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 -=|

On Wed, May 23, 2007 at 02:35:47PM +0200, Jan Michael wrote: > In the described stress situation it tooks about an average of 4 > seconds to make the following to function calls, which provide me the > cpuTime of a domain > > dom_old = virDomainLookupByID(conn_old, listOfDomains > [i]); > ret = virDomainGetInfo(dom_old, &info_old); It is the virDomainLookupByID call which is killing your performance here - it has to go to XenD, and XenD does *incredibly* stupid stuff talking to xenstored http://lists.xensource.com/archives/html/xen-devel/2007-04/ msg00663.html So each call takes ~1 second under normal conditions, so 4 seconds isn't surprising under high load. This is one of the reasons why the Xen driver in libvirt tries to talk directly to the hypervisor if at all possible - the HV impl of most calls is at least x1000 faster than the XenD impl. virConnectListDomains & virDomainGetInfo both have impls which talk to the HV so they are very fast to execute. The virDomainLookupByID has no HV impl, since we need to talk to XenD to find the guests name & UUID info. > Here are the stats from my latest measurement: > > old cpuTime new cpuTime > > Domain-0: 3s 4294835190ms 3s 4294849513ms > stornode: 5s 580501ms 6s 4294550691ms > worknode: 6s 4294546809ms 5s 582761ms > > That leads to results in cpu utilisation computation for the node, > which are much lower, around 75%, than the real value (100%) would > be. > > One solution would be to add the measured time make those calls to > used cpuTime. But this in turn can cause calculations of to high > values because I don't really know in which point in time the value > is written to the structure. The approach I'd recommend is to make sure you avoid calling virDomainLookupByID. If you've got a simple loop like pseduo-code... forever() { ids = virConnectListDomains() foreach (id in ids) { dom = virDomainLookupByID(id) info = virDomainGetInfo(dom) } } Then, you need to pull the virDomainLookupByID out of the inner loop. Basically cache the 'virDomainPtr' handles - you can detect new domains, or shutdown domains after each call to virConnectListDomains(). So with correct caching of handles, you should only need to suffer the performance hit from virDomainLookupByID once per guest - the first time it starts > Nevertheless is xentop showing me every time the correct cpu- > utilisation of each of my domains. So that I conclude, that this > problem must have something to do with libvirt API. Its just libvirt exposing the undering inadequacies of XenD & XenStoreD impl & performance :-( 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 -=|