The PSBLAS librarary is based on the Single Program Multiple Data (SPMD) programming model: each process participating in the computation performs the same actions on a chunk of data. Parallelism is thus data-driven.

Because of this structure, many subroutines coordinate their action
across the various processes, thus providing an implicit
synchronization point, and therefore *must* be
called simultaneously by all processes participating in the
computation. This is certainly true for the data allocation and
assembly routines, for all the computational routines and for some of
the tools routines.

However there are many cases where no synchronization, and indeed no communication among processes, is implied; for instance, all the routines in sec. 3.4 are only acting on the local data structures, and thus may be called independently. The most important case is that of the coefficient insertion routines: since the number of coefficients in the sparse and dense matrices varies among the processors, and since the user is free to choose an arbitrary order in builiding the matrix entries, these routines cannot imply a synchronization.

Throughout this user's guide each subroutine will be clearly indicated as:

**Synchronous:**- must be called simultaneously by all the processes in the relevant communication context;
**Asynchronous:**- may be called in a totally independent manner.