Re: No c2-c7 states on core i7

[Jindrich Makovicka <makovick@gmail.com>]

On Mon, 23 Nov 2009 20:26:53 +0100
Erik Slagter <erik@slagter.name> wrote:

>  > 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.

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.

-- 
Jindrich Makovicka