

> On Wed, Jul 21, 2010 at 03:40:44PM -0400, Jeff Moyer wrote:
>> Vivek Goyal <vgoyal(a)redhat.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(a)redhat.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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