Menu
I recently started to learn how the CPU and the operating system works, and I am a bit confused about the operation of a single-CPU machine with an operating system that provides multitasking.
Supposing my machine has a single CPU, this would mean that, at any given time, only one process could be running.
Now, I can only assume that the scheduler used by the operating system to control the access to the precious CPU time is also a process.
Thus, in this machine, either the user process or the scheduling system process is running at any given point in time, but not both.
So here's a question:
Once the scheduler gives up control of the CPU to another process, how can it regain CPU time to run itself again to do its scheduling work? I mean, if any given process currently running does not yield the CPU, how could the scheduler itself ever run again and ensure proper multitasking?
So far, I had been thinking, well, if the user process requests an I/O operation through a system call, then in the system call we could ensure the scheduler is allocated some CPU time again. But I am not even sure if this works in this way.
On the other hand, if the user process in question were inherently CPU-bound, then, from this point of view, it could run forever, never letting other processes, not even the scheduler run again.
Supposing time-sliced scheduling, I have no idea how the scheduler could slice the time for the execution of another process when it is not even running?
I would really appreciate any insight or references that you can provide in this regard.
254
HARDWARE SUPPORT: THE TIMER INTERRUPT The addition of a timer interrupt gives the OS the ability to regain control of the CPU even if processes act in a non- cooperative fashion. Thus, this hardware is key in helping the OS maintain control of the system.
CPU Scheduling is a process of determining which process will own CPU for execution while another process is on hold. The main task of CPU scheduling is to make sure that whenever the CPU remains idle, the OS at least select one of the processes available in the ready queue for execution.
The short-term scheduler (also known as the CPU scheduler) decides which of the ready, in-memory processes is to be executed (allocated a CPU) after a clock interrupt, an I/O interrupt, an operating system call or another form of signal.
Operating System has to define which process the CPU will be given. In Multiprogramming systems, the Operating system schedules the processes on the CPU to have the maximum utilization of it and this procedure is called CPU scheduling. The Operating System uses various scheduling algorithm to schedule the processes.
The OS sets up a hardware timer (Programmable interval timer or PIT) that generates an interrupt every N milliseconds. That interrupt is delivered to the kernel and user-code is interrupted.
It works like any other hardware interrupt. For example your disk will force a switch to the kernel when it has completed an IO.
60
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
