Re: [Xen-devel] [PATCH 1/2] cpu steal time accounting

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 22 Feb 2006, at 14:27, Rik van Riel wrote:

> > Is accounting user/system time an unnecessary extra? I guess we 
> > already
> > do it by sampling at tick granularity anyway?
> >
> > Should 'steal time' include blocked time when the guest had no work to
> > execute?
> No, this is idle time.  If the guest had no work to do,
> it wasn't suffering from contention of the CPU.
But the 'vcpu_time' you read out of Xen excludes time spent 
blocked/unrunnable. Won't you end up accounting  that as it it were 
involuntary preemption? Also:
 1. What if a guest gets preempted for lots of short time periods (less 
than a jiffy). Then some arbitrary time in the future is preempted for 
long enough to activate you stolen-time logic. Won't you end up 
incorrectly accounting the accumulated short time periods?
 2. Is the Xen provided 'vcpu_time', divided down into jiffies, even 
comparable with the kstats that you sum? What about accumulated 
rounding errors in 'vcpu_time' and the kstats causing relative drift 
between them over time?
 -- Keir


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel