netdev - Re: Flow Control and Port Mirroring Revisited

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Date:	Thu, 13 Jan 2011 15:47:19 +0900
From:	Simon Horman <horms@...ge.net.au>
To:	Jesse Gross <jesse@...ira.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	virtualization@...ts.linux-foundation.org, dev@...nvswitch.org,
	virtualization@...ts.osdl.org, netdev@...r.kernel.org,
	kvm@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: Flow Control and Port Mirroring Revisited

On Mon, Jan 10, 2011 at 06:31:55PM +0900, Simon Horman wrote:
> On Fri, Jan 07, 2011 at 10:23:58AM +0900, Simon Horman wrote:
> > On Thu, Jan 06, 2011 at 05:38:01PM -0500, Jesse Gross wrote:
> > 
> > [ snip ]
> > > 
> > > I know that everyone likes a nice netperf result but I agree with
> > > Michael that this probably isn't the right question to be asking.  I
> > > don't think that socket buffers are a real solution to the flow
> > > control problem: they happen to provide that functionality but it's
> > > more of a side effect than anything.  It's just that the amount of
> > > memory consumed by packets in the queue(s) doesn't really have any
> > > implicit meaning for flow control (think multiple physical adapters,
> > > all with the same speed instead of a virtual device and a physical
> > > device with wildly different speeds).  The analog in the physical
> > > world that you're looking for would be Ethernet flow control.
> > > Obviously, if the question is limiting CPU or memory consumption then
> > > that's a different story.
> > 
> > Point taken. I will see if I can control CPU (and thus memory) consumption
> > using cgroups and/or tc.
> 
> I have found that I can successfully control the throughput using
> the following techniques
> 
> 1) Place a tc egress filter on dummy0
> 
> 2) Use ovs-ofctl to add a flow that sends skbs to dummy0 and then eth1,
>    this is effectively the same as one of my hacks to the datapath
>    that I mentioned in an earlier mail. The result is that eth1
>    "paces" the connection.

Further to this, I wonder if there is any interest in providing
a method to switch the action order - using ovs-ofctl is a hack imho -
and/or switching the default action order for mirroring.

> 3) 2) + place a tc egress filter on eth1
> 
> Which mostly makes sense to me although I am a little confused about
> why 1) needs a filter on dummy0 (a filter on eth1 has no effect)
> but 3) needs a filter on eth1 (a filter on dummy0 has no effect,
> even if the skb is sent to dummy0 last.
> 
> I also had some limited success using CPU cgroups, though obviously
> that targets CPU usage and thus the effect on throughput is fairly course.
> In short, its a useful technique but not one that bares further
> discussion here.
> 
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