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