Module 2.1: CPU Scheduling

1. Module 2.1: CPU Scheduling • Scheduling Types • Scheduling Criteria • Scheduling Algorithms • Performance Evaluation K. Salah

2. CPU SCHEDULING • The basic problem is as follows: How can OS schedule the allocation of CPU cycles to processes in system, to achieve “good performance”? • Components of CPU scheduling subsystem of OS: • Dispatcher – gives control of the CPU to the new process • Scheduler - selects next process from those in main memory (short-term scheduler) • Swapper - manages transfer of processes between main memory and secondary storage (medium-term scheduler) • Long-Term Scheduler - in a batch system, determines which and how many jobs to admit into system. K. Salah

3. Types of Scheduling Algorithms • Preemptive: process may have CPU taken away before completion of current CPU burst (e.g. end of CPU quantum.) • Non-preemptive: processes always run until CPU burst completes • Static Priority • Dynamic Priority K. Salah

4. Performance Criteria for Scheduling • Scheduling (as an Optimization task): How to best order the ready queue for efficiency purposes. • CPU utilization: % of time CPU in use • Throughput: # of jobs completed per time unit • Turnaround Time: wall clock time required to complete a job • Waiting Time: amount of time process is ready but waiting to run • Response Time: in interactive systems, time until system responds to a command • Response Ratio: (Turnaround Time)/(Execution Time) -- long jobs should wait longer • The overhead of a scheduling algorithm (e.g., data kept about execution activity, queue management, context switches) should also be taken into account. Kleinrock's Conservation Law: • No matter what scheduling algorithm is used, you cannot help one class of jobs without hurting the other ones. • Example: A minor improvement for short jobs (say, on waiting time) causes a disproportionate degradation for long jobs. K. Salah

5. Optimization Criteria • Max CPU utilization • Max throughput • Min turnaround time • Min waiting time • Min response time K. Salah

6. Basic Scheduling Algorithm • FCFS - First-Come, First-Served • Non-preemptive • Ready queue is a FIFO queue • Jobs arriving are placed at the end of queue • first job in queue runs to completion of CPU burst • Advantages: simple, low overhead • Disadvantages: long waiting time, inappropriate for interactive systems, large fluctuations in average turnaround time are possible. K. Salah

7. FCFS Example • Pid Arr CPU Start Finish Turna Wait Ratio---+---+---+-----+------+-----+----+----- A 0 3 0 3 3 0 1.0 B 1 5 3 8 7 2 1.4 C 3 2 8 10 7 5 3.5 D 9 5 10 15 6 1 1.2 E 12 5 15 20 8 3 1.6---+---+---+-----+------+-----+----+----- A 0 1 0 1 1 0 1.00 B 0 100 1 101 101 1 1.01 C 0 1 101 102 102 101 102.00 D 0 100 102 202 202 102 2.02 K. Salah

8. RR - Round Robin • Preemptive version FCFS • Treat ready queue as circular • arriving jobs placed at end • first job in queue runs until completion of CPU burst, or until time quantum expires • if quantum expires, job again placed at end K. Salah

9. Properties of RR Advantages: simple, low overhead, works for interactive systems Disadvantages: if quantum too small, too much time wasted in context switching; if too large, approaches FCFS.Typical value: 10 - 100 msecRule of thumb: choose quantum so that large majority (80-90%) of jobs finish CPU burst in one quantum K. Salah

10. SJF - Shortest Job First • non-preemptive • ready queue treated as a priority queue based on smallest CPU-time requirement • arriving jobs inserted at proper position in queue • shortest job (1st in queue) runs to completion Advantages: provably optimal w.r.t. average turnaround time Disadvantages: in general, unimplementable. Also, starvation possible! Can do it approximately: use exponential averaging to predict length of next CPU burst ==> pick shortest predicted burst next! K. Salah

11. Exponential Averaging tn+1= a tn+ (1 - a) t n tn+1 : predicted length of next CPU burst tn : actual length of last CPU burst tn : previous prediction a = 0 implies make no use of recent history (t n+1 = t n) a = 1 implies tn+1 = tn (past prediction not used). a = 1/2 implies weighted (older bursts get less and less weight). K. Salah

12. Prediction of the Length of the Next CPU Burst K. Salah

13. SRTF - Shortest Remaining Time First • Preemptive version of SJF • Ready queue ordered on length of time till completion (shortest first) • Arriving jobs inserted at proper position • shortest job runs to completion (i.e. CPU burst finishes) or until a job with a shorter remaining time arrives (i.e. placed in the ready queue.) K. Salah

14. Performance Evaluation • Deterministic Modeling (vs. Probabilistic) Look at behavior of algorithm on a particular workload, and compute various performance criteria Example: workload - Job 1: 24 units Job 2: 3 units Job 3: 3 units • Gantt chart for FCFS: | Job 1 | Job 2 | Job 3 | 0 24 27 30 Total waiting time: 0 + 24 + 27 = 51 Average waiting time: 51/3 = 17 Total turnaround time: 24 + 27 + 30 = 81 Average turnaround time: 81/3 = 27 K. Salah

15. RR and SJF • Chart for RR with quantum of 3: | Job 1 | Job 2 | Job 3 | Job 1 |0 3 6 9 30 Total waiting time: 6 + 6 + 3 = 15 Avg. waiting time: 15 / 3 = 5 • Chart for SJF: | Job 2 | Job 3 | Job 1 | 0 3 6 30 Total waiting time: 6 + 0 + 3 = 9 Avg. waiting time: 9 / 3 = 3 • Can see that SJF gives minimum waiting time. RR is intermediate. (This can be proved in general.) K. Salah

16. HPF - Highest Priority First • general class of algorithms • each job assigned a priority which may change dynamically • may be preemptive or non-preemptive • Problem: how to compute priorities? • SJF is a special case of priority; the longer the CPU burst, the lower the priority. • Priority can be internally computed, e.g., CPU burst vs. I/O burst. Or it can be externally defined depending on the importance of the process, (e.g. using nice command in Unix). • Effective Priority = System (Internal) + User defined • System: type of process + age (dynamically changes) K. Salah

17. Windows XP Priorities • 6 priority classes (shown with Task Manager) • 7 default relative priorities/values (shown with Process Explorer) • 16-31 (time critical) • 1-16 (others) K. Salah

18. Multilevel Queue Scheduling K. Salah

19. Multilevel Feedback Queue • A process is admitted to one class of queues • Schedule top queue processes first • A process can move between the various queues; aging can be implemented this way • Multilevel-feedback-queue scheduler defined by the following parameters: • number of queues • scheduling algorithms for each queue • method used to determine when to upgrade a process • method used to determine when to demote a process • method used to determine which queue a process will enter when that process needs service K. Salah

20. Example of Multilevel Feedback Queue • Attractive scheme for I/O bound jobs • Three queues: • Q0 – RR with time quantum 8 milliseconds • Q1 – RR time quantum 16 milliseconds • Q2 – FCFS • Scheduling • A new job enters queue Q0which is servedFCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1. • At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2. K. Salah

21. Multilevel Feedback Queues K. Salah

22. Further Readings • When is processor affinity used? • In Windows XP, what is the default base priority for a process when it gets created? K. Salah