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    SubjectRe: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself
    On 05/19/2010 10:39 AM, Rusty Russell wrote:
    >
    > I think we're talking about the last 2 entries of the avail ring. That means
    > the worst case is 1 false bounce every time around the ring.

    It's low, but why introduce an inefficiency when you can avoid doing it
    for the same effort?

    > I think that's
    > why we're debating it instead of measuring it :)
    >

    Measure before optimize is good for code but not for protocols.
    Protocols have to be robust against future changes. Virtio is warty
    enough already, we can't keep doing local optimizations.

    > Note that this is a exclusive->shared->exclusive bounce only, too.
    >

    A bounce is a bounce.

    Virtio is already way too bouncy due to the indirection between the
    avail/used rings and the descriptor pool. A device with out of order
    completion (like virtio-blk) will quickly randomize the unused
    descriptor indexes, so every descriptor fetch will require a bounce.

    In contrast, if the rings hold the descriptors themselves instead of
    pointers, we bounce (sizeof(descriptor)/cache_line_size) cache lines for
    every descriptor, amortized.

    --
    Do not meddle in the internals of kernels, for they are subtle and quick to panic.



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