Virtio/Block/Latency

This page describes how virtio-blk latency can be measured. The aim is to build a picture of the latency at different layers of the virtualization stack for virtio-blk.

Benchmarks

Single-threaded read or write benchmarks are suitable for measuring virtio-blk latency. The guest should have 1 vcpu only, which simplifies the setup and analysis.

Instrumenting the stack

Guest

The single-threaded read/write benchmark prints the mean time per operation at the end. This number is the total latency including guest, host, and QEMU. All latency numbers from layers further down the stack should be smaller than the guest number.

Guest virtio-pci

The virtio-pci latency is the time from the virtqueue notify pio write until the vring interrupt. The guest performs the notify pio write in virtio-pci code. The vring interrupt comes from the PCI device in the form of a legacy interrupt or a message-signaled interrupt.