Tuesday, July 8, 2014

Kilowatts, Control Cables, and Cooling

The Retro-Computing Society of RI is located in the Atlantic Mills in the Olneyville neighborhood of Providence. We have a 1,500 sq. ft. facility in a mixed-use complex that was originally a worsted mill during the industrial revolution. It suits our needs quite well with amenities like a loading dock and a freight elevator. As I described in the previous installment, the system weighs more than a half ton. It was surprisingly easy to move as it has well designed casters that allow it to be smoothly rolled around. Now that it has arrived we need a plan to plug it in. Our electrical panel was a bit under-powered for running the Cray.

The back of the Cray
The back of the J916 with the two cabinets bolted together. Each cabinet has an AC power entry box which we had removed to prevent damage to the power cord during the move.
The system requires three-wire, single-phase, at 240 volts. A fully configured Processor Cabinet could draw a maximum of 4,200 watts while a full Peripheral Cabinet could draw up to of 3,600 watts. Our J916 is a moderately configured system which is about one half full. There is plenty of space for expansion to add additional drive bays, for example. There is a helpful Electrical Requirements Worksheet in the Preparing for a System Installation manual. For this configuration we calculate that both cabinets draw a total of about 2,000 watts or so. The property manager scheduled an electrician to upgrade our service and install the receptacles. We also made other changes to make it easier to power up some of our other machines that we haven't been able to run recently. The work was completed on June 30, 2014.

Cray manuals
Some assembly required.
While we were waiting for the scheduled electrical upgrade we began to reassemble the system. We had split the two cabinets, which were bolted together, to make it easier and safer to move. The cables between the cabinets that had been unplugged were reconnected. There are two Y1 Channel cables between the I/O Subsystem and Processor 0. But, most of the cables are for the Central Control Unit (CCU) which monitors for fault conditions.

Central Control Unit
The Central Control Unit Panel. Photo by Dave Fischer.
According to a an archived Cray press release from March 13, 1995:
The CRAY J916 systems begin volume shipments this month, at U.S. list prices ranging from $225,000 to $1.5 million for systems with four to 16 processors...
We're not sure how much this system actually cost new, but from the above quote we estimate that the list price of our 8 processor J916 was likely close to $850,000 in 1996. That is equivalent to about $1.3 million today taking into account inflation.

The handy Worksheet for calculating electrical requirements also has a section for air conditioning. The Processing Cabinet could produce a maximum of 12,000 BTU/hour and the Peripheral Cabinet 14,000 BTU/hour. Again, ours is a moderately configured system so, as with power consumption, the numbers are lower. We estimate just over 7,000 BTU/hour. To put that in perspective: this Cray could heat nearly 1,000 gallons of water by 1 degree Fahrenheit per hour while running if everything were turned on.

Between the list price and the heat generation it becomes clear why there is a sophisticated CCU to monitor for temperature faults. There is a loud rush of air from the cooling blowers when the system powers up. The Processing Cabinet blower can move 1,200 cubic feet of air per minute. The Peripheral Cabinet about 1,400 CFM. If any of fault conditions occur there is a piezoelectric buzzer that sounds an alarm.

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