netdev - Re: [ovs-dev] Flow Control and Port Mirroring

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Date:	Mon, 8 Nov 2010 13:59:42 +0900
From:	Simon Horman <horms@...ge.net.au>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	virtualization@...ts.linux-foundation.org,
	Jesse Gross <jesse@...ira.com>, dev@...nvswitch.org,
	virtualization@...ts.osdl.org, netdev@...r.kernel.org,
	kvm@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [ovs-dev] Flow Control and Port Mirroring

On Mon, Nov 08, 2010 at 01:41:23PM +1030, Rusty Russell wrote:
> On Sat, 30 Oct 2010 01:29:33 pm Simon Horman wrote:
> > [ CCed VHOST contacts ]
> > 
> > On Thu, Oct 28, 2010 at 01:22:02PM -0700, Jesse Gross wrote:
> > > On Thu, Oct 28, 2010 at 4:54 AM, Simon Horman <horms@...ge.net.au> wrote:
> > > > My reasoning is that in the non-mirroring case the guest is
> > > > limited by the external interface through wich the packets
> > > > eventually flow - that is 1Gbit/s. But in the mirrored either
> > > > there is no flow control or the flow control is acting on the
> > > > rate of dummy0, which is essentailly infinate.
> > > >
> > > > Before investigating this any further I wanted to ask if
> > > > this behaviour is intentional.
> > > 
> > > It's not intentional but I can take a guess at what is happening.
> > > 
> > > When we send the packet to a mirror, the skb is cloned but only the
> > > original skb is charged to the sender.  If the original packet is
> > > delivered to localhost then it will be freed quickly and no longer
> > > accounted for, despite the fact that the "real" packet is still
> > > sitting in the transmit queue on the NIC.  The UDP stack will then
> > > send the next packet, limited only by the speed of the CPU.
> > 
> > That would explain what I have observed.
> 
> I can't find the thread (what is ovs-dev?),

Sorry, yes its on ovs-dev.
http://openvswitch.org/pipermail/dev_openvswitch.org/2010-October/003806.html

> but I think the tap device
> has this fundamental feature: you can blast as many packets as you want
> through it.
> 
> If that's a bad thing, we have to look harder...

There does seem to be flow control in the non-mirrored case.
So I suspect its occurring at the skb level but that breaks down when
a clone occurs. It would seem that fragment level flow control would
help this problem (which is basically what Xen's netback/netfront has),
but by this point I am speculating wildly.  I'll try and find out exactly
where the problem is occurring in order for us to have a more informed
discussion.
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