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Re: [Qemu-devel] [PATCH 5/5] cpu: throttle: fix throttle time slice
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [PATCH 5/5] cpu: throttle: fix throttle time slice |
Date: |
Tue, 4 Apr 2017 16:44:42 +0100 |
User-agent: |
Mutt/1.8.0 (2017-02-23) |
* Paolo Bonzini (address@hidden) 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 / address@hidden / Manchester, UK