netdev - Re: [RFC PATCH v8 00/16] Provide a zero-copy method on KVM virtio-net.

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Message-ID: <4C525CD2.5080604@redhat.com>
Date:	Fri, 30 Jul 2010 08:02:10 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Shirley Ma <mashirle@...ibm.com>
CC:	xiaohui.xin@...el.com, netdev@...r.kernel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, mst@...hat.com, mingo@...e.hu,
	davem@...emloft.net, herbert@...dor.hengli.com.au,
	jdike@...ux.intel.com
Subject: Re: [RFC PATCH v8 00/16] Provide a zero-copy method on KVM virtio-net.

  On 07/30/2010 01:31 AM, Shirley Ma wrote:
>
>> Our goal is to improve the bandwidth and reduce the CPU usage.
>> Exact performance data will be provided later.
> I did some vhost performance measurement over 10Gb ixgbe, and found that
> in order to get consistent BW results, netperf/netserver, qemu, vhost
> threads smp affinities are required.
>
> Looking forward to these results for small message size comparison. For
> large message size 10Gb ixgbe BW already reached by doing vhost smp
> affinity w/i offloading support, we will see how much CPU utilization it
> can be reduced.
>
> Please provide latency results as well. I did some experimental on
> macvtap zero copy sendmsg, what I have found that get_user_pages latency
> pretty high.
>

get_user_pages() is indeed slow.  But what about get_user_pages_fast()?

Note that when the page is first touched, get_user_pages_fast() falls 
back to get_user_pages(), so the latency needs to be measured after 
quite a bit of warm-up.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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