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SubjectRe: [Xen-devel] [PATCH net-next v2 0/9] xen-netback: TX grant mapping with SKBTX_DEV_ZEROCOPY instead of copy

On 2013/12/13 7:48, 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)
Sounds good.
Is the TX throughput gotten between one vm and one bare metal box? or
between multiple vms and bare metal? Do you have any test results with

> Based on my investigations the packet get only copied if it is delivered to
> Dom0 stack, which is due to this [2] patch. That's a bit unfortunate, but
> luckily it doesn't cause a major regression for this usecase. In the future
> we should try to eliminate that copy somehow.
> There are a few spinoff tasks which will be addressed in separate patches:
> - grant copy the header directly instead of map and memcpy. This should help
> us avoiding TLB flushing
> - use something else than ballooned pages
> - fix grant map to use page->index properly
> I will run some more extensive tests, but some basic XenRT tests were already
> passed with good results.
> I've tried to broke it down to smaller patches, with mixed results, so I
> welcome suggestions on that part as well:
> 1: Introduce TX grant map definitions
> 2: Change TX path from grant copy to mapping
> 3: Remove old TX grant copy definitons and fix indentations
> 4: Change RX path for mapped SKB fragments
> 5: Add stat counters for zerocopy
> 6: Handle guests with too many frags
> 7: Add stat counters for frag_list skbs
> 8: Timeout packets in RX path
> 9: Aggregate TX unmap operations
> v2: I've fixed some smaller things, see the individual patches. I've added a
> few new stat counters, and handling the important use case when an older guest
> sends lots of slots. Instead of delayed copy now we timeout packets on the RX
> path, based on the assumption that otherwise packets should get stucked
> anywhere else. Finally some unmap batching to avoid too much TLB flush
> [1]
> [2]
> Signed-off-by: Zoltan Kiss <>
> _______________________________________________
> Xen-devel mailing list

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