Coretemp seems to be off quite a bit

[Jeremy Chadwick]

On Tue, Oct 07, 2008 at 02:25:17PM +0100, Frank Shute wrote:
> On Mon, Oct 06, 2008 at 09:39:40AM -0500, Kirk Strauser wrote:
> >
> > I have a Gigabyte motherboard with an Intel ICH-9 chipset, and a  
> > 3.0GHz Core 2 Duo (E8400).  The coretemp sysctls seem to always show  
> > 50C as the baseline temperature:
> > 
> > $ sysctl dev.cpu | grep temp
> > dev.cpu.0.temperature: 50
> > dev.cpu.1.temperature: 50
> > 
> > This is with a big PSU fan, a good CPU fan, a clean heatsink, and two  
> > case fans aimed the right direction (front fan pulling cool air in,  
> > rear fan pushing warm air out).  If I reboot and go into the BIOS, I  
> > get numbers around 42-43C.  I know it's kind of hard to compare  
> > directly, but the coretemp numbers are from a totally idle system with  
> > powerd scaling it back to 373MHz, so it should be as cool as when  
> > sitting idle in the BIOS screens.  When I work the system hard, like  
> > running "make -j4 buildworld", I see temperatures up around 63-64C,  
> > and I'm almost positive that's not right.
> > 
> > Any ideas why coretemp and the BIOS would show such different numbers?
> 
> To add some numbers, I've got an E6550 on a Gigabyte GA-P35-DS3L
> (ICH9) and I get:
> 
> $ sysctl dev.cpu | grep temp
> dev.cpu.0.temperature: 24
> dev.cpu.1.temperature: 28
> 
> Ambient room temp: 23°C
> 
> That's running powerd and the machine idle, standard heatsink/fan
> combo, 1x12cm case fan.
> 
> Your's might run hotter (higher clock speed? Mine: 2.33GHz) but I
> wouldn't expect it to run *so* much hotter.
> 
> I'd expect your numbers to be right if all the ACPI stuff is working.

Clarification here is needed:

coretemp(4) has nothing to do with ACPI.  It gets thermal statistics
from the processor by talking *directly* to the processor with specific
opcodes, the results returned in specific CPU registers.  It does not
rely on ACPI.

-- 
| Jeremy Chadwick                                jdc at parodius.com |
| Parodius Networking                       http://www.parodius.com/ |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.              PGP: 4BD6C0CB |