Thread by @vagnerpilar

CPU Scheduler #WindowsInternals (1/9)

The system scheduler controls multitasking by determining which of the competing threads receives the next processor time slice. (2/9)

There is no single “scheduler” module or routine, the code is spread throughout the kernel in which scheduling-related events occur. The routines that perform these duties are called the kernel’s dispatcher (3/9)

The scheduler determines which thread runs next using scheduling priorities. When a thread is selected to run, it runs for an amount of time called a quantum. (4/9)

A quantum is the length of time a thread is allowed to run before another thread at the same priority level (or higher, which can occur in a multiprocessor system) is given a turn to run. Windows schedules at the thread granularity. (5/9)

This approach makes sense when you consider that processes don’t run but only provide resources and context in their threads run. Because scheduling decisions are made strictly on a thread basis, no consideration is given to what process the thread belongs to. (6/9)

Very early MS-DOS and Microsoft Windows systems were non-multitasking, and as such did not feature a scheduler. Windows 3.1x used a non-preemptive scheduler, meaning that it did not interrupt programs. (7/9)

It relied on the program to end or tell the OS that it didn't need the proc so that it could move on to another process. This is usually called cooperative multitasking. (8/9)

Windows 95 introduced a rudimentary preemptive scheduler; however, for legacy support opted to let 16 bit applications run without preemption. (9/9) #WindowsRules