How processor get to know to switch process with high prioirity process?

Question

I red that, process scheduler will replace the process that is currently processing by cpu with high priority process. At any point only one process will be executed by processor in that case where the scheduler is running to notify cpu about high priority process, when cpu is busy in executing low priority process ?

Answer 1

The process scheduler is the component of the operating system that is responsible for deciding whether the currently running process should continue running and, if not, which process should run next.

To help the scheduler monitor processes and the amount of CPU time that they use, a programmable interval timer interrupts the processor periodically (typically 50 or 60 times per second) . This timer is programmed when the operating system initializes itself. At each interrupt, the operating system's scheduler gets to run and decide whether the currently running process should be allowed to continue running or whether it should be suspended and another ready process allowed to run. This is the mechanism used for preemptive scheduling.

So,basically,the process scheduler runs in the same main memory, when active , but are only activated after getting invoked by interrupts. Hence,they aren't all time running.

BTW,that was a great conceptual question to answer.Best wishes for your topic.

Answer 2

The higher-priority thread/process will preempt the lower-priority thread when an interrupt causes the scheduler to be run to decide on what set of threads to run next, and the scheduler algorithm decides that the lower-priority thread needs to be replaced by the higher-priority one.

Interrupts come in two flavours:

1. Software interrupts, (syscalls) from threads that are already running and change the state of threads, eg. by signaling an event, mutex or semaphore upon which another thread is waiting, and so making it ready to run.

2. Hardware interrupts that cause a driver to run and that driver chooses to invoke the scheduler on exit because an I/O operation has completed or some timeout interval has expired that needs to change the set of running threads, (eg. disk, NIC, KB, mouse, timer).