Re: No c2-c7 states on core i7

Erik Slagter
2009-11-24T13:55:06Z

> Most of the overclockers disable C3/C6/C7 anyway for stability reasons,
> probably because sudden changes in core voltage disturb the CPU
> operation. My i5 Lynnfield desktop was actually unusable with C3/C6/C7
> even with default settings (fsck reporting random errors etc.). I
> ended up disabling the higher C-states and just raised the Base clock
> so I do not need the Turbo mode. As long as you do not disable C1,
> the difference in idle power consumption is rather small.

This is very interesting.

I really would not want to disable the higher C states because according
to intel marketing, but also according to tech sheets these promise to
save a lot of energy by progressively completely turning off parts of
the core(s).

What you describe is actually exactly what I see, power usage differs
greatly from C0 to C1E (between 150 and 100 wats, 50%...), after that,
in higher C states, the power usage doesn't change that much.

It seems I have a combination of a very stable power supply and a very
stable motherboard because I have all features enabled that make the
power supply fluctuate (c3-c7, cpufreq), and even more, until yesterday
I used to use overclocking to 3.2 Ghz with very slightly raised cpu
voltage, that combination has been running for months now without any
flaw or any related kernel message.

Yesterday I raised the clock to 3.4 Ghz and I set all voltages to
"normal" and it seems to run fine. I will have to check under really
heavy load yet, I will do that tonight. My motherboard has an option
"power line load calibration", I guess that has something to do with it :-)

Interesting: I pulled out a "modern" (but cheapo, no fans) pcie vga card
and replaced it with a very old pci card (I guess from 1996) and that
saves me 20 watts! That's more power than I problably ever could save
using C states!

BTW on my ancient athlon-mp board, I could enable c2/c3 (although not
supported by the bios) and that would save about ~40 watts of power. But
it looks I won't see such a difference on modern CPU's, maybe because of
the dynamic frequency scaling and enhanced c1? Is there anyone who can
give me an indication of what to expect in powersavings by c2-c7 on
modern cpu's?