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    SubjectRe: cfq-iosched: tiobench regression
    Hi Shaohua,
    On Thu, Dec 24, 2009 at 1:55 AM, Shaohua Li <> wrote:
    > df5fe3e8e13883f58dc97489076bbcc150789a21
    > b3b6d0408c953524f979468562e7e210d8634150
    > The coop merge is too aggressive. For example, if two tasks are reading two
    > files where the two files have some adjecent blocks, cfq will immediately
    > merge them. cfq_rq_close() also has trouble, sometimes the seek_mean is very
    > big. I did a test to make cfq_rq_close() always checks the distence according
    > to CIC_SEEK_THR, but still saw a lot of wrong merge. (BTW, why we take a long
    > distence far away request as close. Taking them close doesn't improve any thoughtput
    > to me. Maybe we should always use CIC_SEEK_THR as close criteria).
    Yes, when deciding if two queues are going to be merged, we should use
    the constant CIC_SEEK_THR.
    > So sounds we need make split more aggressive. But the split is too lazay,
    > which requires to wait 1s. Time based check isn't reliable as queue might not
    > run at given time, so uses a small time isn't ok.
    1s is too much, but I wouldn't abandon a time based approach. To fix
    the problem of queue not being run, you can consider a slice. If at
    the end of the slice, the queue is seeky, you split it.

    > I'm thinking changing the split
    > check based on requests number instead of time. That is if several continuous
    > requests are regarded as seeky, the coop queue is split. See blow RFC patch.
    > How many count a queue should be split after need more consideration,
    > below patch just uses an arbitary number.  This reduce about 5% performance
    > lost when doing tio 32 threads sequential read.


    dott. Corrado Zoccolo
    PhD - Department of Computer Science - University of Pisa, Italy
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