netdev - Re: [PATCHv4] virtio-spec: virtio network device multiqueue support

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Message-ID: <20120919061249.GA24564@redhat.com>
Date:	Wed, 19 Sep 2012 09:12:49 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Tom Herbert <therbert@...gle.com>,
	Jason Wang <jasowang@...hat.com>, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	pbonzini@...hat.com, levinsasha928@...il.com, rick.jones2@...com
Subject: Re: [PATCHv4] virtio-spec: virtio network device multiqueue support

On Wed, Sep 19, 2012 at 11:10:10AM +0930, Rusty Russell wrote:
> Tom Herbert <therbert@...gle.com> writes:
> > On Tue, Sep 11, 2012 at 10:49 PM, Rusty Russell <rusty@...tcorp.com.au>wrote:
> >> Perhaps Tom can explain how we avoid out-of-order receive for the
> >> accelerated RFS case?  It's not clear to me, but we need to be able to
> >> do that for virtio-net if it implements accelerated RFS.
> >
> > AFAIK ooo RX is possible with accelerated RFS.  We have an algorithm that
> > prevents this for RFS case by deferring a migration to a new queue as long
> > as it's possible that a flow might have outstanding packets on the old
> > queue.  I suppose this could be implemented in the device for the HW
> > queues, but I don't think it would be easy to cover all cases where packets
> > were already in transit to the host or other cases where host and device
> > queues are out of sync.
> 
> Having gone to such great lengths to avoid ooo for RFS, I don't think
> DaveM would be happy if we allow it for virtio_net.
> 
> So, how *would* we implement such a thing for a "hardware" device?  What
> if the device will only change the receive queue if the old receive
> queue is empty?
> 
> Cheers,
> Rusty.
> 

I think that would do it in most cases.  Or if we want to be more
exact we could delay switching a specific flow until no
outstanding rx packets for this flow. Not sure it's worth the
hassle.

-- 
MST
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