What is the function of the OS scheduler?

Answer 1

The scheduler's job is deciding when each process gets a shot at CPU time and for how long. While their approaches are different, they all follow roughly the same strategy:

First, assume that the CPU can run only one process at a time. CPUs with multiple cores are treated as multiple CPUs that can only run a single process or thread. Therefore the scheduler will determine that X amount of processes can be in a "running" state, where X is the number of cores or CPUs.

Second, assume that any process waiting for I/O is not going to accomplish anything while on the CPU, so do not schedule the process until its associated I/O operation has been COMPLETED by the hardware in question.

Third, assume that not all processes are equal in importance. Some processes really do not need much CPU time, so do they really need to preempt a process that does need plenty of CPU?

Fourth (In preemptive scheduling.), the kernel and not the processes, have full control over when context switching occurs, including when a process's time slice expires. Further, a process can and should be able to preempt itself when asking for I/O so that the kernel can arrange the operation with the drivers.

Finally, assume that all processes are flawed and just waiting to try and take each other's stuff. The scheduler itself will not implement memory protection, but it does need to know how to interpret whatever memory management is in place for context switching purposes.