Imbalanced IRQs on virtio devices

Question

I noticed in the top on my linux server that one cpu had far more number of software interrupts than all other 7 cores. Digging further, I noticed that this core is pinned to a particular irq which happens to be a virtio device. Infact each core has an affinity toward a particular virtio device

virtio0-config
virtio0-control
virtio0-event
virtio0-request
virtio2-config
virtio2-input.0
virtio2-output.0
virtio3-config
virtio3-input.0
virtio3-output.0
virtio4-config
virtio4-input.0
virtio4-output.0

In this list, virtio4-input.0 has in particular very high number of interrupts and I am not able to figure out what is special about this particular device. any clues will be very helpful.. The machine in question is a nutanix VM running on a linux host.

Answer 1

iirc, that's your virtualised KVM network device (virto4), and -input.0 is it's input queue. I don't know why, but it the interrupts appear to only be handled by one CPU. You can read more on someone's investigation, and attempts to spread the IRQ handling over multiple CPUs here: http://www.9bitwizard.eu/packets-part-2