Schedulers are specializing systems. Software that manages process scheduling in a variety of ways. Moreover, The main task is to select the jobs to perform in the system. And then to decide which process to run.

There are three types of schedulers:

  1. Firstly Long-Term Scheduler
  2. Secondly, Medium-Term Scheduler
  3. Thirdly, Short-Term Scheduler

Long-Term Scheduler in OS:

Also known as job scheduler.

So, A long-term scheduler determines which program allowed on the system for processing.

Then, It select the process to place in the ready queue.

The long-term scheduler basically specifies the priority, In which processes must place in the main memory.

Processes of the long-term scheduler are kept in the ready state because the process is ready to execute in this state. Waiting for calls of execution from the CPU takes time and this is called a long-term scheduler in os.

Mid-Term Scheduler in OS :

Medium-term scheduling is a part of swapping.

So, It removes processes from memory.

Then, It reduces the level of multiprogramming.

The medium-term scheduler is responsible for maintaining the swapped-out processes.

Moreover, A running process will stop upon I / O request. A suspended process cannot make any progress towards completion.

Short-Term Scheduler in OS:

Also called a CPU scheduler.

It is a change of ready state to running state of process.

The CPU scheduler selects one of the processes that are ready to execute and assigns the CPU to one of them.

It sets the priority in which processes are in ready queue allocating the central processing unit (CPU) time for their execution.

Short-term schedulers, also known as dispatchers, determine which process to pursue next. Short-term schedulers are faster than long-term schedulers.

An operating system uses two types of scheduling processes:

Preemptive process:

In preemptive scheduling policy.

a low priority process has to be suspend its execution.

if high priority process is waiting in the same queue for its execution.

Non – Preemptive process:

In a non – preemptive scheduling policy, executed on a first come first serve basis.

which means the next process executed only when the currently running process finishes its execution.

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