Messages in this thread | | | Date | Thu, 13 Mar 2014 18:23:04 +0000 | From | Zoltan Kiss <> | Subject | Re: [PATCH net-next v7 0/9] xen-netback: TX grant mapping with SKBTX_DEV_ZEROCOPY instead of copy |
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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 >> >switch) > 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 bit >> >unfortunate, but luckily it doesn't cause a major regression for this usecase. > Numbers? I've checked that back in November:
https://lkml.org/lkml/2013/11/5/288
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.
Zoli
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