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    SubjectRe: [PATCH 2/2] cfq: allow dispatching of both sync and async I/O together
    Jens Axboe <> writes:

    > On 21/06/10 21.49, Jeff Moyer wrote:
    >> Hi,
    >> In testing a workload that has a single fsync-ing process and another
    >> process that does a sequential buffered read, I was unable to tune CFQ
    >> to reach the throughput of deadline. This patch, along with the previous
    >> one, brought CFQ in line with deadline when setting slice_idle to 0.
    >> I'm not sure what the original reason for not allowing sync and async
    >> I/O to be dispatched together was. If there is a workload I should be
    >> testing that shows the inherent problems of this, please point me at it
    >> and I will resume testing. Until and unless that workload is identified,
    >> please consider applying this patch.
    > The problematic case is/was a normal SATA drive with a buffered
    > writer and an occasional reader. I'll have to double check my
    > mail tomorrow, but iirc the issue was that the occasional reader
    > would suffer great latencies since service times for that single
    > IO would be delayed at the drive side. It could perhaps just be
    > a bug in how we handle the slice idling on the read side when the
    > IO gets delayed initially.
    > So if my memory is correct, google for the fsync madness and
    > interactiveness thread that we had some months ago and which
    > caused a lot of tweaking. The commit adding this is
    > 5ad531db6e0f3c3c985666e83d3c1c4d53acccf9 and was added back
    > in July last year. So it was around that time that the mails went
    > around.

    OK. Thanks a ton for the pointers! I really appreciate it!


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