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SubjectRe: [net-next RFC V5 3/5] virtio: intorduce an API to set affinity for a virtqueue
On Fri, Jul 27, 2012 at 04:38:11PM +0200, Paolo Bonzini wrote:
> Il 05/07/2012 12:29, Jason Wang ha scritto:
> > Sometimes, virtio device need to configure irq affiniry hint to maximize the
> > performance. Instead of just exposing the irq of a virtqueue, this patch
> > introduce an API to set the affinity for a virtqueue.
> >
> > The api is best-effort, the affinity hint may not be set as expected due to
> > platform support, irq sharing or irq type. Currently, only pci method were
> > implemented and we set the affinity according to:
> >
> > - if device uses INTX, we just ignore the request
> > - if device has per vq vector, we force the affinity hint
> > - if the virtqueues share MSI, make the affinity OR over all affinities
> > requested
> >
> > Signed-off-by: Jason Wang <jasowang@redhat.com>
>
> Hmm, I don't see any benefit from this patch, I need to use
> irq_set_affinity (which however is not exported) to actually bind IRQs
> to CPUs. Example:
>
> with irq_set_affinity_hint:
> 43: 89 107 100 97 PCI-MSI-edge virtio0-request
> 44: 178 195 268 199 PCI-MSI-edge virtio0-request
> 45: 97 100 97 155 PCI-MSI-edge virtio0-request
> 46: 234 261 213 218 PCI-MSI-edge virtio0-request
>
> with irq_set_affinity:
> 43: 721 0 0 1 PCI-MSI-edge virtio0-request
> 44: 0 746 0 1 PCI-MSI-edge virtio0-request
> 45: 0 0 658 0 PCI-MSI-edge virtio0-request
> 46: 0 0 1 547 PCI-MSI-edge virtio0-request
>
> I gathered these quickly after boot, but real benchmarks show the same
> behavior, and performance gets actually worse with virtio-scsi
> multiqueue+irq_set_affinity_hint than with irq_set_affinity.
>
> I also tried adding IRQ_NO_BALANCING, but the only effect is that I
> cannot set the affinity
>
> The queue steering algorithm I use in virtio-scsi is extremely simple
> and based on your tx code. See how my nice pinning is destroyed:
>
> # taskset -c 0 dd if=/dev/sda bs=1M count=1000 of=/dev/null iflag=direct
> # cat /proc/interrupts
> 43: 2690 2709 2691 2696 PCI-MSI-edge virtio0-request
> 44: 109 122 199 124 PCI-MSI-edge virtio0-request
> 45: 170 183 170 237 PCI-MSI-edge virtio0-request
> 46: 143 166 125 125 PCI-MSI-edge virtio0-request
>
> All my requests come from CPU#0 and thus go to the first virtqueue, but
> the interrupts are serviced all over the place.
>
> Did you set the affinity manually in your experiments, or perhaps there
> is a difference between scsi and networking... (interrupt mitigation?)
>
> Paolo


You need to run irqbalancer in guest to make it actually work. Do you?


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