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

[Zhihua Che]

在 2012年4月17日 上午4:11，Eric Blake <eblake at redhat.com> 写道：
> 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).


After reading your post, I did the below experiment again.
I started the domain by issuing 'start ubuntu-1' and shutted down my
domain by issuing 'destroy ubuntu-1'.
(BTW, I cannot shutdown my domain by 'shutdown ubunut-1'. I guess my
domain image got something wrong. I wish this didn't affect the
experiment)

Here are my two sample values.

start ubunt-1
cpu0+cpu1 11239925290
          cpu0 6034491893
          cpu1 5205433307

destroy and start ubunu-1
cpu0+cpu1 10621566430
          cpu0 5403373809
          cpu1 5218192621

I can't determine whether these value reseted because total number is
lower than that before restarting while one cpu is larger or lower.

These values really confuse me.

I check the kvm process id through ps. I'm sure the  two running
domain are assigned two different process id. I guess that means they
run as different qemu process as you mentioned.

>
> 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
>