On Thu, Mar 24, 2011 at 10:46:49AM +1030, Rusty Russell wrote: > On Tue, 22 Mar 2011 13:36:50 +0200, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > > On Mon, Mar 21, 2011 at 11:03:07AM -0700, Shirley Ma wrote: > > > On Fri, 2011-03-18 at 18:41 -0700, Shirley Ma wrote: > > > > > > + /* Drop packet instead of stop queue for better > > > > performance > > > > > */ > > > > > > > > > > I would like to see some justification as to why this is the right > > > > > way to go and not just papering over the real problem. > > > > > > > > Fair. KVM guest virtio_net TX queue stop/restart is pretty expensive, > > > > which involves: > > > > > > > > 1. Guest enable callback: one memory barrier, interrupt flag set > > > > > > Missed this cost: for history reason, it also involves a guest exit from > > > I/O write (PCI_QUEUE_NOTIFY). > > > > OK, after some research, it looks like the reason was the tx timer that > > qemu used to use. So the hack of avoiding the add_buf call will > > avoid this kick and so break these hosts. > > I guess we can add a feature bit to detect a new host > > and so avoid the kick. We are running low on feature bits > > unfortunately, but just fo testing, could you quantify the difference > > that this makes using the following patch: > > Performance would suffer for those ancient qemus if we didn't do this, > but it wouldn't be fatal to them. > > I think we should just remove it; the standard certainly doesn't mention > it. > > Cheers, > Rusty. I agree here. Anthony, agree? -- MST -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
