Astronomical real-time streaming signal processing on a Blue Gene/L supercomputer

LOFAR is the first of a new generation of radio telescopes, that combines the signals from many thousands of simple, fixed antennas, rather than from expensive dishes. Its revolutionary design and unprecedented size enables observations in a frequency range that could hardly be observed before, and allows the study of a vast amount of new science cases.In this paper, we describe a novel approach to process realtime, streaming telescope data in software, using a supercomputer. The desire for a flexible and reconfigurable instrument demands a software solution, where traditionally customized hardware was used. This, and LOFAR's exceptional real-time, streaming signalprocessing requirements compel the use of a supercomputer. We focus on the LOFAR CEntral Processing facility (CEP), that combines the signals of all LOFAR stations. CEP consists of a 12,288-core IBM Blue Gene/L supercomputer, embedded in several conventional clusters.We describe a highly optimized implementation that will do the bulk of the central signal processing on the Blue Gene/L, namely PolyPhase Filtering, Delay Compensation, and Correlation. Measurements show that we reach exceptionally high computational performance (up to 98% of the theoretical floating-point peak performance). We also discuss how we handle external I/O performance limitations into and out of the Blue Gene/L, to obtain sufficient bandwidth for LOFAR.