netdev - Re: Flow Control and Port Mirroring Revisited

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Date:	Thu, 13 Jan 2011 10:45:38 -0500
From:	Jesse Gross <jesse@...ira.com>
To:	Simon Horman <horms@...ge.net.au>
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 Thu, Jan 13, 2011 at 1:47 AM, Simon Horman <horms@...ge.net.au> wrote:
> 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.

I'm not sure that there is a way to do this that is correct in the
generic case.  It's possible that the destination could be a VM while
packets are being mirrored to a physical device or we could be
multicasting or some other arbitrarily complex scenario.  Just think
of what a physical switch would do if it has ports with two different
speeds.
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