[Qemu-discuss] Disparity between host and guest CPU utilization during disk IO benchmark

[Nagarajan, Padhu (HPE Storage)]

I was running an 8K random-read fio benchmark inside the guest with iodepth=32. 
The device used inside the guest for the test was a virtio-blk device with 
iothread enabled, mapped on to a raw block device on the host. While this 
workload was running, I took a snapshot of the CPU utilization reported by the 
host and the guest. The guest had 4 cores. top inside guest shows 3 idle cores 
and one core being 74% utilized by fio (active on core 3). The host had 12 
cores and three cores were completely consumed by three qemu threads. top 
inside host shows three qemu threads, each utilizing the CPU core to a near 
100%. These threads are "CPU 1/KVM", "CPU 3/KVM" and "IO iothread1". The CPU 
utilization story on the host side is the same, even if I run a light fio 
workload inside the guest (for ex. iodepth=1). 

Why do I see two "CPU/KVM" threads occupying 100% CPU, even though only one 
core inside the guest is being utilized ? Note that I had 'accel=kvm' turned on 
for the guest.

Why does the CPU/KVM thread on the host use 100% CPU, even though the 
corresponding guest core utilization is far less ?

The host operating environment was Debian 8 and the guest was CentOS 7.3. I was 
running qemu-2.8.1 on the host. The guest VM had 'accel=kvm' turned on. I've 
attached an image showing the snapshot from 'top' on host (left) and guest 
(right) at the same point in time.

~Padhu.

host_guest_cpu_util.jpg
Description: host_guest_cpu_util.jpg