Intel® Xeon Phi™ coprocessor Power Management Part 3: An Intuitive Description of Power States Using Stick Figures and Light Bulbs


This is the fourth installment of a series of blogs on Power Management for the Intel® Xeon Phi™ coprocessor.

For those of you who have read my blog presenting an intuitive introduction to the Intel® Xeon Phi™ coprocessor, The Intel Xeon Phi coprocessor: What is it and why should I care? PART 3: Splitting Hares and Tortoises too, I irreverently referred to “diligent high tech workers who labor ceaselessly for their corporate masters”. Let’s take this description a little further. In Figure A, we have one such diligent high tech worker. He is analogous to one coprocessor CPU/HW thread.

Figure A Diligent high tech worker, i.e. an Intel® Xeon Phi™ HW thread


There are 4 HW threads to a core. See Figure B. It’s pretty obvious so I’m not going to bother with a multipage boring description of what it means. There is also that mysterious light bulb. The light bulb represents the infrastructure that supports the core, such as timing and power circuits.

Figure B Diligent high tech workers in a room, i.e. a Intel® Xeon Phi™ coprocessor core



So what does all this have to do with power management? Though it is sometimes assumed by the lower paid liberal arts students that engineers are unimaginative and boring, you and I know that, though boring we may be, we are not unimaginative. With this in mind, I ask you to visualize that on every one of those desks is a computer and a desk light.

The Core in C0: When at least one of the high tech workers is diligently working at their task. (I.e. At least one of the core’s CPUs/HW threads is executing instructions.)

CPU Executing a HALT: When one of those diligent workers finishes his task, he turns out his desk lamp, shuts down his computer, and leaves. (I.e. One of the HW threads executes a HALT instruction.)

Entering Core-C1: When all four diligent workers finish their tasks, they all execute HALT instructions. The last one finishing turns off the lights. (i.e. The core is clock gated.)



Entering Core-C6: Yes, I know it’s blatantly obvious, but I like talking to myself. As time proceeds, everyone leaves for lunch. Since no one is in the offices, we can shut things down even further in the rooms (i.e. power gating). Remember, though, that they are coming back after lunch so anything shut down must be able to be powered back up quickly.


Figure C A building full of diligent high tech workers, i.e. an Intel® Xeon Phi™ coprocessor



POWER MANAGEMENT: Package Auto-C3, Package Deep-C3 and Package C6

Now I’m going to stretch this analogy a little bit, but since it is fun, I’m going to keep on going.

Let’s expand this very creative analogy. Imagine if you will, a building with many rooms, 60+ in point of fact. See Figure C. Yes, I know that here in Silicon Valley, diligent high tech workers work in luxurious cubes, not stuffy offices. Unfortunately, the analogy breaks down at that point so I am sticking with communal offices.

Entering Package Auto-C3: Everyone has left the floor, so the movement sensor automatically shut off the floor lights. (I.e. the coprocessor power management software clock gates the Uncore and other support circuitry on the silicon).

Entering Package Deep-C3: It’s the weekend so facilities (i.e. the MPSS Coprocessor Driver Power Management module) shuts down the air condition and phone services. (I.e. the host reduces the coprocessor’s VccP and has it ignore interrupts.)

Entering Package C6: It’s Christmas week shutdown and forced vacation time, so facilities turns off all electricity, air condition, phones, servers, elevators, toilets, etc. (I.e. the host turns off the coprocessor’s VccP and shuts down its monitoring of PCI Express* traffic.)

POWER MANAGEMENT: Getting obsessive

Having fun with this analogy, I was thinking of extending it further into industrial campuses (a node containing multiple coprocessors), international engineering divisions (clusters with each node containing multiple coprocessors) and contracting with external partners (distributed WAN processing). Sanity and common sense prevailed and I leave the analogy as is.



For those of you with a passion for power management, check out the Intel® Xeon Phi™ Coprocessor Software Developer’s Guide. It has state diagrams and other goodies. I recommend sections 2.1.13, “Power Management”, and all of section 3.1, “Power Management (PM)” for your late night reading.

Another great reference is, “Intel® Xeon Phi™ Coprocessor: Datasheet”. Its URL is

You can find the three previous blogs in this series at:

NOTE: As previously in my blogs, any illustrations can be blamed solely on me as no copyright has been infringed or artistic ability shown.

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