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    SubjectRe: [PATCH] virtio-ring: Use threshold for switching to indirect descriptors
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    On Tue, 2011-11-29 at 16:58 +0200, Avi Kivity wrote:
    > On 11/29/2011 04:54 PM, Michael S. Tsirkin wrote:
    > > >
    > > > Which is actually strange, weren't indirect buffers introduced to make
    > > > the performance *better*? From what I see it's pretty much the
    > > > same/worse for virtio-blk.
    > >
    > > I know they were introduced to allow adding very large bufs.
    > > See 9fa29b9df32ba4db055f3977933cd0c1b8fe67cd
    > > Mark, you wrote the patch, could you tell us which workloads
    > > benefit the most from indirect bufs?
    > >
    >
    > Indirects are really for block devices with many spindles, since there
    > the limiting factor is the number of requests in flight. Network
    > interfaces are limited by bandwidth, it's better to increase the ring
    > size and use direct buffers there (so the ring size more or less
    > corresponds to the buffer size).
    >

    I did some testing of indirect descriptors under different workloads.

    All tests were on a 2 vcpu guest with vhost on. Simple TCP_STREAM using
    netperf.

    Indirect desc off:
    guest -> host, 1 stream: ~4600mb/s
    host -> guest, 1 stream: ~5900mb/s
    guest -> host, 8 streams: ~620mb/s (on average)
    host -> guest, 8 stream: ~600mb/s (on average)

    Indirect desc on:
    guest -> host, 1 stream: ~4900mb/s
    host -> guest, 1 stream: ~5400mb/s
    guest -> host, 8 streams: ~620mb/s (on average)
    host -> guest, 8 stream: ~600mb/s (on average)

    Which means that for one stream, guest to host gets faster while host to
    guest gets slower when indirect descriptors are on.

    --

    Sasha.



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