User Mode Scheduler (UMS)

User Mode Scheduler (UMS) is a feature in SQL Server 2000. UMS is used within SQL Server to schedule task. It is comparative to Windows Task Scheduler. Windows Task Scheduler is preemptive tasking which means that Windows prevents a single thread from monopolizing a processor. UMS, by contrast, relies on threads to yield voluntarily. If a SQL Server worker thread does not voluntarily yield, it will likely prevent other threads from running.

UMS serves as a thin layer between the server and the operating system that provides much of the same functionality offered by the Win32 thread and scheduling primitives, but it does so without requiring as many transitions into kernel mode or as many context switches.