[ltp] Power saving: self-compiled vs. binary

[Laurent Gilson]

Hello,

> For sure both systems have their own advantages and I do not want to  
> start a new flame-war.

I think thatīs too late now...

> The question I am interested in is whether there are significant  
> advantages concerning power saving when you compile your system from  
> scratch with optimization flags for your CPU or not.

It depends on you software. Things like the kernel, office, webbrowsers,  
email-clients... and so on do not use advanced stuff. There are no  
commands in sse2&Co that may be used for these. Audio/video-players,  
3D-games/simulations, ... do use these extentions.

And it depends on the CPU-design:

1. Not all parts of a cpu are optimized in the same way. Are you sure the  
sse-unit is optimised for power ? Or has intel optimized it for speed ?

2. If i cpu does not need some parts, it can switch off these.

3. Not all extentions really exist in-silico. Most older fp-extentions are  
provided by the normal fp-pipeline. Look at AMD docs (intel is not  
publishing any information about the internas). So the transistors at work  
are the same for older, non-sse code.


And on the laptop: a undervolted, underclocked CPU consumes way less  
energy then ... the display or the WLAN.

> What do you think about that?

The energy needed to recompile office/kernel is way, way higher then the  
overall change in energy consumption.

But itīs OK for audio/video, 3D,... (since the speedup means lower  
execution times <=> more time in C1/2/3...).

cu