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SubjectRe: [PATCH net-next v5 0/9] xen-netback: TX grant mapping with SKBTX_DEV_ZEROCOPY instead of copy
On 19/02/14 09:50, Ian Campbell wrote:
> 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.


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