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    SubjectRe: [RFC PATCH 1/1] vhost: TX used buffer guest signal accumulation
    On Thu, Oct 28, 2010 at 10:14:22AM -0700, Shirley Ma wrote:
    >
    > > Two ideas:
    > > 1. How about writing out used, just delaying the signal?
    > > This way we don't have to queue separately.
    >
    > This improves some performance, but not as good as delaying
    > both used and signal. Since delaying used buffers combining
    > multiple small copies to a large copy, which saves more CPU
    > utilization and increased some BW.

    Hmm. I don't yet understand. We are still doing copies into the per-vq
    buffer, and the data copied is really small. Is it about cache line
    bounces? Could you try figuring it out?

    > > 2. How about flushing out queued stuff before we exit
    > > the handle_tx loop? That would address most of
    > > the spec issue.
    >
    > The performance is almost as same as the previous patch. I will resubmit
    > the modified one, adding vhost_add_used_and_signal_n after handle_tx
    > loop for processing pending queue.
    >
    > This patch was a part of modified macvtap zero copy which I haven't
    > submitted yet. I found this helped vhost TX in general. This pending
    > queue will be used by DMA done later, so I put it in vq instead of a
    > local variable in handle_tx.
    >
    > Thanks
    > Shirley

    BTW why do we need another array? Isn't heads field exactly what we need
    here?



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