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What is Scheduling ?

  • Scheduling is an essential part of a Multiprogramming operating systems.
  • Such operating systems allow more than one process to be loaded into the executable memory at a given time and the loaded process shares from the CPU using time multiplexing.
  • The Operating System maintains the following important process scheduling queues −
    • Job queue − This queue preserves all the processes in the system.
    • Ready queue − This queue keeps a set of all processes residing in main memory, ready and waiting to execute. A new process is always placed in this queue.
    • Device queues − The processes which are blocked due to inaccessibility of an I/O device establish this queue.
Scheduling

What is Scheduling

  • The OS can use different procedures to manage each queue (FIFO, Round Robin, Priority, etc.).
  • The OS scheduler determines how to move from processes between the ready and run queues which can only have one entry per processor core on the system; in the above diagram, it has been combined with the CPU.

Schedulers are of three types:

  • Long-Term Scheduler
  • Short-Term Scheduler
  • Medium-Term Scheduler

Long Term Scheduler:

  • It is also called a job scheduler.
  • It selects processes from the queue and loads into memory for execution.
  • The long-term scheduler may not be accessible or minimal.
  • No long-term scheduler in the time-sharing operating systems.
  • Use of long-term scheduler when a process changes the state from new to ready.

Short Term Scheduler:

  • Short term scheduler is also known as CPU scheduler.
  • It is used to improve system performance.
  • It is change from ready state to running state of the process.
  • Short term scheduler is faster than compared to long term scheduler.

Medium Term Scheduler:

  • Medium-term scheduling is a part of swapping.
  • Medium Term Scheduler has removed the processes from the memory.
  • To remove the process from memory and make space to the other processes, the postponed process is moved to the secondary storage.
  • This process is called swapping.
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