Power hunger

Today I read an article on power demands in the datacenter. The issues discussed in the article are similar to those I was able to discuss with Sun customers at the Sun User's Performance Group (SUPerG) conference in Washington, DC recently. But it reminded me also of my early career...

In the summer of 1981 I worked for NASA at the Marshall Space Flight Center in the Spacelab Systems Division. One of my jobs was to analyse the Spacelab-1 mission timeline for periods of maximum power usage.

Spacelab is a collection of laboratory modules which fit in the shuttle orbiter payload bay. The shuttle has 3 auxilliary power units (APUs) (fuel cells) which each can produce 7 kiloWatts (kW) of power. Two APUs are available to the payload bay. Though a single APU provided enough power to return the shuttle safely to earth, mission rules required that all three APUs be functional to run spacelab. In other words, if we lost an APU, we would sacrifice the scientific experiments, and not endanger the crew. With our power budget set at 7 kW, we needed to know whether we would break the budget during a spacelab mission.

Spacelab was a multinational effort to create a reusable laboratory in space. This was during the lull between Skylab and what became known as the International Space Station. The basic idea was to put a bunch of 19" racks in a climate controlled environment and allow scientists to manage and run experiments. Prior to Spacelab, spacecraft were very much custom built and experiments were crammed into corners and crevices as room was available. Thus, each experiment was custom built for the mission it flew on. Incidentally, we did fly one electrophoresis experiment on the shuttle which originally flew on an Apollo mission; we basically strapped it in a crevice in the shuttle mid-deck. The promise of using standard 19" racks for housing experiments theoretically reduced the cost of creating experiments and thus would allow many more scientists to fly experiments than ever before. To further extend the opportunity, Spacelab was a joint effort between NASA and the European Space Agency (ESA). Thus Spacelab was a globally accessible space-borne laboratory. Cool vision.

Of course, the details of making all of this work was left to us engineers. The very first Spacelab-1 mission was quite a diverse collection of experiments from all over the world. And it all needed to be integrated. My division was responsible for this integration and we performed much of the analysis needed to ensure overall mission success.

From a power perspective, some of the experiments didn't consume much juice. Others were ovens (literally) and required lots of juice. Given the 7 kW power budget, we had to ensure that all of the power-hungry ovens were not turned on at the same time. Once we solved that relatively easy scheduling problem, we had to prove that at no time during the mission would the dozens of active experiments cause us to break the power budget. This was a perfect job for a computer. Herman Hight, another Mississippi State grad, and I wrote a FORTRAN-4+ program which would analyze the total power consumption of a Spacelab mission at all points in the mission timeline. This is mildly difficult and involves some additional fudging because some of the experiments were started by an astronaut flipping a switch rather than some automated start process. Hence, there were times which we identified where an astronaut could be early or late to start an experiment and blow the budget. These risky times were identified and fed into the mission experiment tests to be performed by the actual crew while still on the ground, just to work out any potential issues.

7 kW today seems like such a little amount of power. A rack full of 1U servers could easily consume 20 kW. A "power desktop" may approach 1.2 kW of real power. The art and science of power budgeting was largely lost during the 1980s-1990s in the computer business. Today, power analysis is perhaps the most important factor in designing datacenters. And it is for this reason that Sun is putting so much emphasis on reducing the power needed by making more use of the computing resources. In particular, chip multithreading (CMT) allows significantly better utilization of the pipeline. If we can replace a dozen dual-core 120 W processors with a single 80 W processor, then the power usage goes way down. And I think that is a good thing.


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