The ILOG CPLEX Parallel Barrier Optimizer achieves significant speedups over its serial counterpart (the ILOG CPLEX Barrier Optimizer described in Solving LP Problems with the Barrier Optimizer, and in Chapter 7, Solving Quadratic Programming Problems) on a wide variety of classes of problems. Consequently, the parallel barrier optimizer will be the best LP choice on a parallel computer more frequently than on a single-processor. For that reason, you should be careful not to apply performance data or experience based on serial optimizers when you are choosing which optimizer to use on a parallel platform.

If you decide to use the parallel barrier optimizer on the subproblems of a MIP, see also other special considerations about nested parallelism in Nested Parallelism.

On Hewlett-Packard (HP) only, the default number of threads used by the ILOG CPLEX Parallel Barrier Optimizer is the number of processors on the computer. You can override this default by resetting the operating system environment variable MP_NUMBER_OF_THREADS before you start ILOG CPLEX.

On Sun only, you must set the UNIX environment variable PARALLEL to a number. This number will be the overriding maximum for the number of threads, subject to licensing limits. The default behavior of the ILOG CPLEX Parallel Barrier Optimizer without this environment variable is to use only one thread.