Re: [PATCH net-next v5 0/9] xen-netback: TX grant mapping with SKBTX_DEV_ZEROCOPY instead of copy

[Ian Campbell]

On Mon, 2014-01-20 at 21:24 +0000, Zoltan Kiss wrote:
> A long known problem of the upstream netback implementation that on the TX
> path (from guest to Dom0) it copies the whole packet from guest memory into
> Dom0. That simply became a bottleneck with 10Gb NICs, and generally it's a
> huge perfomance penalty. The classic kernel version of netback used grant
> mapping, and to get notified when the page can be unmapped, it used page
> destructors. Unfortunately that destructor is not an upstreamable solution.
> Ian Campbell's skb fragment destructor patch series [1] tried to solve this
> problem, however it seems to be very invasive on the network stack's code,
> and therefore haven't progressed very well.
> This patch series use SKBTX_DEV_ZEROCOPY flags to tell the stack it needs to
> know when the skb is freed up. That is the way KVM solved the same problem,
> and based on my initial tests it can do the same for us. Avoiding the extra
> copy boosted up TX throughput from 6.8 Gbps to 7.9 (I used a slower
> Interlagos box, both Dom0 and guest on upstream kernel, on the same NUMA node,
> running iperf 2.0.5, and the remote end was a bare metal box on the same 10Gb
> switch)
> Based on my investigations the packet get only copied if it is delivered to
> Dom0 stack,

This is not quite complete/accurate since you previously told me that it
is copied in the NAT/routed rather than bridged network topologies.

Please can you cover that aspect here too.

Ian.

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