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Lecturer 5: Process Scheduling

Lecturer 5: Process Scheduling. Process Scheduling Criteria & Objectives Types of Scheduling Long term Medium term Short term CPU Scheduling Algorithms First-Come, First-Served (FCFS) Shortest Job First (SJF) Priority (PRI) Round-Robin (RR). CPU Scheduling.

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Lecturer 5: Process Scheduling

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  1. Lecturer 5: Process Scheduling • Process Scheduling • Criteria & Objectives • Types of Scheduling • Long term • Medium term • Short term • CPU Scheduling Algorithms • First-Come, First-Served (FCFS) • Shortest Job First (SJF) • Priority (PRI) • Round-Robin (RR)

  2. CPU Scheduling Scheduling: deciding which process are given access to resources from moment to moment.

  3. Chapter 5: CPU Scheduling • Always want to have CPU (or CPU’s) working • Usually many processes in ready queue • Ready to run on CPU • Focus on a single CPU here • Need strategies for • Allocating CPU time • Selecting next process to run • Deciding what happens after a process completes a system call, or completes I/O • Short-term scheduling • Must not take much CPU time to do the scheduling

  4. CPU & I/O Bursts • Process execution is nothing but a sequence of CPU & I/O bursts • Processes alternate between these two states

  5. CPU Burst & I/O Burst Alternation (Fig. 5.1) In general, most programs have short CPU bursts What happens when the I/O burst occurs? What is the process state at that time?

  6. Types of Scheduling • Long-term scheduling • the decision to add to pool of processes to be executed • Mid-term scheduling • the decision to add to the number of processes that are partially or fully in memory • Short-term scheduling • decision as to which available process will be executed • I/O scheduling • decision as to which process’s pending request shall be handled by an available I/O device

  7. Short termCPU Scheduler • Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them • CPU scheduling decisions may take place when a process: 1. Switches from running to waiting state 2. Switches from running to ready state 3. Switches from waiting to ready 4. Terminates • Scheduling under 1 and 4 is non-preemptive • All other scheduling is preemptive

  8. Preemptive & Non-preemptive Scheduling • SO what is Preemptive & Non-preemptive scheduling? • Non-preemptive • Once CPU is allocated to a process, it holds it till it • Terminates (exits) • Blocks itself to wait for I/O • Requests some OS service

  9. Preemptive & Non-preemptive Scheduling • Preemptive • Currently running process may be interrupted and moved to the ready state by the OS • Windows 95 introduced preemptive scheduling

  10. Dispatcher • Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves: • switching context • switching to user mode • jumping to the proper location in the user program to restart that program (IP) • Dispatch latency – time it takes for the dispatcher to stop one process and start another running

  11. For schedulingCriteria & Objectives • CPU utilization – keep the CPU as busy as possible • Throughput – # of processes that complete their execution per time unit • Turnaround time – amount of time to execute a particular process (total time spent on the system) • 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, not output

  12. Optimization Criteria Max CPU utilization Max throughput Min turnaround time Min waiting time Min response time

  13. CPU Scheduling Algorithms • First-Come, First-Served (FCFS) • Complete the jobs in order of arrival • Shortest Job First (SJF) • Complete the job with shortest next CPU burst • Priority (PRI) • Processes have a priority • Allocate CPU to process with highest priority • Round-Robin (RR) • Each process gets a small unit of time on CPU (time quantum or time slice)

  14. FCFS: First-Come First-Served • Ready queue data structure: a FIFO queue • Assuming we have (at least) a multiprogrammed system • Example 1 • Draw Gantt chart • Compute the average waiting time for processes with the following next CPU burst times and ready queue order: • P1: 20 • P2: 12 • P3: 8 • P4: 16 • P5: 4 • Waiting time: • Time period spent in the ready queue (assume processes terminate) • Also calculate average waiting time across processes • Assume 0 context switch time E.g., time units are milliseconds CPU burst times

  15. FCFS: First-Come First-Served P1 P2 P3 P4 P5 0 20 32 40 56 60 Solution: Gantt Chart Method • Waiting times? P1: 0 P2: 20 P3: 32 P4: 40 P5: 56 • Average wait time: 148/5 = 29.6

  16. FCFS: First-Come First-Served Advantage: Relatively simple algorithm Disadvantage: long waiting times

  17. Scheduling Algorithms: First-Come, First-Served (FCFS) • Example 2: Three processes arrive in order P1, P2, P3. • P1 burst time: 24 • P2 burst time: 3 • P3 burst time: 3 • Waiting Time • P1: 0 • P2: 24 • P3: 27 • Completion Time: • P1: 24 • P2: 27 • P3: 30 • Average Waiting Time: (0+24+27)/3 = 17 • Average Completion Time: (24+27+30)/3 = 27 P1 P2 P3 27 30 24 0

  18. Scheduling Algorithms: First-Come, First-Served (FCFS) • What if their order had been P2, P3, P1? • P1 burst time: 24 • P2 burst time: 3 • P3 burst time: 3

  19. Scheduling Algorithms: First-Come, First-Served (FCFS) • What if their order had been P2, P3, P1? • P1 burst time: 24 • P2 burst time: 3 • P3 burst time: 3 • Waiting Time • P1: 0 • P2: 3 • P3: 6 • Completion Time: • P1: 3 • P2: 6 • P3: 30 • Average Waiting Time: (0+3+6)/3 = 3 (compared to 17) • Average Completion Time: (3+6+30)/3 = 13 (compared to 27) P2 P3 P1 6 30 3 0

  20. SJF • Provably shortest average wait time • BUT: What do we need to actually implement this?

  21. P5 P3 P2 P4 P1 0 4 12 24 40 60 SJF Solution • Waiting times (how long did process wait before being scheduled on the CPU?): P1: 40 P2: 12 P3: 4 P4: 24 P5: 0 • Average wait time: 16 (Recall: FCFS scheduling had average wait time of 29.6)

  22. Priority Scheduling • Have to decide on a numbering scheme • 0 can be highest or lowest

  23. Starvation Problem • Priority scheduling algorithms can suffer from starvation (indefinite waiting for CPU access) • In a heavily loaded system, a steady stream of higher-priority processes can result in a low priority process never receiving CPU time • I.e., it can starve for CPU time • One solution: aging • Gradually increasing the priority of a process that waits for a long time

  24. Which CPU Scheduling Algorithms Can be Preemptive? • FCFS (First-come, First-Served) • Non-preemptive • SJF (Shortest Job First) • Can be either • Choice when a new (shorter) job arrives • Can preempt current job or not • Priority • Can be either • Choice when a processes priority changes or when a higher priority process arrives

  25. RR (Round Robin) Scheduling • Used in time-sharing or multi-tasking systems • typical kind of scheduling algorithm in a contemporary general purpose operating system • Method • Give each process a unit of time (time slice, quantum) of execution on CPU • Then move to next process in ready queue • Continue until all processes completed • Necessarily preemptive • Requires use of timer interrupt • Time quantum typically between 10 and 100 milliseconds • Linux default appears to be 100ms

  26. RR (Round Robin) Scheduling: Example • CPU job burst times & order in queue • P1: 20 • P2: 12 • P3: 8 • P4: 16 • P5: 4 • Draw Gantt chart, and compute average wait time • Time quantum of 4 • Like our previous examples, assume 0 context switch time

  27. completes completes completes completes completes P1 P2 P3 P4 P5 P1 P2 P3 P4 P1 P2 P4 P1 P4 P1 0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 Solution • Waiting times: P1: 60 - 20 = 40 P2: 44 - 12 = 32 P3: 32 - 8 = 24 P4: 56 - 16 = 40 P5: 20 - 4 = 16 • Average wait time: 30.4

  28. P1 P2 P3 P1 P1 P1 P1 P1 0 10 14 18 22 26 30 4 7 Example 2of RR with Time Quantum = 4 ProcessBurst Time P1 24 P2 3 P3 3 The Gantt chart is:

  29. P1 P2 P3 P1 P1 P1 P1 P1 0 10 14 18 22 26 30 4 7 Example of RR with Time Quantum = 4 ProcessBurst Time P1 24 P2 3 P3 3 • Waiting Time: • P1: (10-4) = 6 • P2: (4-0) = 4 • P3: (7-0) = 7 • Completion Time: • P1: 30 • P2: 7 • P3: 10 • Average Waiting Time: (6 + 4 + 7)/3= 5.67 • Average Completion Time: (30+7+10)/3=15.67

  30. Example 3 of RR with Time Quantum = 20 • Waiting Time: • P1: (68-20)+(112-88) = 72 • P2: (20-0) = 20 • P3: (28-0)+(88-48)+(125-108) = 85 • P4: (48-0)+(108-68) = 88 • Completion Time: • P1: 125 • P2: 28 • P3: 153 • P4: 112 • Average Waiting Time: (72+20+85+88)/4 = 66.25 • Average Completion Time: (125+28+153+112)/4 = 104.5 A process can finish before the time quantum expires, and release the CPU.

  31. Performance Characteristics of Scheduling Algorithms • Different scheduling algorithms will have different performance characteristics • RR (Round Robin) • Average waiting time often high • Good average response time • Important for interactive or timesharing systems • SJF • Best average waiting time • Some overhead with respect to estimates of CPU burst length & ordering ready ‘queue’

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