[Libvir] Question on acquiring cpuTime in struct _virDomainInfo
Daniel P. Berrange
berrange at redhat.com
Thu May 10 16:32:53 UTC 2007
On Thu, May 10, 2007 at 05:41:33PM +0200, Jan Michael wrote:
> Hi everyone,
> using libvirt I'm trying to calculate cpu utilization of a node in
> percent. But sometimes values beyond 100.0% are being calculated.
> This is because a domain spend more time on a cpu than time is
> elapsed in the meantime.
> A short explanation of the way how cpu utilization is computed in my
> 1. - open two connections with
> conn_cur/conn_old = virConnectOpenReadOnly(NULL);
> 2. - get current time
> gettimeofday(&time_old, NULL);
> - get domain by id with
> dom_old = virDomainLookupByID(conn_old, id)
> - get domain information
> virDomainGetInfo(dom_old, &info_old);
> 3. - sleep a second
> 4. - doing same stuff like in 2. but with _cur
> 5. - compute cpu utilization by dividing used cputime by elapsed time
> and multiply with 100
> Am I right if I suppose that cpuTime for _virDomainInfo structure
> will be directly acquired from the hypervisor in virDomainGetInfo
> (dom_old, &info_old) or is it already present with getting the domain
> itself? Is there any better solution of doing this, which is more
This is the best approach - the algorithm you summarized is basically
the same as I use in virt-manager. The reason it sometimes goes above
100% is just due to timing / schedular variations
1. get timeofday
2. get cputime for domA
3. sleep a while
4. get timeofday
5. get cputime for domA
We're basically looking at the ratio of 4-1, against 5-2. It would
be 100% accurate if you could guarentee no time elapased between
steps 1 & 2, or between steps 4 & 5, but there's always some latency
in there, so occassionally you might end up calculating a value that
is a tiny bit over 100%. In virt-manager I deal with this by simply
rounding down to 100 if this occurs.
Based on the hypercalls which are available to us, I don't see any
way to avoid this scenario. Then again it is not like we really need
millisecond precision in caculating CPU usage so I don't think its
a problem worrying about too much.
> And another general question:
> The monitoring utility of xen, called xentop, provides also
> statistics about networking and vbds. Are there any plans to provide
> this values by libvirt in the future?
I'd like to see the ability to track network & disk I/O stats.
No one has so far stepped forward to suggest an API or implmentation,
but I'd welcome anyone interested in taking a look at this area.
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