I had an interesting question come across my desk a few days ago: “Is it still worthwhile to understand T-states?” My first response was to think, “Huh? What the heck is a T-state?”
Doing a little more research, I discovered that, yes, there is something called a T-state, and no, it really isn’t relevant any more, at least for mainline Intel® processors.
Let me say this again: T-States are no longer relevant!
Now that the purely practical people have drifted off to other more “relevant” activities, here’s something for all you power management history buffs.
A T-state was once known as a Throttling state. Back in the days before C and P states, T-states existed to save processors from burning themselves up when things went very badly, such as when the cooling fan failed while the processor was running as fast as she could. If a simple well placed temperature sensor registered that the junction temperature was reaching a level that could cause damage to the package or its contents, the HW power manager would place the processor in different T-States depending upon temperature; the higher the temperature, the higher the T-State.
As you probably already guessed, the normal run state of the processor was T0. When the processor entered a higher T-state, the manager would clock gate the cores to slowdown execution and allow the processor to “relax” and cool. For example, in T1 the HW power manager might clock gate 12% of the cycles. In rough terms, this means that the core will run for 78% of the time and sleep for the rest. T2 might clock gate 25% of the cycles, etc. In the very highest T-state, over 90% of the cycles might be clock gated. (See Figure.)
Note that in contrast to P-states, the voltage and frequency are not changed. Also, using T-states the application runs slower not because the processor is running slower, but because it is suspended for some percent of the time. In some ways, you can think of a T-state as being like a clock gated C1 state with the processor not being idle, i.e. it is still doing something useful.
In the figure above, the top most plot shows the runtime of a compute intensive workload if no thermal overload occurs. The bottom shows the situation with T states (i.e. before P states), where the processor begins to toggle between running and stopped states to cool down the processor. The middle is what happens in current processors, where the frequency/voltage pair is reduced allowing the processor to cool.
For those of you who have borne with me for the history lesson, there are a few more practical reasons you should be at least aware of T-states.
(1) Some technical literature now uses the term “throttling states” to mean P-states, not T-states.
(2) Some power management data structures, such as some defined by ACPI, still include an unused T-state field. Many inquiries about T-states originate from this little fact.
(3) I suspect that T-states are still relevant in some embedded processors