Taken for Granted

ESL, embedded processors, and more

“Fight the Power!” – No, “Fight the Energy!”

Filed under: Uncategorized — September 30, 2011 @ 1:43 am

For years I have observed a simple confusion between power and energy leading designers to jump to odd conceptions about design alternatives.

Power consumption is expressed in units such as milliWatts (mW) consumed over some suitably short interval of time.  To quote our modern oracle, Wikipedia,

In its technical sense, power is not at all the same as energy, but is the rate at which energy is converted (or, equivalently, at which work is performed)

In evaluating designs, a unit such as milliWatts per MegaHertz (mW/MHz) allows one to normalise different designs – assuming one can run two designs at the same rate (MHz), then the one that consumes more instantaneous power per MHz can be thought of, in some sense, as more “power-consuming”.

But this is a rather crude metric.  Not all milliWatts are the same.   Design A may actually accomplish more computational “work” than Design B even if it consumes more power.  If Design A does double the amount of computation than Design B but consume only 10% more power, then for a given set of computations (e.g. to process a frame or token of data according to some algorithm), running the algorithm on A will take fewer cycles or time periods than on B.  In fact, 1/2 the number of cycles.  Since each cycle on A consume 10% more ‘power’ than B, then in terms of energy, A will consume 55% of the energy of B (0.5 * 1.10).   Thus A is more efficient than B.  We assume of course that when not doing useful work, these devices are powered down in some way and consumption drops to near zero.

If devices are plugged into the mains, then consuming more or less energy is a cost, but possibly not a huge cost (e.g. running the computer I am using right now is a cost, but not very large).   Of course, running Google’s data centres requires a large amount of energy and efficiency is all important there.

If devices consume a high level of instantaneous power (peak power), then running them without melting may require expensive packaging, expensive (and energy consuming) cooling, or a combination of both.  So for the devices running the large servers in the data centres, and the processors in desktop machines, peak power is also quite important.

But when we turn to mobile device design, such as for smartphones, tablets and all kinds of battery powered devices, energy trumps power every time.  Making the assumption that peak power for the designs is kept controlled (a big challenge in itself) and does not require exotic cooling or expensive packaging, what is key in these devices is energy:   power consumption multiplied by the time required to play a piece of music or video or read several pages of a book or to look up something on the web.  When a device is battery powered, and peak power is kept under control, it is energy consumption that determines how long the battery lasts in typical use scenarios and how often you have to tether the device to the wall for a new charge of soma.

So in mobile, while mW/MHz is one useful metric, it is not the key one.  Efficiency of how you use the mW is key to lowering joules per song, or joules per video, or whatever else is draining your batteries.

Fight the Power?  Yes, to some extent, but in mobile “Fight the Energy!”

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