i stumbled upon some infomaion i had forgotten about in my old and dusty digital library concerning the little known "laptop mode"
you will need to be running a 2.6 kernel, that means ethier OZ or PDOXROM betas, then again 2.4 might have support for this feaure as well, see if ls -l /proc/sys/vm/laptop* returns anything, if it does then you are in luck
you can get the files from here:
with no compiling needed, having make installed will help but i am sure you can work out how to install it without make
a guide to its use can be found here:
dont forget the hdparm -S 4 /dev/hda (i have mine set to -S 1) so that the drive turns off
i also recomend enabling the write cache on your microdrive, i have found that i lose about 10kb/s off the maximum transfer rate and cpu usage goes up a bit however there ****MAY**** be a good reason to using it and it is not going to cause any problems to enable its hdparm -W 1 /dev/hda
also heed the warning about syslog, turn off the automatic sync, if you are worried about crashees and want infomation on what went wrong on OZ then i recomend you learn the sysrq key combo to force a filesystem flush even if the kernel has hung to make sure no data is lost, it might not work in all cases
i havent done benchmarks but what do you have to losse hope someone can do 3 benchmarks, playing MP3s, watching a looping video and sitting there that should give us a good indicator to several loads from which we can work out battery life gains
another trick that can help out is to turn on read ahead with hdparm, the reason bieng tht if you listen to mp3s then there is a good chance that more of the song will go into memorey so the hard drive dosnt spin up as often
now to increses the cache for the above you can swap out part of some running prorams to disks (the ones that dont normally get called). to do this activate a swapfile somewhere and:
echo 100 > /proc/sys/vm/swappiness
this causes linux to be more agressive in what it swaps to disk leaving more mem free for caching. it also helps with programs that request more mem than they use and just sit on it.
one of the last tricks is to change the backlight brightness, it does help.
and last but not least, if you are running pdaxrom then get a propper cpufreq scaling daemon like cpufreqd (http://cpufreqd.sourceforge.net/) this one is the best and most flexible in my opinon, it takes a bit of effrot to learn the config stuff but it pays off in its ability to apply policies acording to remaining batter life, running programs (think mplayer) cpu load, AC power status and even temprature
if you are just playing MP3s using a decoder like libmad (ie low cpu usage intiger bassed) then try running the device at 100Mhz, you can get huge gains when you go below 100Mhz for some work loads (under wince i gained upto nearly an hour with an dell x30) and if you can you can try lowering the ram bus speed, but i suspect the best way to reduce power hungry ram usage is to not request as much data from it (ie be idle for longer, the data sheets confirm this) and to fiddle with the refresh rate (danger, can cause data coruption and requires a custom kernel)
anyway, these hacks and more should be intergrated with the openPDA project, as well as another one or two that cant be done with the xscales