Saturday, July 04, 2009

Cooling, Undervolting, and Greening Gimli

So here's the best answer so far. I don't want to make an unsightly hole in my computer case's side wall (see prior post), but I need to get air flow under control for best cooling my Core i7 920 chip. The solution is to use some 4-inch plastic air hose with wire reinforcing to direct supply air from one of the two front air inlet fans (lower right on photo) to the CPU cooler fan. This minimizes the "blow back" of hot air from the CPU cooler outlet to the cooler inlet. This solution still works well when the case is buttoned up. We don't have to worry about the acoustic and radio frequency noise that would leak out through any new holes.

The hose is available at hardware stores for clothes dryer connections.

So, the airflow and CPU cooling is about as good as it is going to get without moving to a more exotic chip cooler scheme. The next step toward controlling operating temperature is to minimize CPU power consumption. My philosophy is to keep the specified CPU performance (i.e., clock rate), but to reduce operating voltage to a safe minimum. This is called "undervolting".

(Many computer tinkerers want to get the highest performance possible from their chips, e.g., by "overclocking". This generally requires operating at higher voltage which in turn means more power dissipation, bigger and noisier cooling systems, etc.)

I devised a simple Python program that loads all 8 "processors" (4 cores x 2 hyperthreads/core) for a specific interval and measures the temperature rise. This is a good test of the cooling system and the power level of the CPU (as a function of core Voltage, clock rate, etc.) It is not meant to be a typical application program, just a source of heat. (Actually, it is such a trivial program -- a tight loop -- that it causes more power drain than typical real programs.)

The standard CPU core Voltage (Vcore) is 1.29375 V. Our Gigabyte GA-EX58-UD4P motherboard provides great flexibility in selecting all kinds of CPU parameters, but I am mainly varying Vcore. Running the test program, I observe a final case temperature (Tc) of 65 C and mean junction temperature (Tj) of 75 C. The AC supply power is about 204 W under load, and 98 W at idle.

I tried successively lower Vcore settings. At Vcore = 0.91250 V, the computer would begin the BIOS display at boot, but would fail before it completed. At Vcore = 0.95000 V, it booted partly into Linux, but it failed before getting to the Ubuntu login. At Vcore = 0.97500 V, we had a successful boot. The supply power was 150 W under load, 97 W at idle. Final temperature was Tc = 53 C, Tj = 57 C.

It is not clear how stable the system would be at 0.975 V under all conditions (room temperature, computing loads, etc.), so I raised the voltage a couple of steps to Vc = 1.01250 V. At this voltage, we ended with Tc = 53 C and Tj = 58 C. AC power under load was 155 W; at idle, 97 W.

From Intel's data, it is not clear what the ultimate "safe" operating temperature of the i7 920 chip is. As we said earlier, it comes down to a subjective judgement about how much stress is acceptable. Less is always better from the standpoint of system stability and longevity. Some of Intel's charts suggest that Tc = 68 C is an acceptable maximum. In that case, our operating maximum of 53 C should be fine. (Note that we must allow for ambient temperature changes. Our measurements here are at Tamb ~ 20 C, but our ambient could rise to ~ 30 C on a hot day.)

For now, the engineering is "done", and we can go back to BOINC running all day without fear. We have also trimmed about 25% from our power bill.
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