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    SubjectRe: [PATCH 2/3] cfq-iosched: Implement a tunable group_idle
    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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