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. -- |=- Red Hat, Engineering, Emerging Technologies, Boston. +1 978 392 2496 -=| |=- Perl modules: http://search.cpan.org/~danberr/ -=| |=- Projects: http://freshmeat.net/~danielpb/ -=| |=- GnuPG: 7D3B9505 F3C9 553F A1DA 4AC2 5648 23C1 B3DF F742 7D3B 9505 -=| -- Libvir-list mailing list Libvir-list@redhat.com https://www.redhat.com/mailman/listinfo/libvir-list
