[International] Power Bits: Hidden Energy Costs
By Ed Sperling

 Memory is everywhere. In fact on most SoCs it accounts for the majority of the chip. But what exactly does it cost in terms of energy?

It all depends on where that memory actually is, but the general rule is that accessing memory requires about the same amount of energy as doing a complex computation, according to Paul Franzon, a professor of electrical and computer engineering at North Carolina State University.

In a presentation at the Berkeley Symposium on Energy Efficient Electronic Systems, Franzon detailed the energy costs of various functions on an SoC compared with a complex computation, which for decades has been the benchmark in computing devices. He said that I/O requires about the same energy as a complex computation, but energy storage requires more because it has to be turned on and off.

In the stacked die world, a TSV with ESD protection requires about three times as much energy as a TSV without ESD protection. And if communications is vertical rather than planar, there can be up to a 60% memory power savings.

So how much energy can be saved using a variety of techniques? Here are the breakdowns:

- Shorter wires: 2% to 13%.
- DRAM on logic: up to 38%.
- Trading area for power: up to 20%
- Heterogeneity: 15%+
- Re-architecting the system: 5% to 60%.

Source: Low Power Engineering Community