Originally Posted by
rge
I have not put this pic up before but i just sent to unclewebb. It is testing of a pentium northwood which has a higher gradient than 65nm (65nm has more copper bands in silicon die and better die attach, etc). But it illustrates the point of idle vs load. It is from yang et al, who has done some testing for intel as well.
Ignore the solid grey top lines, it is software estimating the die temp of a pentium northwood by estimating power usage/leakage, and then converting to temps.
The red scribble is mine.
-Ambient at idle = bottom red arrow (bottom broken line).
-Thermocouple in IHS at idle = middle red arrow (middle broken line labeled spreader by authors, temp was double checked with FLIR for accuracy).
-CPU diode between cores at idle = the top red arrow (top broken line labeled On-die by authors)
-There is no DTS sensor. The solid grey lines above diode is software guessing hot spot temps mathematically. At idle DTS same temp as cpu diode (confirmed on 65nm intel testing at idle, there is no offset between those at idle)
At idle (blowing up bitmap with ruler)
-IHS is 3.5-4C higher than ambient.
-The cpu diode (and DTS if had one) is 3C higher than IHS (compare this with intels 5C gradient IHS to tjmax).
-So the cpu diode (and the DTS on die) are 6-7C higher than ambient, at stock clock, stock volts, with stock heatsink. (But northwood is hot cpu, less copper bands on die, less conductivity, worse die attach. But power density is higher on 45nm so, again left to guess for 45/65, but that gives you a ballpark.)
At load
-5-7C difference between IHS (spreader) and cpu diode (on die), and that is constant on load for most programs.
-There is no DTS sensor, each grey solid line are software guessing hotspots in different die locations.. DTLB, L1, etc, you can see the varying temp gradients. (But intel did actual die sensor measurements on 65nm, and found lower gradients, but also 65nm would have lower gradients because of improvements and increased copper bands.
So at idle with EIST/speedstep enabled, or undervolted, I would still be using 5-7C above ambient for stock cooling. For different cooling computronix had a pretty good formula that worked for mine. On water I calibrated my cpu diode 3C above water temp, which came to 3C above IR reading at DTS=0, at idle, underclocked...close enough.