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    SubjectRe: [PATCH 1/2] cfq: always return false from should_idle if slice_idle is set to zero
    On Mon, Jun 21, 2010 at 03:49:48PM -0400, Jeff Moyer wrote:
    > Hi,
    >
    > In testing a competing fsync-ing process and a sequential reader on
    > mid-grade storage, I found that cfq was incapable of achieving the I/O
    > rates of deadline, no matter how it was tuned. Investigation, and insight
    > from Vivek (mostly the latter), led to identifying that we were still
    > idling for the last queue in the service tree.
    >
    > Modifying cfq_should_idle to not idle when slice_idle is set to zero got
    > us much closer to the performance of deadline for this workload. I have
    > one follow-on patch that gets us on-par with deadline, but I think this
    > patch stands alone.
    >
    > Comments, as always, are appreciated.
    >

    Looks good to me Jeff. This does take away one functionality and that is
    ability to get group service differentation even with slice_idle=0.
    cfq_should_idle() was making sure that we don't select the next queue
    from next group.

    But I think a better way to fix that would be to introduce a new tunable
    say group_idle. That way if we are running on some powerful storage box
    and don't want the overhead of idling, we can do slice_idle=0 and not idle
    among the cfqq's with-in group but still do idle a bit on group and get
    the group service differentiation.

    I have a small patch for implementing group_idle. I will post that once
    you patch is in Jens's tree.

    Thanks
    Vivek

    > Cheers,
    > Jeff
    >
    > Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
    > ---
    > block/cfq-iosched.c | 4 ++++
    > 1 files changed, 4 insertions(+), 0 deletions(-)
    >
    > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
    > index 5ff4f48..572a050 100644
    > --- a/block/cfq-iosched.c
    > +++ b/block/cfq-iosched.c
    > @@ -1842,6 +1842,10 @@ static bool cfq_should_idle(struct cfq_data *cfqd, struct cfq_queue *cfqq)
    > if (prio == IDLE_WORKLOAD)
    > return false;
    >
    > + /* Don't idle if slice idling is disabled by the user */
    > + if (cfqd->cfq_slice_idle == 0)
    > + return false;
    > +
    > /* We do for queues that were marked with idle window flag. */
    > if (cfq_cfqq_idle_window(cfqq) &&
    > !(blk_queue_nonrot(cfqd->queue) && cfqd->hw_tag))
    > --
    > 1.6.5.2
    >
    > --
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