weird interrupts

[Arjan van de Ven]

Kalmár Dániel wrote:

> I have 300-700 wakeups per second, which seems really high. I have no idea why, even if I switch everything off, and nothing shows more than 5 interrups in the detailed list, I still get 300 wakeups per sec. Any idea why?

something might be doing DMA to your cpu, like USB or wireless

> 
> I've read somewhere that JFS uses 30% less CPU and disk power than ext3 or reiserfs. Is this true? I've converted to JFS a few days ago, but I'm not sure if there's actually any difference.

I haven't looked into this.
> 
> Does turning off one of the CPUs (hotplugging works on my core 2 duo) save power? I did some tests and it seems like it doesn't affect power usage at all (but the 2nd core is reported to be turned off by the kernel).

unfortunately that doesn't seem to actually help at all; or worse, it 
often seems to hurt performance. I've not played with it much yet,
it's on my list somewhere but there are other things there as well ;)

> 
> Does lowering the cpu frequency save a lot of power? 

it saves power for not-idle; when you're idle it won't save you much 
power.

> My default setting is the conservative governor, but when I run the laptop on battery I usually set the frequency to the lowest possible constantly. Should I keep doing this or just stick to conservative?

> 
> Some people on this list said that ondemand is better for intel CPUs. I know that it's better performance-wise (as it switches to the highest possible frequency faster), but is it better power-saving-wise?

it's better power wise as well; it's a bit complex to explain, but 
it's better to execute the code you need to execute at full speed, and 
then really quickly go idle, than it is to execute at a much lower speed.

Maybe a simple example (I plucked these numbers out of the air, they 
don't represent any real cpu that exists) will help:
Say you have a cpu that consumes 40 Watts at full speed, and 30 Watts 
at half speed, and 4 Watts when idle.
You have something to do, lets say mp3 decoding of 1 second of audio, 
and that takes a full second at half speed, and one second at full speed.

At full speed decode + idle, that is half a second at 40 watts (20 
Joules) and half a second at 4 Watts (2 Joules); total is 22 Joules.
At half speed decode, that is a full second at 30 Watts, so 30 Joules.

So, what ondemand does would cost 22 Joules, while a "hit the exact 
frequency" governer would cost you 30 Joules.....