Figure I: Inside an Atom
Because of the enormous computing requirements of QCD calculations, it is described as a 'Grand Challenge' computing application. A grand challenge problem is one which cannot be solved in a reasonable amount of time on currently available parallel supercomputers. For the last three decades a number of commercial and custom-built parallel machines have been employed to solve QCD calculations and they have successfully produced a number of results. The natural parallelism in QCD applications, resulting from its lattice formulation (called lattice QCD), makes QCD one of the first applications to be tried on early parallel computers. Yet the QCD calculations still challenge the power of high-end supercomputers; they continue to push the boundaries of supercomputer architecture, QCD algorithms and parallel software techniques.
Figure II: Lattice QCD
The lattice formulation of the QCD calculation makes this application ideally suited for implementation on a parallel machine. This is because a regular lattice can be decomposed and distributed evenly on a four-dimensional physical network, among parallel processing nodes (Figure III). The communication requirements of the QCD calculations are local i.e. neighbouring nodes along the four space-time dimensions communicate; global broadcast and reduction operations are rare. The code requires intensive use of double-precision floating-point calculations.
Figure III: Mapping of Lattice on to Physical Processors