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

[Anthony L. <an...@co...>]

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 <ali...@us...>
>