When SQL server starts,each physical processor is allocated one SQLOS scheduler; -The SQLOS scheduler helps schedule work tasks by using non-preemptive scheduling.This means that a TASK is usually allowed to run until its quantum has been reached or until it suspends (it needs to wait)on some synchronization event. -The SQLOS schedulers also provide support for non-preemptive IOs,which are asynchronus disk IOs that complete during a scheduler context switch (therefore the asynchronous nature).The scheduler HELPS with synchronization in a manner that if a task needs to wait for a resource, the thread running the task is removed from the scheduler and placed in a waiting list until the resource becomes available.Thils HELPS ensure tahat other tasks can get the work done rather than being BLOCKED behind the one WAITING for a resource,such as a LOCK or disk IO to complete. -A TASK in SQLOS is a unit of execution that is associated with either a thread or a fiber.It is the thread that is assigned to the CPU for execution. A fiber is a lightweight execution context that also helps REDUCE THE COST OF CONTEXT SWITCHING.Due to various performance issues seen with the use of fiber mode,microsoft recommends using the thread mode unless extensive,thorough testing shows that fiber mode will meet performance service-level agreements(SLAs) without any undue side effects.
