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

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The UMS Scheduler Lists

Each UMS scheduler maintains five lists that support the work of scheduling threads: a worker list, a runnable list, a waiter list, an I/O list, and a timer list. Each of these plays a different role, and nodes are frequently moved between lists.

The Worker List

The worker list is the list of available UMS workers. A UMS worker is an abstraction of the thread/fiber concept and allows either to be used without the rest of the code being aware of which is actually being used under the covers. As I said, one of the design goals of UMS was to provide support for fibers in such a way as not to require the core engine code to be concerned with whether the system was using fibers or threads. A UMS worker encapsulates a thread ...

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