Process Schedulers and Process Queue in OS

Process Schedulers

There are various types of Process Schedulers:

1. Long Term Scheduler
2. Short Term Scheduler
3. Medium Term Scheduler                                           

Long Term Scheduler: - Long term scheduler is also referred to as Job Scheduler. The task of the Long-term scheduler is to select the process from secondary storage and hold them in the ready queue, which is in the primary storage. The main task performed by the long-term scheduler is to select the faultless combination of IO-bound and CPU bound processes from the job pool. If any situation arises when job schedulers want to select more than one IO processes then due to this, there may be a possibility that all the jobs enter into the blocked state, and maximum of the time, the CPU will remain in ideal state, and due to this, the degree of multi-programming gets reduced. Therefore, the long-term scheduler must choose both IO and CPU bound processes efficiently and appropriately.

Short Term Scheduler: - The task performed by the short-term scheduler is to choose one of the jobs from the ready queue and send it for execution to the CPU. Short Term Scheduler means a CPU scheduler. In Short term scheduler, we use a scheduling algorithm to choose the job for execution. In some cases, the job of short-term scheduler may be critical such as if the CPU burst time of a selected job is high. Then the other jobs will have to wait for a long time in the ready queue. This problem is called Starvation.

Difference between Long-Term Scheduler, Short-Term Scheduler, and Middle-Term Scheduler

S.No	Long-term scheduler	Short-term scheduler	Middle-term scheduler
1.	The Long-term scheduler means a Job Scheduler.	A short-term scheduler means a CPU Scheduler.	Middle-term means a process is swapping scheduler.
2.	The speed of the Long-term scheduler is less than the short-term scheduler.	The speed of the short-term scheduler is faster than the long-term scheduler, and Medium-term scheduler.	The speed of the Middle-term scheduler is between the short-term scheduler and the long-term scheduler.
3.	The long-term scheduler first chooses the process from memory, and after that, it loads the process from memory then executes it.	The short-term scheduler chooses those processes that are ready to execute.	The medium-term scheduler helps to reintroduce the process in the memory so that the process execution can be continued again.
4.	In a time-sharing system, the Long-term scheduler does not exist.	In a time-sharing system, Short-term scheduler is less exist.	A Medium-term scheduler is just like a part of Time-sharing systems.

Medium Term Scheduler: - The Medium-term scheduler is responsible for having the process suspended and resumed. It primarily switches (moving processes from main memory to disk and disk to main memory). In the medium-term scheduler, swapping is used to increase the performance of the process mix or due to changes in the requirements of memory that have over-engaged available memory that needs memory to be released.

Process Queue

The operating system handles a number of queues for a process state. The process-related to program control block (PCB) is also stored in a queue. When the process switches the state from one state to another state, then the PCB (Program Control Block) of that process is also getting unlinked from the queue and then link to another state queue in which the transition takes place.

There are various types of queue:

1. Job Queue
2. Ready Queue
3. Waiting Queue

1. Job Queue: - All processes are placed in the job queue at the beginning. It is kept in secondary storage. The long-term scheduler selects a few of the jobs and places them in the primary storage.
2. Ready Queue: - The primary memory holds the ready queue. In ready queue with the help of the short-term scheduler, we select the job for the execution from the ready queue, and then we will dispatch it to the CPU.
3. Waiting Queue: - In the ready queue, if the process requires doing some operations means any input/output operation to finished its execution, then it is must that the operating system switches the process state from running to the waiting state. The context (Program Control Block) is stored in the waiting18 queue that will be used by the processor when the IO is done.