Supercomputing Reaches New Heights

December 22, 2012

I’ve written here before about the semi-annual Top 500 ranking of the world’s supercomputer installations, based on their performance on a computational benchmark.  The site has a report of a new system that, while it does not qualify for inclusion in the Top 500 list, has a distinction of its own: it is located at an elevation of 5,000 meters  (16,400 feet) in the Andes in northern Chile, making it the highest system in existence.

The system is installed at the site of the Atacama Large Millimeter/submillimeter Array (ALMA) telescope, the most elaborate ground-based telescope in history.  ALMA, an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile.  The giant telescope’s main array uses 50 dish antennas, each 12 meters (39.3 feet) in diameter, separated by as much as 16 kilometers (10 miles).  There is also a smaller array of four 12-meter and twelve 7-meter (23 feet) antennas.  ALMA functions as an interferometer, which means that the signals from all the antennas in use must be processed together in order to be useful.

The computing system, called the ALMA Correlator, contains 134 million processors, and can handle date from up to 64 antennas simultaneously.  In doing this, it performs approximately 17 quadrillion (1.7 × 1016) operations per second.  Because it is a specialized system, it is not directly comparable to the supercomputers in the Top 500 list (which are ranked on the basis of the LINPACK benchmark).  Nonetheless, the per-operation time is of the same order as that of the TITAN system, which is currently ranked number one of the Top 500, at 1.76 × 1016 floating point operations per second.  (The European Southern Observatory has published an announcement of the ALMA Correlator with more details.)

The radiation wavelengths (millimeter and sub-millimeter) that ALMA studies come from some of the coldest objects in the universe.  Because these wavelengths are significantly absorbed by water vapor, the observatory is located at one of the highest and driest places on earth, the high plateau at Chajnantor, in northern Chile.

Apart from the logistical difficulties involved in building an observatory in such a remote place, the high altitude and correspondingly thin atmosphere create other problems.  Because the air is so thin, the air flow needed to cool the system is approximately  twice that which would be needed at sea level.  Standard hard disk drives rely on “floating” the read/write heads above the platters on an air cushion; that doesn’t work at this altitude, so the system must be diskless.  Human performance is affected, too; a photo accompanying the article shows a technician working on the machine and wearing a supplemental oxygen supply.  (I have never worked at 16,000 feet, but I can say from personal experience that walking 50 yards at a 10,000 foot elevation is a noticeable effort.)  The site is also in a zone of regular seismic activity, so the system must be able to withstand earthquake vibrations.

The ALMA observatory is scheduled to be completed in late 2013, but it has already begun making some observations.  This is fascinating science; in effect, it gives us a “time machine” with which we can observe some of the earliest, and most distant, objects in the universe.

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