The applications in this field can be divided into two major classes, those that require global communications and those that need only local communications. Applications requiring only local communications involve explicit gas dynamics on regular grids with only zero-dimensional microphysics, such as reaction networks and equation of state calculations at each grid point. Applications which use global communication include implicit gas dynamics methods, adaptive grids, radiative transfer, self-gravity calculations, and N-body tree codes. These two set of applications have somewhat different machine requirements, which will be addressed separately below.
For applications which use only local communications, virtually any number
of processors would be used. Certainly processors would present
no significant difficulty and perhaps even
processors could be
used effectively on some applications. Only nearest neighbor communications
would be involved and a sufficiently large number of operations can be
done at each grid point between communications steps that issues of
latency and communication bandwidth become less critical. Applications
of this type should run efficiently on any balanced architecture
and obtain a significant fraction of peak speed.
For applications requiring global communications, computers with a smaller number of faster processors will be easier to use. These applications also will require faster communications with lower latency to run efficiently. It seems unlikely that these applications will achieve a large fraction of peak speed using conventional hardware and software approaches. There is a high probability that applications in this class will become increasingly important in the future as algorithms for multidimensional implicit gas dynamics, multidimensional radiative transfer, and adaptive grids become more popular.