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Re: [Xen-devel] [PATCH net-next v7 0/9] xen-netback: TX grant mapping with SKBTX_DEV_ZEROCOPY instead of copy

On 13/03/14 10:08, Ian Campbell wrote:
On Thu, 2014-03-06 at 21:48 +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 AMD
>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
Do you have any other numbers? e.g. for a modern Intel or AMD system? A
slower box is likely to make the difference between copy and map larger,
whereas modern Intel for example is supposed to be very good at copying.
Performance team made a lot of measurements, I've added Marcus to comment on that. With the latest version and tip net-next kernel I could see even ~9.3 Gbps peak throughput on the same AMD box, which is the practical maximum for 10G cards. However with older guests I couldn't reach that. A lot depends on netfront and TCP stack, e.g. the tcp_limit_output_bytes sysctl can cause an artificial cap. Perf team now has 40 Gbps NICs I guess, it would be interesting to see how does this perform there. I just checked the intrahost guest-to-guest throughput with 2 upstream kernel, I could get out 5.6-5.8 Gbps at most.

>Based on my investigations the packet get only copied if it is delivered to
>Dom0 IP stack through deliver_skb, which is due to this [2] patch. This affects
>DomU->Dom0 IP traffic and when Dom0 does routing/NAT for the guest. That's a 
>unfortunate, but luckily it doesn't cause a major regression for this usecase.
I've checked that back in November:


Originally it was 5.4 vs with my patch it was 5.2. I've checked DomU to Dom0 iperf again, about the same still with my series.


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