Re: No c2-c7 states on core i7

Erik Slagter
2009-11-23T19:26:51Z

> No, that isn't normal. C-states on modern processors generally
> save a lot of energy.
>
> If you run powertop and you find that you are over 99% idle
> and you save no energy compared to when you are 0% idle
> (say a copy of "cat /dev/zero> /dev/null" for each core)
> then something is wrong with your system.

I just removed a large story here, I guess the table from my other
message is a lot more informative.

> It is possible, but as soon as you reverse engineer and over-ride
> something in the BIOS, you are on very thin ice. Presumably
> the BIOS engineer made a concious decision to disable C-states
> when you over-clock your board and had a reason to do so.

As long as nothing gets fried that's no problem for me.

> Maybe the more important question is what measurable benefit
> you get when you over-clock your board, and if you really need that...

I can understand your doubts on this matter, but I think I do have a
legitimate reasoning. This is a server that almost all of the time does
next to nothing. Load 0.02 or similar. It needs to be running 24/24
though because it receives e-mail and answers the telephone. So that's
why I want it to be low on power usage. On the other hand I need to
transcode movie clips to h264 very regularly. I can use every 4*2 core
for the process using x264 and indeed it works very fast. Also I noticed
that every mhz higher clock means shorter encoding time, all
(virtual-)cores get completely loaded. The "normal" speed of the 920 is
2.8 Ghz (or in fact 2.63 Ghz) and a change to 3.4 Ghz really does make a
difference in encoding speed, theoretically 30%, in practise even more.

Intel would really make me happy if they would design a processor with
four or more cores and a chipset that would implement C7 and also could
scale from 100 Mhz to 3.6 Ghz, in two or three steps, I wouldn't mind,
and then would use something like 20 watts in idle, like my laptop does.