Re: [Xen-devel] CPU Usage Discrepancies

["Pradeep Vincent" <pradeep.vincent@xxxxxxxxx>]

> > Does your /proc/stat analysis include time spent in the kernel?
Yes.. it does..

- Pradeep Vincent

On 3/6/07, Pradeep Vincent <pradeep.vincent@xxxxxxxxx> wrote:
> With a trivial workload like "ls -R /" I see as much as 30% diff and
> with other workloads I see that xm reports twice what /proc/stat
> reports. Sounds too high to me.
>
> Linux counts all the nanosecs not accounted by the hypervisor towards
> "stolen" or "blocked" as its own usage. This should include all the
> time spent in the hypervisor in the context of a particular Vcpu - The
> hypervisor counts nsecs as "stolen" or "blocked" only after the Vcpu's
> state is changed (from running to something else)  So most part of the
> hypervisor's CPU usage should be accounted for the same way by xm and
> by /proc/stat on guests as they both use the same "stolen" and
> "blocked" nsecs as accounted for and maintained by the hypervisor.
>
> Like you said context switch overhead isn't accounted for accurately
> but hypervisor's cpu usage accounting suffers from the same problem
> and to the same extent. Even if this isn't the case,  context switch
> cpu usage can't account for this big a difference.
>
> - Pradeep Vincent
>
> On 3/4/07, Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> wrote:
> > Does your /proc/stat analysis include time spent in the kernel?
> >
> > Another possibility here is that, if your guest blocks a lot, you will see
> > that Linux counts the guest as 'running' for less of the context-switch path
> > than Xen does. This will cause Linux's estimate of time used to be less than
> > Xen's. There's not much to be done about that: in general Xen has more
> > knowledge of what is actually going on, including precisely when a switch of
> > control happens, and the numbers from xentop will be more accurate than
> > numbers generated by the guest itself (particularly with frequently-blocking
> > workloads). Although it depends on what you're interested in measuring -- if
> > you care about the amount of time spent doing useful application work (as
> > opposed to context switching) then you might be more interested in the Linux
> > stats because Xen will include more time spent in the Linux and Xen context
> > switch paths.
> >
> >  -- Keir
> >
> > On 2/3/07 23:42, "Pradeep Vincent" <pradeep.vincent@xxxxxxxxx> wrote:
> >
> > > I see serious discrepancies between Cpu usage as reported by /proc/stat on
> > > Xen3
> > > virts and Cpu usage as reported by the hypervisor via "xm" tool
> > > (cpu_time). The problem exists on Intel and AMD platforms - 1 Vcpu and
> > > multiple Vcpu slots - 1 Physical CPU and multiple Physical CPU hosts.
> > >
> > > The skew is pronounced with workloads that "sleep-wake-sleep-wake" at
> > > a high frequency while workloads that hog the CPU don't exhibit this
> > > problem as much.
> > >
> > > Anybody seen this ? Any insights ?
> > >
> > > http://bugzilla.xensource.com/bugzilla/show_bug.cgi?id=882 has all the
> > > details.
> > >
> > > - Pradeep Vincent
> > >
> > > _______________________________________________
> > > Xen-devel mailing list
> > > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > > http://lists.xensource.com/xen-devel
> >
> >
> >
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