Re: [Qemu-devel] [PATCH 5/5] cpu: throttle: fix throttle time slice

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Paolo Bonzini (pbonzini@redhat.com) wrote:
> 
> 
> > > So I'm inclined _not_ to take your patch.  One possibility could be to
> > > do the following:
> > > 
> > > - for throttling between 0% and 80%, use the current algorithm.  At 66%,
> > > the CPU will work for 10 ms and sleep for 40 ms.
> > > 
> > > - for throttling above 80% adapt your algorithm to have a variable
> > > timeslice, going from 50 ms at 66% to 100 ms at 100%.  This way, the CPU
> > > time will shrink below 10 ms and the sleep time will grow.
> 
> Oops, all the 66% should be 80%.
> 
> > It seems odd to have a threshold like that on something that's supposedly
> > a linear scale.
> 
> I futzed a bit with the threshold until the first derivative of the CPU
> time was zero at the threshold, and the result was 80%.  That is, if you
> switch before 80%, the CPU time slice can grow to more than 10 ms right
> after the threshold, and then start shrinking.
> 
> > > It looks like this: http://i.imgur.com/lyFie04.png
> > > 
> > > So at 99% the timeslice will be 97.5 ms; the CPU will work for 975 us
> > > and sleep for the rest (10x more than with just your patch).  But I'm
> > > not sure it's really worth it.
> > 
> > Can you really run a CPU for 975us ?
> 
> It's 2-3 million clock cycles, should be doable.

I wasn't really worrying about the CPU running, I was more worried
about timer resolution in stopping it.  If you're timer isn't that accurate
in starting/stopping the CPU then the errors might be so large as to
make that a bit odd.

Dave

> Paolo
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK