Re: [PATCH 2/4] Revert virtio tap hack

Anthony Liguori
2008-05-04T04:03:47Z

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Anthony Liguori wrote:
> While it has served us well, it is long overdue that we eliminate the
> virtio-net tap hack. It turns out that zero-copy has very little impact on
> performance. The tap hack was gaining such a significant performance boost
> not because of zero-copy, but because it avoided dropping packets on receive
> which is apparently a significant problem with the tap implementation in QEMU.
>

FWIW, attached is a pretty straight forward zero-copy patch. What's
interesting is that I see no change in throughput using this patch. The
CPU is pegged at 100% during the iperf run. Since we're still using
small MTUs, this isn't surprising. Copying a 1500 byte packet that we
have to bring into the cache anyway doesn't seem significant. I think
zero-copy will be more important with GSO though.

Regards,

Anthony Liguori

> Patches 3 and 4 in this series address the packet dropping issue and the net
> result is a 25% boost in RX performance even in the absence of zero-copy.
>
> Also worth mentioning, is that this makes merging virtio into upstream QEMU
> significantly easier.
>
> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
>

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