On Tue, 2008-11-18 at 14:57 +0000, Daniel P. Berrange wrote: > On Mon, Nov 17, 2008 at 06:39:45PM -0800, Chris Wright wrote: > > Below is a simple PoC for setting a large MTU size on a tap device. > > With this we are able to improve net i/o throughput substantially (~40% > > improvement on TX and ~130% improvement on RX). This is just RFC because > > it's hardcoded to an MTU of 9000 for any tap device. Thoughts on the > > best way to add this kind of support? > > Well if we want it to be configurable per guest then we'd add it > to the XML, in the <target> device field, alongside the VIF > name, eg > > <interface type='network'> > <source network='default'/> > <target dev='vnet7' mtu='9000'/> > <mac address="11:22:33:44:55:66"/> > </interface> Yeah, I was just about to suggest that. > libvirt can handle this stuff directly for QEMU and LXC drivers > but for Xen, VIF setup is done by an /etc/xen/scripts/vif-bridge > script. AFAIK, XenD has no config param for MTU, but there's > no reason we can't write a patch for XenD to accept an MTU param > and pass that through to vif-bridge. If large MTUs are useful > for Xen's netback of course... > > If we don't need this configurable, is there any downside to setting > a MTU of 9000 for all TAP devices we create. I assume that PTMUD > will ensure that the guest only sends packets <= 1500 if the physical > NIC connected to the bridge doesn't have such a large MTU, or if the > guest doesn't do PMTUD, then the bridge code will do fragementation > as needed ? My take on it is that it needs to be an opt in thing - if path MTU discovery was sufficient to deal with all cases then we'd probably have an mtu > 1500 everywhere ... Herbert? Cheers, Mark. -- Libvir-list mailing list Libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list