netdev - Re: [PATCHv5] virtio-spec: virtio network device RFS support

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Date:	Thu, 6 Dec 2012 20:03:14 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Jason Wang <jasowang@...hat.com>, <rusty@...tcorp.com.au>,
	<virtualization@...ts.linux-foundation.org>,
	<netdev@...r.kernel.org>, <kvm@...r.kernel.org>
Subject: Re: [PATCHv5] virtio-spec: virtio network device RFS support

On Thu, 2012-12-06 at 10:13 +0200, Michael S. Tsirkin wrote:
> On Wed, Dec 05, 2012 at 08:39:26PM +0000, Ben Hutchings wrote:
> > On Mon, 2012-12-03 at 12:58 +0200, Michael S. Tsirkin wrote:
> > > Add RFS support to virtio network device.
> > > Add a new feature flag VIRTIO_NET_F_RFS for this feature, a new
> > > configuration field max_virtqueue_pairs to detect supported number of
> > > virtqueues as well as a new command VIRTIO_NET_CTRL_RFS to program
> > > packet steering for unidirectional protocols.
> > [...]
> > > +Programming of the receive flow classificator is implicit.
> > > + Transmitting a packet of a specific flow on transmitqX will cause incoming
> > > + packets for this flow to be steered to receiveqX.
> > > + For uni-directional protocols, or where no packets have been transmitted
> > > + yet, device will steer a packet to a random queue out of the specified
> > > + receiveq0..receiveqn.
> > [...]
> > 
> > It doesn't seem like this is usable to implement accelerated RFS in the
> > guest, though perhaps that doesn't matter.
> 
> What is the issue? Could you be more explicit please?
> 
> It seems to work pretty well: if we have
> # of queues >= # of cpus, incoming TCP_STREAM into
> guest scales very nicely without manual tweaks in guest.
> 
> The way it works is, when guest sends a packet driver
> select the rx queue that we want to use for incoming
> packets for this slow, and transmit on the matching tx queue.
> This is exactly what text above suggests no?

Yes, I get that.

> >  On the host side, presumably
> > you'll want vhost_net to do the equivalent of sock_rps_record_flow() -
> > only without a socket?  But in any case, that requires an rxhash, so I
> > don't see how this is supposed to work.
> > 
> > Ben.
> 
> Host should just do what guest tells it to.
> On the host side we build up the steering table as we get packets
> to transmit. See the code in drivers/net/tun.c in recent
> kernels.
> 
> Again this actually works fine - what are the problems that you see?
> Could you give an example please?

I'm not saying it doesn't work in its own way, I just don't see how you
would make it work with the existing RFS!

Since this doesn't seem to be intended to have *any* connection with the
existing core networking feature called RFS, perhaps you could find a
different name for it.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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