On 11/17/2014 07:58 PM, Michael S. Tsirkin wrote: > On Mon, Nov 17, 2014 at 01:22:07PM +0200, Gleb Natapov wrote: >> > On Mon, Nov 17, 2014 at 12:38:16PM +0200, Michael S. Tsirkin wrote: >>> > > On Mon, Nov 17, 2014 at 09:44:23AM +0200, Gleb Natapov wrote: >>>> > > > On Sun, Nov 16, 2014 at 08:56:04PM +0200, Michael S. Tsirkin wrote: >>>>> > > > > On Sun, Nov 16, 2014 at 06:18:18PM +0200, Gleb Natapov wrote: >>>>>> > > > > > Hi Michael, >>>>>> > > > > > >>>>>> > > > > > I am playing with vhost multiqueue capability and have a question about >>>>>> > > > > > vhost multiqueue and RSS (receive side steering). My setup has Mellanox >>>>>> > > > > > ConnectX-3 NIC which supports multiqueue and RSS. Network related >>>>>> > > > > > parameters for qemu are: >>>>>> > > > > > >>>>>> > > > > > -netdev tap,id=hn0,script=qemu-ifup.sh,vhost=on,queues=4 >>>>>> > > > > > -device virtio-net-pci,netdev=hn0,id=nic1,mq=on,vectors=10 >>>>>> > > > > > >>>>>> > > > > > In a guest I ran "ethtool -L eth0 combined 4" to enable multiqueue. >>>>>> > > > > > >>>>>> > > > > > I am running one tcp stream into the guest using iperf. Since there is >>>>>> > > > > > only one tcp stream I expect it to be handled by one queue only but >>>>>> > > > > > this seams to be not the case. ethtool -S on a host shows that the >>>>>> > > > > > stream is handled by one queue in the NIC, just like I would expect, >>>>>> > > > > > but in a guest all 4 virtio-input interrupt are incremented. Am I >>>>>> > > > > > missing any configuration? >>>>> > > > > >>>>> > > > > I don't see anything obviously wrong with what you describe. >>>>> > > > > Maybe, somehow, same irqfd got bound to multiple MSI vectors? >>>> > > > It does not look like this is what is happening judging by the way >>>> > > > interrupts are distributed between queues. They are not distributed >>>> > > > uniformly and often I see one queue gets most interrupt and others get >>>> > > > much less and then it changes. >>> > > >>> > > Weird. It would happen if you transmitted from multiple CPUs. >>> > > You did pin iperf to a single CPU within guest, did you not? >>> > > >> > No, I didn't because I didn't expect it to matter for input interrupts. >> > When I run iperf on a host rx queue that receives all packets depends >> > only on a connection itself, not on a cpu iperf is running on (I tested >> > that). > This really depends on the type of networking card you have > on the host, and how it's configured. > > I think you will get something more closely resembling this > behaviour if you enable RFS in host. > >> > When I pin iperf in a guest I do indeed see that all interrupts >> > are arriving to the same irq vector. Is a number after virtio-input >> > in /proc/interrupt any indication of a queue a packet arrived to (on >> > a host I can use ethtool -S to check what queue receives packets, but >> > unfortunately this does not work for virtio nic in a guest)? > I think it is. > >> > Because if >> > it is the way RSS works in virtio is not how it works on a host and not >> > what I would expect after reading about RSS. The queue a packets arrives >> > to should be calculated by hashing fields from a packet header only. > Yes, what virtio has is not RSS - it's an accelerated RFS really. Strictly speaking, not aRFS. aRFS requires a programmable filter and needs driver to fill the filter on demand. For virtio-net, this is done automatically in host side (tun/tap). There's no guest involvement. > > The point is to try and take application locality into account. > Yes, the locality was done through (consider a N vcpu guest with N queue): - virtio-net driver will provide a default 1:1 mapping between vcpu and txq through XPS - virtio-net driver will suggest a default irq affinity hint also for a 1:1 mapping bettwen vcpu and txq/rxq With all these, each vcpu get its private txq/rxq paris. And host side implementation (tun/tap) will make sure if the packets of a flow were received from queue N, if will also use queue N to transmit the packets of this flow to guest. -- 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
