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SubjectRe: [net-next RFC V5 3/5] virtio: intorduce an API to set affinity for a virtqueue
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


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