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    SubjectRe: [RFC PATCH]vhost-blk: In-kernel accelerator for virtio block device
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    On Thu, Jul 28, 2011 at 4:44 PM, Stefan Hajnoczi <stefanha@gmail.com> wrote:
    > On Thu, Jul 28, 2011 at 3:29 PM, Liu Yuan <namei.unix@gmail.com> wrote:
    >
    > Did you investigate userspace virtio-blk performance?  If so, what
    > issues did you find?
    >
    > I have a hacked up world here that basically implements vhost-blk in userspace:
    > http://repo.or.cz/w/qemu/stefanha.git/blob/refs/heads/virtio-blk-data-plane:/hw/virtio-blk.c
    >
    >  * A dedicated virtqueue thread sleeps on ioeventfd
    >  * Guest memory is pre-mapped and accessed directly (not using QEMU's
    > usually memory access functions)
    >  * Linux AIO is used, the QEMU block layer is bypassed
    >  * Completion interrupts are injected from the virtqueue thread using ioctl
    >
    > I will try to rebase onto qemu-kvm.git/master (this work is several
    > months old).  Then we can compare to see how much of the benefit can
    > be gotten in userspace.

    Here is the rebased virtio-blk-data-plane tree:
    http://repo.or.cz/w/qemu-kvm/stefanha.git/shortlog/refs/heads/virtio-blk-data-plane

    When I run it on my laptop with an Intel X-25M G2 SSD I see a latency
    reduction compared to mainline userspace virtio-blk. I'm not posting
    results because I did quick fio runs without ensuring a quiet
    benchmarking environment.

    There are a couple of things that could be modified:
    * I/O request merging is done to mimic bdrv_aio_multiwrite() - but
    vhost-blk does not do this. Try turning it off?
    * epoll(2) is used but perhaps select(2)/poll(2) have lower latency
    for this use case. Try another event mechanism.

    Let's see how it compares to vhost-blk first. I can tweak it if we
    want to investigate further.

    Yuan: Do you want to try the virtio-blk-data-plane tree? You don't
    need to change the qemu-kvm command-line options.

    Stefan
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