Andrei F (andrei.delete@this.anandtech.com) on September 16, 2021 8:12 am wrote:
> David Kanter (dkanter.delete@this.realworldtech.com) on September 15, 2021 11:46 am wrote:
> > > > Isn't the point to deliver max perf with min energy?
> > >
> > > The point is that your window of user experience is 16/8ms. If the workload completes
> > > interactively to fill that "QoS" at the current frequency without going over a utilisation
> > > threshold in that sliding window, you *do not want to go any higher*.
> >
> >
> > Wouldn't you want to run at lower voltage/frequency if possible?
>
> My point is you're already running at the lowest frequency without failing to deliver the QoS/frame.
> If the performance window is smaller, then you'd be running at higher frequencies to service the
> higher load periods, and low frequencies during lower load periods, which costs more energy than
> running medium frequency the whole time and spreading out the load over the frame.

My point is that even within 10ms, the optimal voltage will probably vary quite a bit (assuming fixed frequency).

> >
> > Generally the point is changing V/F should be inexpensive to enable using the optimal V/F combo.
> >
> > > So your 120MHz FIVR is completely and utterly pointless. You would be wasting energy at higher
> > > voltages for no gain in user performance.
> >
> > Shifting to low voltage fast seems valuable, right?
>
> It doesn't because of the above reasons. You should be running
> at the "just right" F/V servicing the workload.

Let's say you are running at 2GHz. 8ms is 16M cycles. The 'best' voltage will vary over those 16M cycles based on activity in the chip, temperature, etc.

> > > Fmax should only every be reached and triggered after
> > > continuous load of 2-3x of user experience window - current mobile phones do that in around 40-50ms,
> > > anything faster than that is waste of energy and battery life. These are not HW limitations, but
> > > learning the hard way what the most efficient way to design battery powered DVFS logic.
> >
> > Would that change if V/F switch took nanoseconds and cost little energy?
> >
> > Seems like you would want to dynamically adjust voltage...even if at fixed frequency.
> >
> > David
>
> If it would take nanoseconds to change V/F then all you're doing is having two states - Fmax and Fmin.
> A that point Fmin becomes meaningless as a state and it's better to just clock gate/power gate. Your
> most prevalent state in which you do computation would be Fmax and you defeat the purpose of DVFS.

Well voltage can change quite rapidly. I think Intel's FIVR is around 1V in 0.5us. So if you wanted to change 50mV, that's probably quite quick.

David