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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 22 Feb 2006, at 17:11, Rik van Riel wrote:

> If the domain is unrunnable, surely there won't be a
> process on the virtual cpu that is runnable?  Or am
> I overlooking something here?
Oh, I see, this is dealt with inside account_steal_time(). No problem 
then.
> >  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?
> This is true.  I'm not sure we'd want to get the vcpu info
> at every timer interrupt though, that could end up being
> too expensive...
Having to call down to Xen to get that information is unfortunate. 
Perhaps we can export it in shared_info, or have the guest register a 
virtual address it would like the info written to.
> In the tests I ran the steal time seemed to work out quite
> well with what I expected it to be, watching /proc/stat from
> inside the guest and xentop from dom0 simultaneously.
>
> The rounding errors happen occasionally (I added printks to
> the if statements catching them), but not all that often...
I think the calculation of delta stolen time would be clearer as:
 ((system_time - prev_system_time) - (vcpu_time - prev_vcpu_time)) / 
NS_PER_TICK
where system_time/vcpu_time become the prev_system_time/prev_vcpu_time 
the next time your logic is triggered.
It has another advantage that it does not subtract quantities that can 
slowly relatively drift over days/weeks.
 -- Keir


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