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    SubjectRe: [RFC PATCH] block: Fix bio merge induced high I/O latency
    On Tue, Jan 20 2009, Jens Axboe wrote:
    > On Mon, Jan 19 2009, Mathieu Desnoyers wrote:
    > > * Jens Axboe (jens.axboe@oracle.com) wrote:
    > > > On Sun, Jan 18 2009, Mathieu Desnoyers wrote:
    > > > > I looked at the "ls" behavior (while doing a dd) within my LTTng trace
    > > > > to create a fio job file. The said behavior is appended below as "Part
    > > > > 1 - ls I/O behavior". Note that the original "ls" test case was done
    > > > > with the anticipatory I/O scheduler, which was active by default on my
    > > > > debian system with custom vanilla 2.6.28 kernel. Also note that I am
    > > > > running this on a raid-1, but have experienced the same problem on a
    > > > > standard partition I created on the same machine.
    > > > >
    > > > > I created the fio job file appended as "Part 2 - dd+ls fio job file". It
    > > > > consists of one dd-like job and many small jobs reading as many data as
    > > > > ls did. I used the small test script to batch run this ("Part 3 - batch
    > > > > test").
    > > > >
    > > > > The results for the ls-like jobs are interesting :
    > > > >
    > > > > I/O scheduler runt-min (msec) runt-max (msec)
    > > > > noop 41 10563
    > > > > anticipatory 63 8185
    > > > > deadline 52 33387
    > > > > cfq 43 1420
    > > >
    > >
    > > Extra note : I have a HZ=250 on my system. Changing to 100 or 1000 did
    > > not make much difference (also tried with NO_HZ enabled).
    > >
    > > > Do you have queuing enabled on your drives? You can check that in
    > > > /sys/block/sdX/device/queue_depth. Try setting those to 1 and retest all
    > > > schedulers, would be good for comparison.
    > > >
    > >
    > > Here are the tests with a queue_depth of 1 :
    > >
    > > I/O scheduler runt-min (msec) runt-max (msec)
    > > noop 43 38235
    > > anticipatory 44 8728
    > > deadline 51 19751
    > > cfq 48 427
    > >
    > >
    > > Overall, I wouldn't say it makes much difference.
    >
    > 0,5 seconds vs 1,5 seconds isn't much of a difference?
    >
    > > > raid personalities or dm complicates matters, since it introduces a
    > > > disconnect between 'ls' and the io scheduler at the bottom...
    > > >
    > >
    > > Yes, ideally I should re-run those directly on the disk partitions.
    >
    > At least for comparison.
    >
    > > I am also tempted to create a fio job file which acts like a ssh server
    > > receiving a connexion after it has been pruned from the cache while the
    > > system if doing heavy I/O. "ssh", in this case, seems to be doing much
    > > more I/O than a simple "ls", and I think we might want to see if cfq
    > > behaves correctly in such case. Most of this I/O is coming from page
    > > faults (identified as traps in the trace) probably because the ssh
    > > executable has been thrown out of the cache by
    > >
    > > echo 3 > /proc/sys/vm/drop_caches
    > >
    > > The behavior of an incoming ssh connexion after clearing the cache is
    > > appended below (Part 1 - LTTng trace for incoming ssh connexion). The
    > > job file created (Part 2) reads, for each job, a 2MB file with random
    > > reads each between 4k-44k. The results are very interesting for cfq :
    > >
    > > I/O scheduler runt-min (msec) runt-max (msec)
    > > noop 586 110242
    > > anticipatory 531 26942
    > > deadline 561 108772
    > > cfq 523 28216
    > >
    > > So, basically, ssh being out of the cache can take 28s to answer an
    > > incoming ssh connexion even with the cfq scheduler. This is not exactly
    > > what I would call an acceptable latency.
    >
    > At some point, you have to stop and consider what is acceptable
    > performance for a given IO pattern. Your ssh test case is purely random
    > IO, and neither CFQ nor AS would do any idling for that. We can make
    > this test case faster for sure, the hard part is making sure that we
    > don't regress on async throughput at the same time.
    >
    > Also remember that with your raid1, it's not entirely reasonable to
    > blaim all performance issues on the IO scheduler as per my previous
    > mail. It would be a lot more fair to view the disk numbers individually.
    >
    > Can you retry this job with 'quantum' set to 1 and 'slice_async_rq' set
    > to 1 as well?
    >
    > However, I think we should be doing somewhat better at this test case.

    Mathieu, does this improve anything for you?

    diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
    index e8525fa..a556512 100644
    --- a/block/cfq-iosched.c
    +++ b/block/cfq-iosched.c
    @@ -1765,6 +1765,32 @@ cfq_update_idle_window(struct cfq_data *cfqd, struct cfq_queue *cfqq,
    }

    /*
    + * Pull dispatched requests from 'cfqq' back into the scheduler
    + */
    +static void cfq_pull_dispatched_requests(struct cfq_data *cfqd,
    + struct cfq_queue *cfqq)
    +{
    + struct request_queue *q = cfqd->queue;
    + struct request *rq, *tmp;
    +
    + list_for_each_entry_safe(rq, tmp, &q->queue_head, queuelist) {
    + if ((rq->cmd_flags & REQ_STARTED) || RQ_CFQQ(rq) != cfqq)
    + continue;
    +
    + /*
    + * Pull off the dispatch list and put it back into the cfqq
    + */
    + list_del(&rq->queuelist);
    + cfqq->dispatched--;
    + if (cfq_cfqq_sync(cfqq))
    + cfqd->sync_flight--;
    +
    + list_add_tail(&rq->queuelist, &cfqq->fifo);
    + cfq_add_rq_rb(rq);
    + }
    +}
    +
    +/*
    * Check if new_cfqq should preempt the currently active queue. Return 0 for
    * no or if we aren't sure, a 1 will cause a preempt.
    */
    @@ -1820,8 +1846,14 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
    */
    static void cfq_preempt_queue(struct cfq_data *cfqd, struct cfq_queue *cfqq)
    {
    + struct cfq_queue *old_cfqq = cfqd->active_queue;
    +
    cfq_log_cfqq(cfqd, cfqq, "preempt");
    - cfq_slice_expired(cfqd, 1);
    +
    + if (old_cfqq) {
    + __cfq_slice_expired(cfqd, old_cfqq, 1);
    + cfq_pull_dispatched_requests(cfqd, old_cfqq);
    + }

    /*
    * Put the new queue at the front of the of the current list,
    --
    Jens Axboe



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