Re: vhost + multiqueue + RSS question.

[Gleb Natapov <gleb@cloudius-systems.com>]

On Tue, Nov 18, 2014 at 11:41:11AM +0800, Jason Wang wrote:
> On 11/18/2014 09:37 AM, Zhang Haoyu wrote:
> >> On Mon, Nov 17, 2014 at 01:58:20PM +0200, 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.
> >>>
> >> OK, if what virtio has is RFS and not RSS my test results make sense.
> >> Thanks!
> > I think the RSS emulation for virtio-mq NIC is implemented in tun_select_queue(),
> > am I missing something?
> >
> > Thanks,
> > Zhang Haoyu
> >
> 
> Yes, if RSS is the short for Receive Side Steering which is a generic
> technology. But RSS is usually short for Receive Side Scaling which was
> commonly technology used by Windows, it was implemented through a
> indirection table in the card which is obviously not supported in tun
> currently.
Hmm, I had an impression that "Receive Side Steering" and "Receive Side
Scaling" are interchangeable. Software implementation for RSS is called
"Receive Packet Steering" according to Documentation/networking/scaling.txt
not "Receive Packet Scaling". Those damn TLAs are confusing.
 
--
			Gleb.