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    SubjectRE: [PATCH v4] io_uring: reduce latency by reissueing the operation
    Date
    From: Jens Axboe
    > Sent: 25 June 2021 01:45
    >
    > On 6/22/21 6:17 AM, Olivier Langlois wrote:
    > > It is quite frequent that when an operation fails and returns EAGAIN,
    > > the data becomes available between that failure and the call to
    > > vfs_poll() done by io_arm_poll_handler().
    > >
    > > Detecting the situation and reissuing the operation is much faster
    > > than going ahead and push the operation to the io-wq.
    > >
    > > Performance improvement testing has been performed with:
    > > Single thread, 1 TCP connection receiving a 5 Mbps stream, no sqpoll.
    > >
    > > 4 measurements have been taken:
    > > 1. The time it takes to process a read request when data is already available
    > > 2. The time it takes to process by calling twice io_issue_sqe() after vfs_poll() indicated that data
    > was available
    > > 3. The time it takes to execute io_queue_async_work()
    > > 4. The time it takes to complete a read request asynchronously
    > >
    > > 2.25% of all the read operations did use the new path.

    How much slower is it when the data to complete the read isn't
    available?

    I suspect there are different workflows where that is almost
    always true.

    David

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