Power Efficient Software Coding

Question

In a typical handheld/portable embedded system device Battery life is a major concern in design of H/W, S/W and the features the device can support. From the Software progra

Answer 1

To avoid polling is a good suggestion.

A microprocessor's power consumption is roughly proportional to its clock frequency, and to the square of its supply voltage. If you have the possibility to adjust these from software, that could save some power. Also, turning off the parts of the processor that you don't need (e.g. floating-point unit) may help, but this very much depends on your platform. In any case, you need a way to measure the actual power consumption of your processor, so that you can find out what works and what not. Just like speed optimizations, power optimizations need to be carefully profiled.

Answer 2

Look at what your compiler generates, particularly for hot areas of code.

Answer 3

On Linux, install powertop to see how often which piece of software wakes up the CPU. And follow the various tips that the powertop site links to, some of which are probably applicable to non-Linux, too.

http://www.lesswatts.org/projects/powertop/

Answer 4

Well, to the extent that your code can execute entirely in the processor cache, you'll have less bus activity and save power. To the extent that your program is small enough to fit code+data entirely in the cache, you get that benefit "for free". OTOH, if your program is too big, and you can divide your programs into modules that are more or less independent of the other, you might get some power saving by dividing it into separate programs. (I suppose it's also possible to make a toolchain that spreas out related bundles of code and data into cache-sized chunks...)

I suppose that, theoretically, you can save some amount of unnecessary work by reducing the number of pointer dereferencing, and by refactoring your jumps so that the most likely jumps are taken first -- but that's not realistic to do as a programmer.

Transmeta had the idea of letting the machine do some instruction optimization on-the-fly to save power... But that didn't seem to help enough... And look where that got them.

Answer 5

Do your work as quickly as possible, and then go to some idle state waiting for interrupts (or events) to happen. Try to make the code run out of cache with as little external memory traffic as possible.

Answer 6

From my work using smart phones, the best way I have found of preserving battery life is to ensure that everything you do not need for your program to function at that specific point is disabled.

For example, only switch Bluetooth on when you need it, similarly the phone capabilities, turn the screen brightness down when it isn't needed, turn the volume down, etc.

The power used by these functions will generally far outweigh the power used by your code.