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Guru's Guide to SQL Server Architecture and Internals, The by Ken Henderson

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The UMS Scheduler

The UMS mechanism for managing the scheduling process and for ensuring that only one thread per processor is active at any given time is called a scheduler. When SQL Server starts, one UMS scheduler is created for each processor in the machine. These schedulers are not affinitized to specific processors by default, but Windows' scheduling algorithms work out such that, over time, with each UMS scheduler allowing just one thread to run, that thread should end up on its own processor.

The worker pool for the server—regardless of whether it consists of threads or fibers—is distributed evenly across the UMS schedulers. This means that if you have max worker threads set to the default of 255 and you have a four-processor machine, ...

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