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Message-ID: <20110114045818.GA29738@redhat.com> Date: Fri, 14 Jan 2011 06:58:18 +0200 From: "Michael S. Tsirkin" <mst@...hat.com> To: Simon Horman <horms@...ge.net.au> 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 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. > > > 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. -- MST -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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