linux-kernel - Re: [Xen-devel] [PATCH net-next RFC 0/5] xen-netback: TX grant mapping instead of copy

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Date:	Wed, 30 Oct 2013 15:16:17 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Zoltan Kiss <zoltan.kiss@...rix.com>
Cc:	ian.campbell@...rix.com, wei.liu2@...rix.com,
	xen-devel@...ts.xenproject.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, jonathan.davies@...rix.com
Subject: Re: [Xen-devel] [PATCH net-next RFC 0/5] xen-netback: TX grant
 mapping instead of copy

On Wed, Oct 30, 2013 at 12:50:15AM +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
> (http://lwn.net/Articles/491522/) 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, which is due to this patch:
> https://lkml.org/lkml/2012/7/20/363
> That's a bit unfortunate, but as far as I know for the huge majority this use
> case is not too important. There are a couple of things which need more
> polishing, see the FIXME comments. I will run some more extensive tests, but
> in the meantime I would like to hear comments about what I've done so far.
> I've tried to broke it down to smaller patches, with mixed results, so I
> welcome suggestions on that part as well:
> 1/5: Introduce TX grant map definitions
> 2/5: Change TX path from grant copy to mapping
> 3/5: Remove old TX grant copy definitons
> 4/5: Fix indentations
> 5/5: Change RX path for mapped SKB fragments

Odd. I don't see #5 patch patch?
> 
> Signed-off-by: Zoltan Kiss <zoltan.kiss@...rix.com>
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@...ts.xen.org
> http://lists.xen.org/xen-devel
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