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    SubjectRe: [PATCH] cfq-iosched: allow groups preemption for sync-noidle workloads
    On Thu, Jun 23, 2011 at 08:21:59PM +0400, Konstantin Khlebnikov wrote:
    > commit v2.6.32-102-g8682e1f "blkio: Provide some isolation between groups" break
    > fast switching between task and journal-thread for very common write-fsync workload.
    > cfq wait idle slice at each cfqq switch, if this task is from non-root blkio cgroup.
    >
    > This patch move idling sync-noidle preempting check little bit upwards and update
    > new service_tree->count check for case with two different groups.
    > I do not quite understand what means these check for new_cfqq, but now it even works.
    >
    > Without patch I got 49 iops and with this patch 798, for this trivial fio script:
    >
    > [write-fsync]
    > cgroup=test
    > cgroup_weight=1000
    > rw=write
    > fsync=1
    > size=100m
    > runtime=10s

    What kind of storage and filesystem you are using? I tried this on a SATA
    disk and I really don't get good throughput. With deadline scheduler I
    get aggrb=103KB/s.

    I think with fsync we are generating so many FLUSH requests that it
    really slows down fsync.

    Even if I use CFQ with and without cgroups, I get following.

    CFQ, without cgroup
    ------------------
    aggrb=100KB/s

    CFQ with cgroup
    --------------
    aggrb=94KB/s

    So with FLUSH requests, not much difference in throughput for this
    workload.

    I guess you must be running with barriers off or something like that.

    Thanks
    Vivek


    >
    > Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
    > ---
    > block/cfq-iosched.c | 14 +++++++-------
    > 1 files changed, 7 insertions(+), 7 deletions(-)
    >
    > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
    > index 3c7b537..c71533e 100644
    > --- a/block/cfq-iosched.c
    > +++ b/block/cfq-iosched.c
    > @@ -3318,19 +3318,19 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
    > if (rq_is_sync(rq) && !cfq_cfqq_sync(cfqq))
    > return true;
    >
    > - if (new_cfqq->cfqg != cfqq->cfqg)
    > - return false;
    > -
    > - if (cfq_slice_used(cfqq))
    > - return true;
    > -
    > /* Allow preemption only if we are idling on sync-noidle tree */
    > if (cfqd->serving_type == SYNC_NOIDLE_WORKLOAD &&
    > cfqq_type(new_cfqq) == SYNC_NOIDLE_WORKLOAD &&
    > - new_cfqq->service_tree->count == 2 &&
    > + new_cfqq->service_tree->count == 1+(new_cfqq->cfqg == cfqq->cfqg) &&
    > RB_EMPTY_ROOT(&cfqq->sort_list))
    > return true;
    >
    > + if (new_cfqq->cfqg != cfqq->cfqg)
    > + return false;
    > +
    > + if (cfq_slice_used(cfqq))
    > + return true;
    > +
    > /*
    > * So both queues are sync. Let the new request get disk time if
    > * it's a metadata request and the current queue is doing regular IO.


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