[libvirt-users] What does cpu_time returned by virDomainGetCPUStats mean?

[Eric Blake]

On 04/16/2012 05:37 AM, Zhihua Che wrote:
> 
>    I thought this value store the run time of the cpu since last boot.

The intent is to store run time of the hypervisor process managing the
guest (which therefore is larger than the amount of time that the guest
thinks it has been running, since the hypervisor has some overhead).
But the API is flexible enough that we can add more statistics, if it
proves easy to collect such additional statistics.

> But I find I was wrong because this value would increase until it
> wraps down and doesn't reset even the domain is restarted.

How are you restarting the guest?  If it is by rebooting the guest
_within the same qemu process_, then no, the numbers won't reset.  Based
on how the cpuacct cgroup works, the numbers should only wrap when you
actually create a new qemu process (actually stop the guest and boot it
fresh in a new qemu process, rather than rebooting the guest within the
same qemu process).

Perhaps we should be improving our XML to track delta usage since a
given point in time, and when we detect a domain reboot, update that
delta point so that the usage will again appear to be 0; allowing a
delta calculation would also let us "track" CPU usage even across domain
migration or managedsave/restore.

> 
>    So, what does this value mean?
> 
>    How can I get the CPU usage of the domain?
> 
>    I found nothing on the API reference doc page:-(. No word is
> related with the meaning of the returned array of virTypedParameter by
> virDomainGetCPUStats().

I found this:

http://libvirt.org/html/libvirt-libvirt.html#VIR_DOMAIN_CPU_STATS_CPUTIME

"cpu usage in nanoseconds, as a ullong"

and looking at libvirt.h, VIR_DOMAIN_CPU_STATS_CPUTIME maps to the
"cpu_time" name of your API call.

If that still isn't enough information, could you help out by submitting
patches to improve our documentation?

-- 
Eric Blake   eblake at redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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