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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

From: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
Date: Thu, 6 Mar 2014 21:48:22 +0000

> 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)
> 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.
> 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've tried to broke it down to smaller patches, with mixed results, so I
> welcome suggestions on that part as well:
> 1: Use skb->cb to store pending_idx
> 2: Some refactoring
> 3: Change RX path for mapped SKB fragments (moved here to keep bisectability,
> review it after #4)
> 4: Introduce TX grant mapping
> 5: Remove old TX grant copy definitons and fix indentations
> 6: Add stat counters for zerocopy
> 7: Handle guests with too many frags
> 8: Timeout packets in RX path
> 9: Aggregate TX unmap operations

Series applied, thanks.

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