• • The currently running process may be interrupted and moved to the ready state by the OS.
  • The resources (mainly CPU cycles) are allocated to the process for the limited amount of time and then is taken away, and the process is again placed back in the ready queue if that process still has CPU burst time remaining.

• • That process stays in ready queue till it gets next chance to execute.
  • The decision to preempt may be performed when,
    • A new process arrives (with a higher priority than the current process)

Preemptive Scheduling

• • An interrupt occurs (fairness)
  • Periodically based on clock interrupt. (Eg: Round Robin)

• More robust, one process cannot monopolize the CPU.
• Fairness. The OS makes sure that CPU usage is the same by all running process.

• Context switch cost: Time taken by the scheduler to suspend the running task, switch the context, and dispatch the new incoming task.

• 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.

• 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.

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