Re: [Libvir] Question on acquiring cpuTime in struct _virDomainInfo
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
case:
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
precise?
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.
Regards,
Dan.
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