netdev - Re: [RFC] kvm tools: Implement multiple VQ for virtio-net

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Date:	Wed, 16 Nov 2011 14:39:28 +0530
From:	Krishna Kumar2 <krkumar2@...ibm.com>
To:	jason wang <jasowang@...hat.com>
Cc:	Asias He <asias.hejun@...il.com>, gorcunov@...il.com,
	kvm@...r.kernel.org, Sasha Levin <levinsasha928@...il.com>,
	mingo@...e.hu, "Michael S. Tsirkin" <mst@...hat.com>,
	netdev@...r.kernel.org, penberg@...nel.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	virtualization@...ts.linux-foundation.org
Subject: Re: [RFC] kvm tools: Implement multiple VQ for virtio-net

jason wang <jasowang@...hat.com> wrote on 11/16/2011 11:40:45 AM:

Hi Jason,

> Have any thought in mind to solve the issue of flow handling?

So far nothing concrete.

> Maybe some performance numbers first is better, it would let us know
> where we are. During the test of my patchset, I find big regression of
> small packet transmission, and more retransmissions were noticed. This
> maybe also the issue of flow affinity. One interesting things is to see
> whether this happens in your patches :)

I haven't got any results for small packet, but will run this week
and send an update. I remember my earlier patches having regression
for small packets.

> I've played with a basic flow director implementation based on my series
> which want to make sure the packets of a flow was handled by the same
> vhost thread/guest vcpu. This is done by:
>
> - bind virtqueue to guest cpu
> - record the hash to queue mapping when guest sending packets and use
> this mapping to choose the virtqueue when forwarding packets to guest
>
> Test shows some help during for receiving packets from external host and
> packet sending to local host. But it would hurt the performance of
> sending packets to remote host. This is not the perfect solution as it
> can not handle guest moving processes among vcpus, I plan to try
> accelerate RFS and sharing the mapping between host and guest.
>
> Anyway this is just for receiving, the small packet sending need more
> thoughts.

I don't recollect small packet performance for guest->local host.
Also, using multiple tuns devices on the bridge (instead of mq-tun)
balances the rx/tx of a flow to a single vq. Then you can avoid
mq-tun with it's queue selector function, etc. Have you tried it?

I will run my tests this week and get back.

thanks,

- KK

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