What exactly is a P-state? (Pt. 1)

What is a P-state?

When someone refers to a P-state, generally only the frequency is talked about. For example, on my Intel® Core™ processor, P0 is 2.3 GHz, and P1 is 980 MHz. In truth, a P-state is both a frequency and voltage operating point. Both are scaled as the P-state increases. 

The effect of reducing frequency

 It's obvious that performance is directly related to frequency. We all know that increasing the frequency, increases a processor's performance. The same applies for decreasing the frequency. If you halve the frequency, a compute bound task runs half as fast. For example, if your task is compute bound, requiring 100 % of the CPU for 1 second at 2 GHz, it will take 2 seconds to execute at 1 GHz. (This is roughly correct. There are a host of other factors influencing runtime, such as cache size and speed, interrupts, etc.) 

Now wait a moment. If we reduce the frequency, increasing the runtime of an application, how does this increase battery life? If we are decreasing the frequency, we are increasing the CPU utilization and reducing the % idle time. See the frequency half of Figure A below. This shouldn't have any effect on the power usage of the processor. It's running all that time anyway. 

You're right, historically. This was the case years ago with those ancient generations old processors. Though I don't know this for a fact, I suspect that in some cases, decreasing the percent idle time might have increased energy usage since increased processor activity increases instantaneous power, and we're decreasing the % idle time (i.e. time of low activity). This is where voltage scaling comes into play. There are two primary reasons for P-states, one is to reduce the peak thermal load, and the other is to save power. 

Reducing peak thermal load

 The reasons why you want to reduce the peak thermal load is pretty obvious. The instantaneous energy usage (power) of the processor is related to its activity. If the processor is very busy, requiring a lot of gates to do a lot of switching, it runs hotter. So reducing the frequency reduces the peak thermal output even if the total energy usage is not reduced. The advantage of reducing peak thermal load has to do with cost. The effectiveness of your cooling is based upon peak power, not average power. (I'm neglecting the effect of thermal inertia.) So if you can reduce peak power, you reduce the cost and size of the equipment having to do the cooling.  

pstatepeakenergy.jpg

 

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suresh babu k.'s picture

http://en.wikipedia.org/wiki/Advanced_Configuration_and_Power_Interface

P-state is performance state whereas c-state is processor state.

Processor states

The CPU power states C0–C3 are defined as follows:

  • C0 is the operating state.
  • C1 (often known as Halt) is a state where the processor is not executing instructions, but can return to an executing state essentially instantaneously. All ACPI-conformant processors must support this power state. Some processors, such as the Pentium 4, also support an Enhanced C1 state (C1E or Enhanced Halt State) for lower power consumption.[24]
  • C2 (often known as Stop-Clock) is a state where the processor maintains all software-visible state, but may take longer to wake up. This processor state is optional.
  • C3 (often known as Sleep) is a state where the processor does not need to keep its cache coherent, but maintains other state. Some processors have variations on the C3 state (Deep Sleep, Deeper Sleep, etc.) that differ in how long it takes to wake the processor. This processor state is optional.
  • Additional states are defined by manufacturers for some processors. For example, Intel's Haswell platform has states up to C10, where it distinguishes core states and package states.[25]
suresh babu k.'s picture

When CPU is idle the power should be less consumed by deactivating the cores. So what is the advantage of reducing the speed and consuming more power?  Radiation is cooled by the CPU cooler fan that runs constant.
 

antony1111111's picture

Hi, i am from china. and i am a bioser, now i am learning CPU PPM , i found it is difficult to learn.

anonymous's picture

Hi from Greece. I am searching on power consumption on each power state for intel core i7 or i5 or i3. Are there any availiable data about how much energy the processor consumes in each p-state?

I am a student in aristotel univercity of thessaloniki and i could use the info for a simulation experiment.

Thanks

anonymous's picture

Hi,

Please see my earlier blog entries on C-states. I hope to be extending this soon.

--Taylor

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