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Pastimes : Dream Machine ( Build your own PC ) -- Ignore unavailable to you. Want to Upgrade?

To: Howard R. Hansen who wrote (1222)	6/9/1998 10:57:00 AM
From: Sean W. Smith	Respond to of 14778

Howard, here is a definition from MS. I would have posted a link but you have be a registerd developer to access it... premium.microsoft.com 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. c 1997 Microsoft Corporation. All rights reserved. Terms of Use. Basically the concept is unless there is an unbalance among processors you should continue to run on the same processor that way you incur no context switch penalty and you have a much higher chance of cache hits which increases overall performance. This concept is around 20 years old. Any good OS book will go into great detail on this concept. Notice microsoft recommends not using it. Because their scheduler is broken. This is a key feature that everyone from Cray, IBM, SGI and Sun have used for years inb their SMP systems... Enjoy, Sean