Originally posted by dete:
Cool thermal management stuff:
"Thermal management isn't really about cooling, it's about noise."
(I'm going to assume that everyone knows the basic details: 9 fans, 4 zones, etc.)
The power supply zone has two fans which run at constant speed. The power supply runs along the bottom of the case and is very thin [probably similar/same to PS in Xserve].
The drive bay fans are controlled based on the exhaust temperature.
The PCI/AGP zone fans are controlled based on the power draw of the cards! They couldn't know where to put a sensor because any one of the cards could be hot, so they just count on the fact that the heat output is equal to the power draw.
The speed of the CPU fans is based on the internal temperature of the chips themselves. The four fan design (two push, two pull) significantly reduces noise since a vast majority of fan noise is due to back pressure. The two pull fans run in tandem, but each push fan is individually linked to one of the CPUs. In a single CPU system, there are only three fans (one push, two pull).
Normally, the machines are running at about 2/3 their total clock speed (for 2GHz machines, this is 1.4GHz), this jumps up to the full speed whenever it's required. The ramp time up or down is ~1ms, but the CPU is running normally during this time, so there is no performance "hiccup". This results in about 60% power/heat savings, which jumps up to about 85% savings if the machine is idle and they "turn on other power saving features". When idle, the CPU fans are barely turning.
The fans have Xserve-like monitoring. They know how much power they are putting into the fan and how many RPM they are running. So, if the fan fails, they know immediately. They also can warn the user if the fan is running more slowly than it should based on the power input (indicative of pending failure).
Tidbits: If you take the plastic air-channelling liner out of the machine, it will go to sleep and won't wake up until the liner is replaced. If the CPU fans are going full-blast and the CPUs are still over-heating, they will throttle back the CPU speed, or even go to sleep, rather than risk blowing the chip. (This is only likely to occur if you are exceeding environmental tolerances, i.e. it's in a stupidly hot room.)
During the Q&A session, Peter Sandon (thx MrNSX!) was asked about other power saving features. His response: "I can talk more about that in a couple of months."
Whew! Three long posts from a single session.