Power Management

Overview

Compared to its predecessors the Core i7 comes with a power control unit (PCU) which is able to shut down the cores. The advantage is obvious: leakage in indle state can be reduced and therefore also energy comsumption. Futher the PCU - which by the way contains as many transistors as a whole 486 CPU - can also deactivate the uncore regions which reduces the power consumption even more.


Minimizing the power consumption in idle-state

On principle the following three circumstances generate dissipation power: processes which run a high frequency cause leakage, high frequency needs high performance referring to golbal clock distribution as well as the logic and the local clock distribution are responsible for the power consumption of a CPU.

Power consumption can be minimized when the operating system tells the CPU that no task is ready for execution. The CPU now goes in to idle mode or a so called C-State. There it waits for futher instructions.

Before Nehalem the following C-states were available:

• C0: CPU ist aktiv
• C1, C2: stops the pipeline and most clocks
• C3: stops the residual clocks
• C4, C5, C6: lower the core voltage/frequency and reduces leakage

Integrated Power Gate

To reduce the voltage of every single core the so calld power gate is needed. This is in other words a switch between VR-output and the current supply. The advantages of this are the following:

• very low on-resistance
• very high off-resistance
• Current can be provieded much faster
• makes C6-state (current regulation) per core possible
• dissipation power near zero

The integrated power gate gives the option to reduce the power consumption in idle state to near zero watts. This can also happen for every core independantly.

Further also the memory subsystem draws less power because the memory clocks can be reduced during requests and low load. Even the QPI links draw less power because they are less active. A similar behaviour is showed by the PCI Express links.

Summarized one can say that the power consumption of the whole platform is going to be lower.


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