Message-ID: <20110114063528.GB10957@verge.net.au>
Date:	Fri, 14 Jan 2011 15:35:28 +0900
From:	Simon Horman <horms@...ge.net.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Jesse Gross <jesse@...ira.com>,
	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
Subject: Re: Flow Control and Port Mirroring Revisited

On Fri, Jan 14, 2011 at 06:58:18AM +0200, Michael S. Tsirkin wrote:
> On Fri, Jan 14, 2011 at 08:41:36AM +0900, Simon Horman wrote:
> > On Thu, Jan 13, 2011 at 10:45:38AM -0500, Jesse Gross wrote:
> > > 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.
> 
> This is actually a bug. This means that one slow connection will affect
> fast ones. I intend to change the default for qemu to sndbuf=0 : this
> will fix it but break your "pacing". So pls do not count on this
> behaviour.

Do you have a patch I could test?

> > > > 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.
> > 
> > Yes, I have considered that case. And I agree that perhaps there
> > is no sensible default. But perhaps we could make it configurable somehow?
> 
> The fix is at the application level. Run netperf with -b and -w flags to
> limit the speed to a sensible value.

Perhaps I should have stated my goals more clearly.
I'm interested in situations where I don't control the application.

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