Effect of nice value on idle vcpu threads consumption
Hello,
I have been experimenting with renicing vcpu threads and found some
oddity. I was expecting a idle vcpu thread to consume close to 0% cpu resource irrespective of its nice value. That was true when nice value was 0 for vcpu threads. However altering nice value of (idle) vcpu threads is causing its cpu consumption to shoot up. Does anyone have a quick answer to this behavior?
More details.
Machine : x3650-M2 w/ 2 Quad-core CPUs (Intel Xeon X5570), HT enabled
Host : RHEL 6 distro w/ 2.6.38-rc5 kernel
Single Guest : w/ 4vcpus (all vcpus pinned to physical cpu 0), 1GB mem
Sles11 distro w/ 2.6.37 kernel
Single guest is booted and kept idle.
When all vcpu threads are at nice 0, here's the consumption (close to 0 for all vcpu threads).
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ P COMMAND 5642 qemu 20 0 1567m 381m 3048 S 1.2 1.2 0:02.56 0 qemu-kvm 5640 qemu 20 0 1567m 381m 3048 S 0.8 1.2 0:12.74 0 qemu-kvm 5641 qemu 20 0 1567m 381m 3048 S 0.8 1.2 0:02.60 0 qemu-kvm 5643 qemu 20 0 1567m 381m 3048 S 0.6 1.2 0:02.76 0 qemu-kvm
Changing nice value for one of the vcpu threads to -20:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ P COMMAND 5640 qemu 0 -20 1567m 381m 3048 R 45.5 1.2 0:19.67 0 qemu-kvm 5641 qemu 20 0 1567m 381m 3048 R 0.4 1.2 0:03.33 0 qemu-kvm 5642 qemu 20 0 1567m 381m 3048 R 0.4 1.2 0:03.16 0 qemu-kvm 5643 qemu 20 0 1567m 381m 3048 R 0.4 1.2 0:03.36 0 qemu-kvm
Changing nice value for another of the vcpu threads to -20:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ P COMMAND 5640 qemu 0 -20 1567m 381m 3048 S 35.7 1.2 0:30.92 0 qemu-kvm 5641 qemu 0 -20 1567m 381m 3048 S 26.1 1.2 0:04.77 0 qemu-kvm 5642 qemu 20 0 1567m 381m 3048 S 0.2 1.2 0:03.29 0 qemu-kvm 5643 qemu 20 0 1567m 381m 3048 S 0.2 1.2 0:03.50 0 qemu-kvm
Is this behavior expected? Any explanation for this behavior?
- vatsa