Re: No c2-c7 states on core i7

[Len Brown <lenb@kernel.org>]

> 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!

Yep, beware graphics card power.

> 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?

On the mobile parts they used to publish the power limits for C-states,
but I don't see these numbers in the core i7 datasheet...

However, many have done comparisons of what you might expect for
idle vs loaded power.  eg.
http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=3634&p=17

In general, I don't view it as a Linux bug that the MB's BIOS
disables c-states when it is over-clocked.  Indeed, it is
quite possibly a BIOS feature.

Len Brown, Intel Open Source Technology Center