4 ms·

Right, because the lowest supported frequency is supposed to be the most energy efficient frequency, Feff, at least on Intel chips. I.e. it is the minimum in th

Right, because the lowest supported frequency is supposed to be the most energy efficient frequency, Feff, at least on Intel chips. I.e. it is the minimum in the joules/work chart. After all, there's no point running at a lower frequency: those frequencies are always dominated regardless of your time/energy preference, so Intel claims to set Fmin to Feff. So lowest freq is a good proxy for "I only care about efficiency". That said, powersave is a bit ambiguous: it's a policy in more than one driver. For example, intel_pstate, the default recommended driver for Intel chips, has powersave and performance governors too, and there powersave is much more sophisticated and can run at high frequencies.

marcan_42 5y ago
Reality doesn't agree that using the lowest frequency is the most efficient approach, as evidenced by that Lenovo presentation :-)
- BeeOnRope 5y ago
  OK, I actually read the Lenovo paper and it seems fine. One big difference with what I'm trying to claim is that what I'm saying considers only the power use of the CPU itself: i.e., the thing whose power use varies with frequency. From the CPU manufacturer's point of view, that's all they can really do. The Lenovo paper is looking at total system power, and the way they calculate efficiency includes the system power in the efficiency calculation. This pushes the efficiency point well above what you'd get from looking at the CPU alone. Both approaches are right some of the time: on a laptop where the rest of the system is running regardless, you probably want to consider only the CPU power in the efficiency calculation: if your CPU work finishes more quickly it's not going to stop the draw from the screen, wifi chip, etc. OTOH on a server if you size your fleet based on the total work, you should really use total power since if your work takes less time you need fewer total servers (or fewer cloud CPU hours, etc) so the system power is also saved.

marcan_42 5y ago

Reality doesn't agree that using the lowest frequency is the most efficient approach, as evidenced by that Lenovo presentation :-)

BeeOnRope 5y ago
OK, I actually read the Lenovo paper and it seems fine. One big difference with what I'm trying to claim is that what I'm saying considers only the power use of the CPU itself: i.e., the thing whose power use varies with frequency. From the CPU manufacturer's point of view, that's all they can really do. The Lenovo paper is looking at total system power, and the way they calculate efficiency includes the system power in the efficiency calculation. This pushes the efficiency point well above what you'd get from looking at the CPU alone. Both approaches are right some of the time: on a laptop where the rest of the system is running regardless, you probably want to consider only the CPU power in the efficiency calculation: if your CPU work finishes more quickly it's not going to stop the draw from the screen, wifi chip, etc. OTOH on a server if you size your fleet based on the total work, you should really use total power since if your work takes less time you need fewer total servers (or fewer cloud CPU hours, etc) so the system power is also saved.