Intel® Xeon Phi™ coprocessor Power Management Turbo Part 1: What is turbo? And how will it affect my horsepower?


This is the first of a series of blogs looking at Turbo: What it is and how it impacts software.

This series discusses basic concepts, terminology, how Turbo relates to thermal profiles, when Turbo is useful, and whether Turbo can impact application design.

This is a follow on to my previous series of blogs on power management.

  • ·         Intel® Xeon Phi™ coprocessor Power Management Pt 0: Introduction and inquiring minds
  • ·         Intel® Xeon Phi™ coprocessor Power Management Part 1: P-States, Reducing power consumption without impacting performance
  • ·         Intel® Xeon Phi™ coprocessor Power Management Part 2a: Core C-States, The Details
  • ·         Intel® Xeon Phi™ coprocessor Power Management Part 2b: Package C-States, The Details
  • ·         Intel® Xeon Phi™ coprocessor Power Management Part 3: An Intuitive Description of Power States Using Stick Figures and Light Bulbs


My plan is to continue this silliness with at least two more series with this power management theme:

(1)    Power management configuration: What can I do? How can I do it? And why would I want to do it?

(2)    A power management example: The gory and boring details.

I cannot guarantee that the titles will stay the same when I actually get around to writing the articles.


Turbo really doesn’t have much to do with horsepower. You are thinking about the “turbocharger,” a device that improves the horsepower of an internal combustion engine by increasing, through overpressure, the amount of fuel/air mixture ignited per stroke. Even so, the “turbo” state of an Intel processor is at least related in concept. In modern Intel processors, “turbo” is a way of briefly accelerating software beyond the design limits of the silicon. It does this by boosting the voltage and overclocking the processor. Does this remind anyone else of a P-state? It should.


Any modern processor has thermal sensors. The reason is simple. If the processor gets too hot, nasty things happen. If you’re lucky, the processor shuts down. If you’re unlucky, it will continue to work – sort of – and will fail at a crucial time resulting in the inevitably loss or corruption of very important data.

The basic idea underlying Turbo is that, if the temperature of the entire processor is cool, you can boost the performance of, i.e. overclock, the processor for a limited amount of time. This is possible because the adjacent silicon and packaging material acts as a heat sink. After the surrounding material heats up and no longer acts as an effective heat sink, the processor’s power management hardware drops the voltage-frequency back to a level of performance that the silicon can maintain indefinitely.


Before we get into further details, I need to make sure we are all using the same words in the same way. What follows is a list of processor and power management terms.

PMAX – maximum power dissipation recommended by the manufacturer (e.g. Intel)

TC (Case Temperature) – temperature measured at the geometric center of the processor package

TC-MAX – maximum Tc recommended by the manufacturer

Tjunction – temperature of a core

Tjunction-MAX – maximum allowed junction temperature before thermal throttling occurs

Hardware (HW) Threads – a HW execution context with its own registers, etc. Multiple HW threads often share the same pipeline. The OS usually sees a HW thread as a separate CPU.

Power Management (PM) Hardware – hardware circuitry that runs on the processor and implements the actual power management. Software controls some of this circuitry (e.g. P-state transitions). Much of it is invisible and not under software control (e.g. thermal throttling to avoid damage).

Power Management Software – software that runs on the processor and provides overarching control over the power management hardware. An example is the OS’s kernel power management module that controls P-state and C-state transitions.

Processor Package – the black package containing any silicon (e.g. cores), substrate, thermally conductive covering, pins, discrete components (e.g. capacitors), etc. Most people think of this black package as the “processor”.

SKU – The SKU is a product version. Though the same product (e.g. Intel® Xeon Phi™ coprocessor), different SKUs have different capabilities and price points.

Software (SW) Threads – what is traditionally thought of as an OS or application thread

TDP (Thermal Design Power) – Intel® provides this value to designers of systems using Intel® processors. It is not the maximum junction temperature or the maximum power the chip can dissipate. It is an average power dissipation for the processor when running typical consumer applications, such as word processors, spreadsheets and video playback. Its intent is as a guideline for platform designers (e.g. designers of laptops).

Thermal budget – the heat capacity available to all or part of a package before exceeding its design specifications

Thermal throttling – limiting the performance of the processor to reduce the Tjunction of its cores and so avoid permanently damaging the processor. This throttling often involves reducing the frequency and voltage of the processor (i.e. increasing the P-state).



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 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.