netdev - Re: Flow Control and Port Mirroring Revisited

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Message-ID: <20110106113052.GA2541@verge.net.au>
Date:	Thu, 6 Jan 2011 20:30:52 +0900
From:	Simon Horman <horms@...ge.net.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	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
Subject: Re: Flow Control and Port Mirroring Revisited

On Thu, Jan 06, 2011 at 12:27:55PM +0200, Michael S. Tsirkin wrote:
> On Thu, Jan 06, 2011 at 06:33:12PM +0900, Simon Horman wrote:
> > Hi,
> > 
> > Back in October I reported that I noticed a problem whereby flow control
> > breaks down when openvswitch is configured to mirror a port[1].
> 
> Apropos the UDP flow control.  See this
> http://www.spinics.net/lists/netdev/msg150806.html
> for some problems it introduces.
> Unfortunately UDP does not have built-in flow control.
> At some level it's just conceptually broken:
> it's not present in physical networks so why should
> we try and emulate it in a virtual network?
> 
> 
> Specifically, when you do:
> # netperf -c -4 -t UDP_STREAM -H 172.17.60.218 -l 30 -- -m 1472
> You are asking: what happens if I push data faster than it can be received?
> But why is this an interesting question?
> Ask 'what is the maximum rate at which I can send data with %X packet
> loss' or 'what is the packet loss at rate Y Gb/s'. netperf has
> -b and -w flags for this. It needs to be configured
> with --enable-intervals=yes for them to work.
> 
> If you pose the questions this way the problem of pacing
> the execution just goes away.

I am aware that UDP inherently lacks flow control.

The aspect of flow control that I am interested in is situations where the
guest can create large amounts of work for the host. However, it seems that
in the case of virtio with vhostnet that the CPU utilisation seems to be
almost entirely attributable to the vhost and qemu-system processes.  And
in the case of virtio without vhost net the CPU is used by the qemu-system
process. In both case I assume that I could use a cgroup or something
similar to limit the guests.

Assuming all of that is true then from a resource control problem point of
view, which is mostly what I am concerned about, the problem goes away.
However, I still think that it would be nice to resolve the situation I
described.
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