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    SubjectRe: [patch,rfc] cfq: merge cooperating cfq_queues
    Corrado Zoccolo <> writes:

    Hi, Corrado! Thanks for looking at the patch.

    > Hi Jeff,
    > I'm not sure that 3 broken userspace programs justify increasing the
    > complexity of a core kernel part as the I/O scheduler.

    I think it's wrong to call the userspace programs broken. They worked
    fine when CFQ was quantum based, and they work well with noop and
    deadline. Further, the patch I posted is fairly trivial, in my opinion.

    > The original close cooperator code is not limited to those programs.
    > It can actually result in a better overall scheduling on rotating
    > media, since it can help with transient close relationships (and
    > should probably be disabled on non-rotating ones).
    > Merging queues, instead, can lead to bad results in case of false
    > positives. I'm thinking for examples to two programs that are loading
    > shared libraries (that are close on disk, being in the same dir) on
    > startup, and end up being tied to the same queue.

    The idea is not to leave cfqq's merged indefinitely. I'm putting
    together a follow-on patch that will split the queues back up when they
    are no longer working on the same area of the disk.

    > Can't the userspace programs be fixed to use the same I/O context for
    > their threads?
    > qemu already has a bug report for it
    > (

    I submitted a patch to dump to address this. I think the SCSI target
    mode driver folks also patched their code. The qemu folks are working
    on a couple of different fixes to the problem. That leaves nfsd, which
    I could certainly try to whip into shape, but I wonder if there are

    >> The next step will be to break apart the cfqq's when the I/O patterns
    >> are no longer sequential.  This is not very important for dump(8), but
    >> for NFSd, this could make a big difference.  The problem with sharing
    >> the cfq_queue when the NFSd threads are no longer serving requests from
    >> a single client is that instead of having 8 scheduling entities, NFSd
    >> only gets one.  This could considerably hurt performance when serving
    >> shares to multiple clients, though I don't have a test to show this yet.
    > I think it will hurt performance only if it is competing with other
    > I/O. In that case, having 8 scheduling entities will get 8 times more
    > disk share (but this can be fixed by adjusting the nfsd I/O priority).

    It may be common that nfsd is the only thing accessing the device, good

    > For the I/O pattern, instead, sorting all requests in a single queue
    > may still be preferable, since they will be at least sorted in disk
    > order, instead of the random order given by which thread in the pool
    > received the request.
    > This is, though, an argument in favor of using CLONE_IO inside nfsd,
    > since having a single queue, with proper priority, will always give a
    > better overall performance.

    Well, I started to work on a patch to nfsd that would share and unshare
    I/O contexts based on the client with which the request was associated.
    So, much like there is the shared readahead state, there would now be a
    shared I/O scheduler state. However, believe it or not, it is much
    simpler to do in the I/O scheduler. But maybe that's because cfq is my
    hammer. ;-)

    Thanks again for your review Corrado. It is much appreciated.

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      Last update: 2009-10-22 02:11    [W:0.024 / U:14.300 seconds]
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