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    SubjectRe: [patch]cfq-iosched: delete deep seeky queue idle logic
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    On Mon, Sep 26, 2011 at 2:55 AM, Shaohua Li <shaohua.li@intel.com> wrote:
    > On Fri, 2011-09-23 at 13:50 +0800, Corrado Zoccolo wrote:
    >> Il giorno 21/set/2011 13:16, "Shaohua Li" <shaohua.li@intel.com> ha
    >> scritto:
    >> >
    >> > On Sat, 2011-09-17 at 03:25 +0800, Corrado Zoccolo wrote:
    >> > > On Fri, Sep 16, 2011 at 8:40 AM, Shaohua Li <shaohua.li@intel.com>
    >> wrote:
    >> > > > On Fri, 2011-09-16 at 14:04 +0800, Corrado Zoccolo wrote:
    >> > > >> On Fri, Sep 16, 2011 at 5:09 AM, Shaohua Li
    >> <shaohua.li@intel.com> wrote:
    >> > > >> > Recently Maxim and I discussed why his aiostress workload
    >> performs poorly. If
    >> > > >> > you didn't follow the discussion, here are the issues we
    >> found:
    >> > > >> > 1. cfq seeky dection isn't good. Assume a task accesses
    >> sector A, B, C, D, A+1,
    >> > > >> > B+1, C+1, D+1, A+2...Accessing A, B, C, D is random. cfq will
    >> detect the queue
    >> > > >> > as seeky, but since when accessing A+1, A+1 is already in
    >> disk cache, this
    >> > > >> > should be detected as sequential really. Not sure if any real
    >> workload has such
    >> > > >> > access patern, and seems not easy to have a clean fix too.
    >> Any idea for this?
    >> > > >>
    >> > > >> Not all disks will cache 4 independent streams, we can't make
    >> that
    >> > > >> assumption in cfq.
    >> > > > sure thing. we can't make such assumption. I'm thinking if we
    >> should
    >> > > > move the seeky detection in request finish. If time between two
    >> requests
    >> > > > finish is short, we thought the queue is sequential. This will
    >> make the
    >> > > > detection adaptive. but seems time measurement isn't easy.
    >> > > >
    >> > > >> The current behaviour of assuming it as seeky should work well
    >> enough,
    >> > > >> in fact it will be put in the seeky tree, and it can enjoy the
    >> seeky
    >> > > >> tree quantum of time. If the second round takes a short time,
    >> it will
    >> > > >> be able to schedule a third round again after the idle time.
    >> > > >> If there are other seeky processes competing for the tree, the
    >> cache
    >> > > >> can be cleared by the time it gets back to your 4 streams
    >> process, so
    >> > > >> it will behave exactly as a seeky process from cfq point of
    >> view.
    >> > > >> If the various accesses were submitted in parallel, the deep
    >> seeky
    >> > > >> queue logic should kick in and make sure the process gets a
    >> sequential
    >> > > >> quantum, rather than sharing it with other seeky processes, so
    >> > > >> depending on your disk, it could perform better.
    >> > > > yes, the idle logic makes it ok, but sounds like "make things
    >> wrong
    >> > > > first (in seeky detection) and then fix it later (the idle
    >> logic)".
    >> > > >
    >> > > >> > 2. deep seeky queue idle. This makes raid performs poorly. I
    >> would think we
    >> > > >> > revert the logic. Deep queue is more popular with high end
    >> hardware. In such
    >> > > >> > hardware, we'd better not do idle.
    >> > > >> > Note, currently we set a queue's slice after the first
    >> request is finished.
    >> > > >> > This means the drive already idles a little time. If the
    >> queue is truely deep,
    >> > > >> > new requests should already come in, so idle isn't required.
    >> > > > What did you think about this? Assume seeky request takes long
    >> time, so
    >> > > > the queue is already idling for a little time.
    >> > > I don't think I understand. If cfq doesn't idle, it will dispatch
    >> an
    >> > > other request from the same or an other queue (if present)
    >> > > immediately, until all possible in-flight requests are sent. Now,
    >> you
    >> > > depend on NCQ for the order requests are handled, so you cannot
    >> > > guarantee fairness any more.
    >> > >
    >> > > >
    >> > > >> > Looks Vivek used to post a patch to rever it, but it gets
    >> ignored.
    >> > > >> >
    >> http://us.generation-nt.com/patch-cfq-iosched-revert-logic-deep-queues-help-198339681.html
    >> > > >> I get a 404 here. I think you are seeing only one half of the
    >> medal.
    >> > > >> That logic is there mainly to ensure fairness between deep
    >> seeky
    >> > > >> processes and normal seeky processes that want low latency.
    >> > > > didn't understand it. The logic doesn't protect non-deep
    >> process. how
    >> > > > could it make the normal seeky process have low latency? or did
    >> you have
    >> > > > a test case for this, so I can analyze?
    >> > > > I tried a workload with one task drives depth 4 and one task
    >> drives
    >> > > > depth 16. Appears the behavior isn't changed w/wo the logic.
    >> > sorry for the delay.
    >> >
    >> > > Try a workload with one shallow seeky queue and one deep (16) one,
    >> on
    >> > > a single spindle NCQ disk.
    >> > > I think the behaviour when I submitted my patch was that both were
    >> > > getting 100ms slice (if this is not happening, probably some
    >> > > subsequent patch broke it).
    >> > > If you remove idling, they will get disk time roughly in
    >> proportion
    >> > > 16:1, i.e. pretty unfair.
    >> > I thought you are talking about a workload with one thread depth 4,
    >> and
    >> > the other thread depth 16. I did some tests here. In an old kernel,
    >> > without the deep seeky idle logic, the threads have disk time in
    >> > proportion 1:5. With it, they get almost equal disk time. SO this
    >> > reaches your goal. In a latest kernel, w/wo the logic, there is no
    >> big
    >> > difference (the 16 depth thread get about 5x more disk time). With
    >> the
    >> > logic, the depth 4 thread gets equal disk time in first several
    >> slices.
    >> > But after an idle expiration(mostly because current block plug hold
    >> > requests in task list and didn't add them to elevator), the queue
    >> never
    >> > gets detected as deep, because the queue dispatch request one by
    >> one. So
    >> > the logic is already broken for some time (maybe since block plug is
    >> > added).
    >> Could be that dispatching requests one by one is harming the
    >> performance, then?
    > Not really. Say 4 requests are running, the task dispatches a request
    > after one previous request is completed. requests are dispatching one by
    > one but there are still 4 requests running at any time. Checking the
    > in_flight requests are more precise for the deep detection.
    >
    What happens if there are 4 tasks, all that could dispatch 4 requests
    in parallel? Will we reach and sustain 16 in flight requests, or it
    will bounce around 4 in flight? I think here we could get a big
    difference.
    Probably it is better to move the deep queue detection logic in the
    per-task queue?
    Then cfq will decide if it should dispatch few requests from every
    task (shallow case) or all requests from a single task (deep), and
    then idle.

    Thanks
    Corrado


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