Save Power 4: Dynamic Voltage and Frequency Scaling (DVFS)

Looking at the derating graph for the standard cell libraries, we see that in the operating region, the frequency/voltage curve is roughly linear. CMOS delay is inversely proportional to supply voltage.

Logic with higher-speed capabilities is smaller which means it consumes greater leakage current which is being wasted while we are halted. Also leakage energy is proportional to supply voltage (or perhaps sublinear with exponent 0.9ish : as we raise voltage, transistors are indeed turned off more, but P=IV is still increasing).

Voltage to a region may be varied dynamically. A higher supply voltage (at fixed f) uses more power (square law) but allows a higher f. Operating region of the frequency/voltage curve is roughly linear. But, logic with higher-speed capabilities is smaller which means it (generally) consumes greater leakage current which is being wasted while we are halted. Let's only raise VCC when we ramp up f: classical DVFS.

For fixed task size, energy use is proportional to V squared, so the DVFS ideal method (for predictable, real-time tasks in low-leakage technology):

1. Adjust clock f for just-in-time completion (e.g. in time to decode the next frame of a video),
2. then adjust VCC so logic just works.

Hence, we aim to clock at the lowest suffcient voltage and clock frequency and for the activity ratio as high as possible and minimal halt cycles.

In general, ramping voltage up linearly with clock frequency (f) results in dynamic power consumption with a cubic dependence on f. But work may be bursty, so DVFS is applied (e.g. by a laptop governor).

For all loads, this DVSF approach achieves peak performance under heavy loads and avoids cubic overhead when idle.

In a server farm processing blade we may be thermally limited, so DVFS will be throttled back by rack-level governors or Intel's RAPL.

Minor caveats (non-examinable):

Combinational logic cannot be clock gated (e.g. PAL and PLA). For large combinational blocks: can dip power supply to reduce static current when block is completely idle (detect with XORs).

So a typical SoC uses not only many dynamic clock gated islands, but also some sub-continents with automatic frequency and voltage variation.

Power isolation originally used on a longer and larger scale (complete continents) but now a lot of power islands are being used.

It is possible to locally and quickly adjust supply voltage with a series transistor - but wasteful compared with an off-chip switched-mode regulator.

An off-chip power supply can be efficiently adjusted, but limited to only a few voltage islands and tens of milliseconds inertia.