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scheduling algorithm


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detail description of priority and round robin scheduling.

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scheduling algorithm

  1. 1. CPU SCHEDULING By- Nitish Kumar Sandhawar Roll-52 Id- 17049 Subject-Operating System PRIORITY AND ROUND ROBIN
  2. 2. CPU SHEDULING – CPU scheduling is a process which allows one process to use the CPU while the execution of another process is on hold (in waiting state) due to due to unavailability of any resources like I/O etc,
  3. 3. CPU SCHEDULING CRITERIA Throughput- Keep the CPU as busy as possible (from 0% to 100%). CPU utilization-No. of processes that complete their execution per time unit. Turn AroundTime-Amount of time to execute a particular process. Response time-Amount of time it takes from when a request was submitted until the first response is produced. Waiting time-Amount of time a process has been waiting in the ready queue.
  4. 4. Different Types of Scheduling Algorithm There are two types of scheduling algorithm :- 1.Preemptive a) Preempted SJF (In some Cases)-also called pre-emptive SJF b) Preempted Priority (In some Cases)-also called pre-emptive Priority c) Round Robin d) Multilevel Queue Scheduling e) Multilevel Feedback Queue – 1.Non-Preemptive a) First Come First Serve b) Non-Preempted SJF
  5. 5. PRIORITY  The Priority scheduling algorithm is a general case of the SJF algorithm. It is a non-Preemptive, pre-emptive algorithm and one of the most common scheduling algorithms in batch systems.  Each process is assigned a priority. Process with highest priority is to be executed first and so on. Process with same priority are executed on FCFS basis.  In Priority algorithm priority (p) is the inverse of the given priority value. The larger the numeric value of priority, the lower the priority, and vice versa.  Priority can be decided based on memory requirements, time
  6. 6. Wait time of each process is as follows − Average Wait Time: (9+5+12+0) / 4 = 6.5 Proce ss Wait Time : Service Time - Arrival Time P0 9 - 0 = 9 P1 6 - 1 = 5 P2 14 - 2 = 12 P3 0 - 0 = 0
  7. 7. – Advantage-  Simplicity  Reasonable support for priority  Suitable for applications with varying and resource requirement. – Disadvantage-  Indefinite blocking or starvation.  A priority scheduling can leave some priority waiting processes indefinitely CPU.
  8. 8. ROUND ROBIN  Round Robin is the preemptive process scheduling algorithm.  Each process is provided a fix time to execute, it is called a quantum.  Once a process is executed for a given time period, it is preempted and other process executes for a given time period.  Context switching is used to save states of preempted processes.  The Round Robin scheduling is especially
  9. 9. Wait time of each process is as follows – Proc ess Wait Time : Service Time - Arrival Time P0 (0 - 0) + (12 - 3) = 9 P1 (3 - 1) = 2 P2 (6 - 2) + (14 - 9) + (20 - 17) = P3 (9 - 3) + (17 - 12) = 11 Average Wait Time: (9+2+12+11) / 4 = 8.5
  10. 10. Advantage- • There is fairness since every process gets equal CPU. • The newly created process is added to end of ready queue. • A round-robin scheduler generally employs time- sharing, giving each job a time slot or quantum. Disadvantage- • Very important jobs wait in line. • Largest job take enough time for completion. • Setting the quantum too short causes too many context switches and lower the CPU efficiency. • Setting the quantum too long may cause poor response