Somewhere on teh interweb Jan Knutar typed:

~misfit~ wrote:

SETI BOINC seems to throttle on time. For instance, I've got my
preferences set to 75% of both CPU cores. It seems that it works on
quarter-second (or similar) chunks of time, 75% being 100% load for
3/4 of a second, then 0% load for the other 1/4, resulting in 75%
average over the whole second.

There's actually no other way to make a CPU hog consume X% of CPU
other than to sleep for a certain amount of time and then work again
for a certain amount of time. Having 1s switching time sounds a bit
coarse to me though, especially since it leaves enough time for the
power saving features of the CPU to try adjust.

That was happening with my machine. Using RightMark CPU Clock Utility in 
monitor mode only, when I had SETI-BOINC using less than 100% of CPU the I 
could see that the CPU was dropping from 11x multiplier to 6x every second 
or less. Through experience I've come to appreciate that things break the 
most when either they're switched on and off a lot of they're 
thermal-cycled. As much as I like to support SETI I wasn't about to do it if 
it was going to shorted the life of my hardware. I'm on an extremely limited 
budget and always try to but with longevity in mind. It'd be a shame to 
off-set that by risking stress-related early failure.

thermal cycling isn't doing my CPU any favours either. Why can't
these code-monkeys write code that simply uses 75% of the CPU
constantly and smoothly rather than just switching between 100% and
0% load?

Everything that runs on the CPU essentially runs at 100% when it
runs. A word processor will occupy the cpu 100% for blinking the
cursor. It's just that the amount of time it spends on the CPU to
turn the cursor off or on is so small, so that in order for any
measurement program to see it as 100% it would have to have an update
rate of a few hundred thousand times per second...

A media player playing your favourite mp3 goes to sleep and asks the
OS to wake it up when there's room in the sound buffer again. The OS
gets notified through an interrupt by the sound chip when it has
played a buffer fragment. The OS wakes up the media player, which
will consume 100% CPU until it has decoded more sound and filled up
the sound buffer again. This probably takes less than a millisecond
though. It might also not have had enough of the mp3 in memory, in
which case it will have asked the OS to give it more data from the
harddrive. The player is forced to sleep again, until the
IDE/Sata/USB/etc controller fires an interrupt telling the OS that
the data it requested is now available, the OS wakes up the player
again and feeds it the requested data, and the player consumes 100%
CPU until it does something again which makes it sleep. CPU usage is
simply an average of how much time is spent sleeping and how much
time is spent running.

You might ask why the programmers didn't choose to switch between
working and not working more frequently? There's a compromise, the
quarter second boinc isn't doing anything, the CPU can spend in a
reduced power state. If boinc was switching 100 times per second,
which would probably be fast enough to make all sorts of measurement
and monitoring programs show a "constant" 75%, then CPU would use
more power since it would only have about 25ms at a time to spend in
a lower power state. Transitioning between the different power states
takes time and power too, so the power saved would be less.

If you want the cycle to be much shorter than 100ms you'd have to
request the OS to use a faster timer interrupt, which also increases
power consumption.

Thanks for the detailed explaination. This is how I remember this group from 
a few years ago. :-)

I guess it's up for debate whether the slower switching causes more
thermal stress on the computer compared to faster switching... The
best solution, as was suggested later in this thread, is probably to
let it run 100% daytime and switch it off for the night, if the
cooling of the computer at all can handle full load.. :)

I have since set it to 100% (both cores). The CPU runs at 44�C constantly 
(love that) in a 20� room. RAC is starting to climb nicely too.

Thanks,
-- 
Shaun.