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

[Paolo Bonzini <pbonzini@redhat.com>]

> > 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.

Paolo