Long Term Scheduler :-

• The job scheduler or long-term scheduler selects processes from the storage pool in the secondary memory and loads them into the ready queue in the main memory for execution.

• The long-term scheduler controls the degree of multiprogramming. It must select a careful mixture of I/O bound and CPU bound processes to yield optimum system throughput. If it selects too many CPU bound processes then the I/O devices are idle and if it selects too many I/O bound processes then the processor has nothing to do.

• The job of the long-term scheduler is very important and directly affects the system for a long time.

Short Term Scheduler :-

• The short-term scheduler selects one of the processes from the ready queue and schedules them for execution. A scheduling algorithm is used to decide which process will be scheduled for execution next.

• The short-term scheduler executes much more frequently than the long-term scheduler as a process may execute only for a few milliseconds.

• The choices of the short term scheduler are very important. If it selects a process with a long burst time, then all the processes after that will have to wait for a long time in the ready queue. This is known as starvation and it may happen if a wrong decision is made by the short-term scheduler.

Medium Term Scheduler :-

• The medium-term scheduler swaps out a process from main memory. It can again swap in the process later from the point it stopped executing. This can also be called as suspending and resuming the process.

• This is helpful in reducing the degree of multiprogramming. Swapping is also useful to improve the mix of I/O bound and CPU bound processes in the memory.