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    SubjectRe: [PATCHv2-RFC 0/2] virtio: put last seen used index into ring itself
    On Mon, May 31, 2010 at 09:36:00AM +0200, Jes Sorensen wrote:
    > On 05/30/10 13:22, Michael S. Tsirkin wrote:
    > > On Fri, May 28, 2010 at 11:56:54AM +0200, Jes Sorensen wrote:
    > >> It looks pretty good to me, however one thing I have been thinking of
    > >> while reading through it:
    > >>
    > >> Rather than storing a pointer within the ring struct, pointing into a
    > >> position within the same struct. How about storing a byte offset instead
    > >> and using a cast to get to the pointer position? That would avoid the
    > >> pointer dereference, which is less effective cache wise and harder for
    > >> the CPU to predict.
    > >>
    > >> Not sure whether it really matters performance wise, just a thought.
    > >
    > > I think this won't work: when PUBLUSH_USED_IDX is negotiated,
    > > the pointer is to within the ring.
    >
    > Hmmm shame, it would be a nice optimization.
    >
    > Maybe it's time to introduce the v2 ring format, rather than having
    > adding more kludges to the existing one?
    >
    > Cheers,
    > Jes

    There has been discussion about a ring format that does not
    use indexes at all. My guess is that would be a good point
    for v2 ring format. But making that a product
    and tuning might take a while. So definitely something to
    keep in mind but I would not want that to block this optimization.

    --
    MST


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