Parallel Programs on Uniprocessors

Parallel Programs on Uniprocessors

It might seem a contradiction, but it is possible to execute some parallel programs in uniprocessors. Obviously you would not do this expecting the best performance. However:

It is possible to restrict and assign the available CPUs of a multiprocessor so that only one CPU is available to execute a given program, even a program intended to use parallel execution. This can arise as a brief transient condition as IRIX dynamically assigns CPUs to programs, or it can arise through the operator using commands such as mpadmin (see the mpadmin(1) reference page).
It is easier to debug a parallel program by running it in the more predictable environment of a uniprocessor.

Most parallel programming models adapt to the available hardware, running concurrently on multiple CPUs (up to some programmer-defined limit) when the CPUs are available, but running on a limited number, or even just one CPU, when necessary. For example, the Fortran programmer can control the number of CPUs used by a MIPpro Fortran 77 program by setting environment variables before the program starts (see "Using Statement-Level Parallelism").