In article o.uk,
"Dave Liquorice" writes:
On Sun, 6 Sep 2009 09:19:06 +0100, PeterC wrote:

As for the power monitor: at first my PC was shown as flicking between
80 - 90W, which is about the same as on the above MM. Turning off the
monitor dropped it to 40 - 50W, which is about right for a 35W monitor.
Next time I turned on the PC it was 190W! The monitor dropped it by
~100W - not bad via a 12V 3.3A PSU.

Did you let it settle or was that the first reading after the switch
on. OK SMPSU's shouldn't have much of an inrush compared to a motor
or transformer based PSU but there will be a bit.

Actually, they are often enormous spikes, but far too short
to be visible on a power monitor or to care about the cost.

If the PC were to run at 90W+ the CPU would fry as it's 65W TDP and is
very rarely at 50%.

The CPU load does affect the power it takes but I'm not convinced
it's a great deal and there will be other stuff taking power like the
north and south bridges, graphics, and memory. Still it does look a
bit odd.

On modern PC systems, CPU can be quite a significant consumer
of power. Also, OS's often don't load or start power management
software particularly early on, so you may well have CPU cores
falling back to a default idle loop spin initially. Later in
startup when an OS checks for CPU-specific modules to load, and
selects more appropriate modules optimised for that particular
CPU rather than generic defaults, then you may get power reduction
if the OS decides it can HLT in the idle loop, and even more so
on modern processors where the scheduler may shut down cores when
it doesn't have enough runnable threads to keep them all busy.
(This is important for the likes of Nehalem where closing down
cores you can't currently make good use of gives extra thermal
headroom to the remaining core(s) and increases the chances of
them being able to run in turbo mode, where they are deliberated
overclocked.)

Also, many BIOS's don't seem to do anything to save power when
the system is sitting in the BIOS itself, and this can result in
peak power draw when you're in the BIOS before the system loads,
and it's just spinning CPUs in its idle loop.

I happen to have a true power meter on an old PIII at the moment.
When switched on, it's about 60W, and stays there whilst you roam
through the BIOS screens, and when you boot the OS (Solaris x86
in this case). As Solaris boots, at a certain point when it cuts
across to Solaris's scheduler, the power consumption drops to 20W
when idle.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]