What exactly is a P-state? (Pt. 1)
By Taylor IoT Kidd, published on January 1, 2015
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.
6 comments
Topsuresh babu k. said on Nov 25,2014
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:
suresh babu k. said on Nov 25,2014
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 said on Apr 16,2012
Hi, i am from china. and i am a bioser, now i am learning CPU PPM , i found it is difficult to learn.
Anonymous said on Sep 2,2010
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 said on Jul 29,2008
Hi,
Please see my earlier blog entries on C-states. I hope to be extending this soon.
--Taylor
senwell_wu said on Jun 28,2008
What about C-state?
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