[ovs-dev] Flow Control and Port Mirroring
horms at verge.net.au
Mon Nov 8 04:59:42 UTC 2010
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 at verge.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.
> 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
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