On Tuesday, October 30, 2007 5:02 AM, tesla-dev-bounces at opensolaris.org wrote:
> Eric Saxe wrote: >> >> Yea, it's a good question. My impression of the T-states, is that in >> the general (common) case the system wouldn't use them, since from my >> understanding their purpose is mainly as a mechanism for quickly (and >> forcibly) throttling the processor clock to bring a thermal situation >> under control. > Let's think about that. T-states represent nothing more than the > ability to hold the CPU for some percentage of clock cycles. > T-states do not > reduce the CPU > clock > and do not reduce the voltage to the CPU. Yes, T-states just forcefully introduce idle cycles in the processor, and these idle cycles don't get into power saving C-states. > > Given a choice between a T-state that halts the CPU for 50% of > the clock > cycles > and a P-state which runs the CPU at 50% of full-speed, the > P-state will > win in > terms of power savings (and thermal safety) every time. Given > that the > thermal > time-constant of a CPU is *much* greater than the time it takes to > switch P-states, I don't think there's a compelling case for T-state > usage *when P-states are supported*. > T-states give a linear power reduction, while P-states give a > geometric power > reduction. That's not ture, T-state doesn't save any energy. > > While T-state switching is of much lower latency, it is no > more forcible > than > switch P-states. > > Where I can see T-states being useful is on systems where there's no > P-state support, or to create intermediate levels between P-states. T-states is only used when processor is over heat. It's not designed for normal power saving. We should avoid T-state whenever we can. >> It's also my understanding that the performance impact of >> the T-states are fairly severe, which begs the question if we would >> ever want the CPU to enter them when not idle. When the CPU is idle, >> I would wonder how T-states would compare to what we get from >> entering the C1 state. Since T-states don't change the voltage, I >> would guess that C1 would actually buy more. Maybe Aubrey or someone >> else can correct me if I'm mistaken... > I don't believe it makes sense to compare T-states to C-states - I'd > compare T-states to P-states. The performance impact of a T-state is > comparable > to that of > a P-state which > runs at the same effective clock rate, while the power savings of a > P-state is greater. That doesn't make sense to me, too. They are not comparable. Because they are in different cases. >> >> Aren't there also cases where the processor could go into a T-state >> without the OS actually knowing? > It's certainly possible that the chipset could start holding the CPU > halted for some percentage of clock cycles, I suppose. > The answer is yes, cpu has a thermal sensor, and it can automatically throttle in overheat case. Thanks, -Aubrey
