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Multiple Processors
Windows NT uses a symmetric multiprocessing (SMP) model to schedule threads on
multiple processors. With this model, any thread can be assigned to any
processor. Therefore, scheduling threads on a computer with multiple processors is
similar to scheduling threads on a computer with a single processor. However, the
scheduler has a pool of processors, so that it can schedule threads to run
concurrently. Scheduling is still determined by thread priority. However, on a
multiprocessor computer, you can also affect scheduling by setting thread affinity
and thread ideal processor, as discussed here.
Thread Affinity
Thread affinity forces a thread to run on a specific subset of processors. Use the SetProcessAffinityMask function to specify thread affinity for all threads of the process. To set
the thread affinity for a single thread, use the SetThreadAffinityMask function. The thread affinity must be a subset of the process affinity. You
can obtain the current process affinity by calling the GetProcessAffinityMask function.
Setting thread affinity should generally be avoided, because it can interfere
with the scheduler's ability to schedule threads effectively across processors.
This can decrease the performance gains produced by parallel processing. An
appropriate use of thread affinity is testing each processor.
Thread Ideal Processor
When you specify a thread ideal processor, the scheduler runs the thread on the specified processor when possible. Use
the SetThreadIdealProcessor function to specify a preferred processor for a thread. This does not
guarantee that the ideal processor will be chosen, but provides a useful hint to the
scheduler.
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