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Processes in a Multiprocessor Environment

In Program 6-1, the processes and their primary (and only) threads run almost totally independently of one another. The only dependence is created at the end of the parent process as it waits for all the processes to complete so that the output files can be processed sequentially. Therefore, the Windows scheduler can and will run the process threads concurrently on the separate processors of an SMP system. This can result in substantial performance improvement when performance is measured as elapsed time to execute the program, and no explicit actions are required to get the performance improvement.

Appendix C shows some typical results. The performance improvement is not linear in terms of the number of processors due to overhead costs and the need to output the results sequentially. Nonetheless, the improvements are worthwhile and result automatically as a consequence of the program design, which delegates independent computational tasks to independent processes.

It is possible, however, to constrain the processes to specific processors if you wish to be sure that other processors are free to be allocated to other critical tasks. This can be accomplished using the processor affinity mask (see Chapter 9) in a job object. Job objects are described in a later section.

Finally, it is possible to create independent threads within a process, and these threads will also be scheduled on separate SMP processors. Chapter 7 describes threads and performance issues related to their use.

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