Messages in this thread | | | Date | Wed, 21 Jul 2010 16:13:18 -0400 | From | Vivek Goyal <> | Subject | Re: [PATCH 2/3] cfq-iosched: Implement a tunable group_idle |
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On Wed, Jul 21, 2010 at 03:40:44PM -0400, Jeff Moyer wrote: > Vivek Goyal <vgoyal@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@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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