Michael Pogue wrote: > Ah, yes -- you're right. > > I went back and calculated the total power cost over N minutes, assuming > a 1-minute CPU task, and in all cases, although a throttled CPU *does* > consume less power, the task overall takes longer to complete -- a factor > which always (for this machine) outweighs the power savings of throttling > itself. Which will always be the case. Let's take a step back think about what represents power consumption in a typical CMOS chip; a large component is is dynamic - pumping electrons into the gate capacitance of MOS transistors, then draining those electrons. Fundamentally, you start with Q = C * V to describe what one cycle of that CMOS gate costs in terms of current, then multiply it by the frequency of operation and you end up with I = C * V * F. Power consumption is V * I; substituting, we get P = C * V^2 * F. This makes a lot of simplifying assumptions about how chips actually work.
So, T-states effectively freeze the CPU for some part of the time, thus reducing the effective value of 'F' but doing no work while held. So, a T-state means that a CPU will consume (roughly) 10% less power while doing 10% less work in a given period of time (there's typically some non-dynamic part of the power consumption due to leakage, too). P-states take advantage of the fact that CPUs running at a lower actual clock rate will operate reliably at a lower supply voltage. If you're willing to do the work more slowly, you can reduce the V and enjoy (reduction)^2 savings. Just for illustration, let's assume we can crank the supply voltage down linearly with respect to the clock rate (a drastic simplification). Very roughly, this means that a chip might be able to consume 75% less power while doing 50% less work in a given period of time. As Andrei and Mike point out, lowering the CPU clock rate and the supply voltage allow a much greater savings in power for a given effective throughput. So, I don't endorse T-states as a power-saving scheme; I was just observing that a chip running at 50% clock rate will get as much work done in a given period of time as a chip running at 100% clock rate but held 50% of the time. The difference is how much power is saved. Dana
