Schedulers are special system softwares which handles process scheduling in various ways.Their main task is to select the jobs to be submitted into the system and to decide which process to run.
Schedulers are of three types
1. Long Term Scheduler
2. Short Term Scheduler
3. Medium Term Scheduler
Long Term Scheduler
It is also called job scheduler. Long term scheduler determines which programs are admitted to the system for processing. Job scheduler selects processes from the queue and loads them into memory for execution. Process loads into the memory for CPU scheduling. The primary objective of the job scheduler is to provide a balanced mix of jobs, such as I/O bound and processor bound. It also controls the degree of multiprogramming. If the degree of multiprogramming is stable, then the average rate of process creation must be equal to the average departure rate of processes leaving the system.
On some systems, the long term scheduler may not be available or minimal. Time-sharing operating systems have no long term scheduler. When process changes the state from new to ready, then there is use of long term scheduler.
Short Term Scheduler
It is also called CPU scheduler. Main objective is increasing system performance in accordance with the chosen set of criteria. It is the change of ready state to running state of the process. CPU scheduler selects process among the processes that are ready to execute and allocates CPU to one of them.
Short term scheduler also known as dispatcher, execute most frequently and makes the fine grained decision of which process to execute next. Short term scheduler is faster than long term scheduler.
Medium Term Scheduler
Medium term scheduling is part of the swapping. It removes the processes from the memory. It reduces the degree of multiprogramming. The medium term scheduler is in-charge of handling the swapped out-processes.
Running process may become suspended if it makes an I/O request. Suspended processes cannot make any progress towards completion. In this condition, to remove the process from memory and make space for other process, the suspended process is moved to the secondary storage. This process is called swapping, and the process is said to be swapped out or rolled out. Swapping may be necessary to improve the process mix.
Comparison between Scheduler
Long Term Scheduler
It is a job scheduler
Speed is lesser than short term scheduler
It controls the degree of multiprogramming
It is almost absent or minimal in time sharing system
It selects processes from pool and loads them into memory for execution
Short Term Scheduler
It is a CPU scheduler
Speed is fastest among other two
It provides lesser control over degree of multiprogramming
It is also minimal in time sharing system
It selects those processes which are ready to execute
Medium Term Scheduler
It is a process swapping scheduler.
Speed is in between both short and long term scheduler.
It reduces the degree of multiprogramming.
It is a part of Time sharing systems.
It can re-introduce the process into memory and execution can be continued.
