What is Scheduler ?



A schedule is a decision maker that selects the processes from one scheduling queue to another or allocates CPU for execution. The Operating System has three types of scheduler:

  1. Long-term scheduler or Job scheduler
  2. Short-term scheduler or CPU scheduler
  3. Medium-term scheduler


Long-term scheduler or Job scheduler

  • The long-term scheduler or job scheduler selects processes from discs and loads them into main memory for execution. It executes much less frequently.
  • It controls the degree of multiprogramming (i.e., the number of processes in memory).
  • Because of the longer interval between executions, the long-term scheduler can afford to take more time to select a process for execution.


Short-term scheduler or CPU scheduler

  • The short-term scheduler or CPU scheduler selects a process from among the processes that are ready to execute and allocates the CPU.
  • The short-term scheduler must select a new process for the CPU frequently. A process may execute for only a few milliseconds before waiting for an I/O request.


Medium-term scheduler

The medium-term scheduler schedules the processes as an intermediate level of scheduling

Processes can be described as either:

✦ I/O-bound process – spends more time doing I/O than computations, many short CPU bursts.

✦ CPU-bound process – spends more time doing computations; few very long CPU bursts.


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