[Libvir] Question on acquiring cpuTime in struct _virDomainInfo

Daniel P. Berrange berrange at redhat.com
Wed May 23 13:30:18 UTC 2007

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


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 :-(  

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