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    SubjectRe: [Qemu-devel] Re: [PATCHv2-RFC 0/2] virtio: put last seen used index into ring itself
    On 05/31/10 09:46, Rusty Russell wrote:
    > On Thu, 27 May 2010 05:20:35 am Michael S. Tsirkin wrote:
    >> Here's a rewrite of the original patch with a new layout.
    >> I haven't tested it yet so no idea how this performs, but
    >> I think this addresses the cache bounce issue raised by Avi.
    >> Posting for early flames/comments.
    >
    > Sorry, not without some evidence that it'll actually reduce cacheline
    > bouncing. I *think* it will, but it's not obvious: the host may keep
    > looking at avail_idx as we're updating last_seen. Or does qemu always
    > look at both together anyway?
    >
    > Can someone convince me this is a win?
    > Rusty.

    Hi Rusty,

    I ran some tests using the vring index publish patch with virtio_blk.
    The numbers are based on running IOZone on a ramdisk passed to the guest
    via virtio. While I didn't see any throughput improvements, I saw a
    20-30% reduction in the VMExit count for the full run. This was measured
    grabbing the VMExit count prior and after the run and calculating the
    difference.

    I have the numbers in a PDF, so I will email it to you in private as I
    don't like sending PDFs to the mailing list. However if anybody else
    wants the numbers feel free to ping me off list and I'll forward them.

    Cheers,
    Jes



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