Dispatcher in OS | Gate Vidyalay

Schedulers in OS-

Schedulers in OS are special system software.
They help in scheduling the processes in various ways.
They are mainly responsible for selecting the jobs to be submitted into the system and deciding which process to run.

There are 3 kinds of schedulers-

The primary objective of long-term scheduler is to maintain a good degree of multiprogramming.

Long-term scheduler is also known as Job Scheduler.
It selects a balanced mix of I/O bound and CPU bound processes from the secondary memory (new state).
Then, it loads the selected processes into the main memory (ready state) for execution.

The primary objective of short-term scheduler is to increase the system performance.

Short-term scheduler is also known as CPU Scheduler.
It decides which process to execute next from the ready queue.
After short-term scheduler decides the process, Dispatcher assigns the decided process to the CPU for execution.

The primary objective of medium-term scheduler is to perform swapping.

Medium-term scheduler swaps-out the processes from main memory to secondary memory to free up the main memory when required.
Thus, medium-term scheduler reduces the degree of multiprogramming.
After some time when main memory becomes available, medium-term scheduler swaps-in the swapped-out process to the main memory and its execution is resumed from where it left off.
Swapping may also be required to improve the process mix.

Long-term scheduler	Medium-term scheduler	Short-term scheduler
It is a job scheduler	It is a process swapping scheduler.	It is a CPU scheduler
It controls the degree of multiprogramming.	It reduces the degree of multiprogramming.	It provides lesser control over degree of multiprogramming.
Speed is lesser than short-term scheduler	Speed is in between the long-term and short-term schedulers.	Speed is fastest among the other two.
It is minimal or almost absent in time sharing system.	It is a part of time sharing system.	It is also minimal in time sharing system.
It selects processes from new state and loads them into ready state.	It swaps-out processes from main memory to secondary memory and later swaps in.	It selects processes from the ready state and assigns to the CPU.
Operates less frequently since processes are not rapidly created.	Operates more frequently than long-term scheduler but less frequently than short-term scheduler.	Operates very frequently to match the speed of CPU since CPU rapidly switches from one process to another

In multiprogramming systems,

Multiple processes may be present in the ready state which are all ready for execution.
Degree of multiprogramming is the maximum number of processes that can be present in the ready state.
Long-term scheduler controls the degree of multiprogramming.
Medium-term scheduler reduces the degree of multiprogramming.

An optimal degree of multiprogramming means average rate of process creation is equal to the average departure rate of processes from main memory.
It is the responsibility of long-term scheduler to maintain a good degree of multiprogramming.

To gain better understanding about Schedulers in OS,

Next Article- Various Times Of Process

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