User Mode vs. Kernel Mode Scheduling
You may be wondering what the advantage of moving the scheduling management inside the SQL Server process is. Wouldn't SQL Server just end up duplicating functionality already provided by Windows? With all the smart people Microsoft must have working on Windows and its scheduler, how likely is it that the SQL Server team could come up with something that much more scalable?
I'll address this in detail below, but the short answer is that SQL Server knows its own scheduling needs better than Windows or any other code base outside the product could hope to. UMS doesn't duplicate the complete functionality of the Windows scheduler, anyway—it implements only the basic features related to task scheduling, timers, ...
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