libvirt users - Apr 2012 - What does cpu_time returned by virDomainGetCPUStats mean?

hi, everyone.

    I find an 'interesting' thing involving virDomainGetCPUStats(). I
call it for cpu usage consumed by a domain and get a array of
virTypedParameter. My system is 2-core and hyperviser return 1
parameter per cpu. So the contents of the array is like this.
    virTypedParameter[0]
    {
        .fiedl = "cpu_time"
        .type = 4
        .value.ul = 51640610899
    }
    virTypedParameter[1]
    {
        .field = "cpu_time"
        .type = 4
        .value.ul = 55302820304
    }

   I thought this value store the run time of the cpu since last boot.
But I find I was wrong because this value would increase until it
wraps down and doesn't reset even the domain is restarted.

   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().

Il giorno Lun 16 Apr 2012 13:37:24 CEST, Zhihua Che ha scritto:
[...]
>    How can I get the CPU usage of the domain?
[...]

As Daniel told me two days ago, cpuTime shows the absolute CPU time
consumed since boot. To get % CPU time, take two readings 'n' seconds
apart and calculate the delta between them.

-- 
RaSca
Mia Mamma Usa Linux: Niente ? impossibile da capire, se lo spieghi bene!
rasca at miamammausalinux.org
http://www.miamammausalinux.org

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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