Downloaded 1,493 times
Uploaded byChankey Pathak
Scheduling algorithms
The document discusses various scheduling algorithms used in operating systems including: - First Come First Serve (FCFS) scheduling which services processes in the order of arrival but can lead to long waiting times. - Shortest Job First (SJF) scheduling which prioritizes the shortest processes first to minimize waiting times. It can be preemptive or non-preemptive. - Priority scheduling assigns priorities to processes and services the highest priority process first, which can potentially cause starvation of low priority processes. - Round Robin scheduling allows equal CPU access to all processes by allowing each a small time quantum or slice before preempting to the next process.
Scheduling algorithms
- 1. Scheduling Criteria CPUutilization – keep the CPU as busy as possible (from 0% to 100%) Throughput – # of processes that complete their execution per time unit Turnaround time – amount of time to execute a particular Process Waiting time – amount of time a process has been waiting in the ready queue Response time – amount of time it takes from when a request was submitted until the first response is produced
- 2. Optimization Criteria MaxCPU utilization Max throughput Min turnaround time Min waiting time Min Response time
- 3. Scheduling Algorithems FirstCome First Serve Scheduling Shortest Job First Scheduling Priority Scheduling Round-Robin Scheduling Multilevel Queue Scheduling Multilevel Feedback-Queue Scheduling
- 4. First Come FirstServe Scheduling (FCFS) Process Burst time P1 24 P2 3 P2 3
- 5. First Come FirstServe Scheduling The average of waiting time in this policy is usually quite long Waiting time for P1=0; P2=24; P3=27 Average waiting time= (0+24+27)/3=17
- 6. First Come FirstServe Scheduling Suppose we change the order of arriving job P2, P3, P1
- 7. First Come FirstServe Scheduling Consider if we have a CPU-bound process and many I/O-bound processes There is a convoy effect as all the other processes waiting for one of the big process to get off the CPU FCFS scheduling algorithm is non-preemptive
- 8. Short job firstscheduling (SJF) This algorithm associates with each process the length of the processes’ next CPU burst If there is a tie, FCFS is used In other words, this algorithm can be also regard as shortest-next-cpu-burst algorithm
- 9. Short job firstscheduling SJF is optimal – gives minimum average waiting time for a given set of processes
- 10. Example Processes Burst time P1 6 P2 8 P3 7 P4 3 FCFS average waiting time: (0+6+14+21)/4=10.25 SJF average waiting time: (3+16+9+0)/4=7
- 11. Short job firstscheduling Two schemes: Non-preemptive – once CPU given to the process it cannot be preempted until completes its CPU burst Preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF)
- 12. Short job firstscheduling- Non-preemptive
- 13. Short job firstscheduling- Preemptive
- 14. Priority Scheduling Apriority number (integer) is associated with each process The CPU is allocated to the process with the highest priority (smallest integer ≡ highest priority) Preemptive Non-preemptive SJF is a special priority scheduling where priority is the predicted next CPU burst time, so that it can decide the priority
- 15. Priority Scheduling Processes Burst time Priority Arrival time P1 10 3 P2 1 1 P3 2 4 P4 1 5 P5 5 2 The average waiting time=(6+0+16+18+1)/5=8.2
- 16. Priority Scheduling Processes Burst time Priority Arrival time P1 10 3 0.0 P2 1 1 1.0 P3 2 4 2.0 P4 1 5 3.0 P5 5 2 4.0 Gantt chart for both preemptive and non-preemptive, also waiting time
- 17. Priority Scheduling Problem: Starvation – low priority processes may never execute Solution : Aging – as time progresses increase the priority of the process
- 18. Round-Robin Scheduling TheRound-Robin is designed especially for time sharing systems. It is similar FCFS but add preemption concept A small unit of time, called time quantum , is defined
- 19. Round-Robin Scheduling Eachprocess gets a small unit of CPU time ( time quantum ), usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue.
- 20.
- 21. Round-Robin Scheduling Ifthere are n processes in the ready queue and the time quantum is q , then each process gets 1/ n of the CPU time in chunks of at most q time units at once. No process waits more than ( n -1) q time units.
- 22. Round-Robin Scheduling Performanceq large => FIFO q small => q must be large with respect to context switch, otherwise overhead is too high Typically, higher average turnaround than SJF, but better response
- 23.
- 24. Multilevel Queue Readyqueue is partitioned into separate queues: foreground (interactive) background (batch) Each queue has its own scheduling algorithm foreground – RR background – FCFS
- 25. Multilevel Queue exampleForeground P1 53 (RR interval:20) P2 17 P3 42 Background P4 30 (FCFS) P5 20
- 26. Multilevel Queue Schedulingmust be done between the queues Fixed priority scheduling; (i.e., serve all from foreground then from background). Possibility of starvation. Time slice – each queue gets a certain amount of CPU time which it can schedule amongst its processes; i.e., 80% to foreground in RR
- 27.
- 28. Multilevel Feedback QueueThree queues: Q 0 – RR with time quantum 8 milliseconds Q 1 – RR time quantum 16 milliseconds Q 2 – FCFS Scheduling A new job enters queue Q0 which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q 1. At Q 1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q 2.
- 29.
- 30. Multilevel Feedback QueueP1 40 P2 35 P3 15
- 31. 5.4 Multiple-Processor SchedulingWe concentrate on systems in which the processors are identical (homogeneous) Asymmetric multiprocessing (by one master) is simple because only one processor access the system data structures. Symmetric multiprocessing , each processor is self-scheduling. Each processor may have their own ready queue.
- 32. Load balancing Onsymmetric multiprocessing systems, it is important to keep the workload balanced among all processors to fully utilized the benefits of having more than one CPU There are two general approached to load balancing: Push Migration and Pull Migration
- 33. Symmetric Multithreading Analternative strategy for symmetric multithreading is to provide multiple logical processors (rather than physical) It’s called hyperthreading technology on Intel processors
- 34. Symmetric Multithreading Theidea behind it is to create multiple logical processors on the same physical processor (sounds like two threads) But it is not software provide the feature, but hardware Each logical processor has its own architecture state, each logical processor is responsible for its own interrupt handling.
- 35.