Red Hat Bugzilla – Bug 508109
qemu-kvm cpu usage spikes on F11 host with idle centos 5.3 guest
Last modified: 2009-08-07 06:46:10 EDT
Created attachment 349426 [details]
VM performance details screenshot
When I run a centos 5.3 x86_64 guest on my F11 x86_64 system with kvm I see the qemu-kvm process cpu usage jump to ~20% (in top, ~50% in virtual-machine-manager) every few seconds even though the guest is completely idle (I even disabled the HAL polling for /dev/hdc). Running a "top" using the console the only process that show up as using cpu time is top itself.
I attached a screenshot which show the behaviour pretty well.
This is the command line the process is running with (started from virtual-machine-manager):
/usr/bin/qemu-kvm -S -M pc -m 512 -smp 1 -name centos5 -uuid 2754a928-6d0c-7833-e0e6-c717de291258 -monitor pty -pidfile /var/run/libvirt/qemu//centos5.pid -boot c -drive file=/var/lib/libvirt/images/centos5.img,if=ide,index=0,boot=on -net nic,macaddr=54:52:00:27:40:99,vlan=0 -net tap,fd=16,script=,vlan=0,ifname=vnet0 -serial pty -parallel none -usb -vnc 127.0.0.1:0
The VM is configured with 1 VCPU and 512mb ram. I also shut down the network interface in the guest using "ifdown eth0" but that didn't change anything.
The host system is a fresh F11 x86_64 install running on a system with a "AMD Athlon(tm) 64 X2 Dual Core Processor 4200+" CPU and 4gb of ram.
The kernel and qemu versions are:
Created attachment 349483 [details]
I attached some oprofile profiling data. I'm not very familiar with the proper usage of oprofile but maybe that data can help shed some light on the source of the problem.
Hard to tell what's going on here
Does booting the guest with divider=10 help at all?
After firing up the VM today the major peaks on the performance graph no longer appeared, only the smaller ones. I updated the machine in the mean time so that might have something to do with it.
Adding "divider=10" further reduces both the value and the frequency of the minor peaks.
Thanks for the update
This just sounds like normal behaviour - the guest may be idle, but it is still handling timer interrupts etc.
Closing, but do re-open if you think there's a genuine qemu/kvm bug here