New Stuff: Power Management

At this year’s IDF the biggest Nehalem disclosures had to do with power management.

Nehalem’s design was actually changed on a fairly fundamental level compared to previous microprocessors. Dynamic domino logic was used extensively in microprocessors like the Pentium 4 and IBM’s Cell processor in order to drive clock speeds up. With Nehalem, Intel has removed all domino logic and moved back to an entirely static CMOS design.

Nehalem’s architects spent over 1 million transistors on including a microcontroller on-die called the Power Control Unit (PCU). That’s around the transistor budget of Intel’s 486 microprocessor, just spent on managing power. The PCU has its own embedded firmware and takes inputs on temperature, current, power and OS requests.

Each Nehalem core gets its own PLL, so each core can be clocked independently - much like AMD’s Phenom processor. Also like Phenom, each core runs off of the same core voltage - the difference between Nehalem and Phenom however is Intel’s use of integrated power gates.

Through close cooperation between Nehalem’s architects and Intel’s manufacturing engineers, Intel managed to manufacture a very particular material that could act as a power gate between the voltage source being fed to a core, and the core itself.

The benefit is that while still using a single power plane/core voltage, individual Nehalem cores can be completely (nearly) shut off when they are in deep sleep states. Currently in a multi-core CPU (AMD or Intel), all cores need to run at the same voltage, which means that leakage power on idle cores is still high just because there’s one or more active cores in the CPU.

Nehalem’s power gates allow one or more cores to be operating in an active state at a nominal voltage, while remaining idle cores can have power completely shut off to them - without resorting to multiple power planes, which would drive up motherboard costs and complexity.

The other benefit of doing this power management on-die is that the voltage ramp up/down time is significantly faster than conventional, off-die, methods. Fast voltage switching allows for more efficient power management.

I mentioned earlier that the PCU monitors OS performance state requests, so it can actually make intelligent decisions about what power/performance state to go into, despite what the OS is telling it. There are some situations where Vista (or any other OS) running an application with a high level of interrupts will keep telling the CPU to go into a low power idle state, only to wake it up very shortly thereafter. Nehalem’s PCU can monitor for these sorts of situations and attempt to more intelligently decide what power/performance states it will instruct the CPU to go into, regardless of what the OS thinks it wants.