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CPU Scheduling Algorithms: Maximizing Performance and Efficiency

Explore the fundamental concepts of CPU scheduling, from basic criteria to optimization algorithms. Learn about FCFS, SJF, priority scheduling, and their impact on system performance. Dive deep into Gantt charts and process execution cycles.

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CPU Scheduling Algorithms: Maximizing Performance and Efficiency

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  1. Chapter 5: CPU Scheduling

  2. Basic Concepts • Maximum CPU utilization obtained with multiprogramming • CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait • CPU burst distribution

  3. Alternating Sequence of CPU And I/O Bursts

  4. Histogram of CPU-burst Times

  5. CPU 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 • Terminates • Scheduling only under 1 and 4 is nonpreemptive • All other scheduling is preemptive

  6. 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 • Dispatch latency – time it takes for the dispatcher to stop one process and start another running

  7. Scheduling Criteria • 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 • 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 (for time-sharing environment)

  8. Scheduling Algorithm Optimization Criteria • Max CPU utilization • Max throughput • Min turnaround time • Min waiting time • Min response time • In most cases we optimize the average measure • In some circumstances we want to optimize the min or max values rather than the average – e.g., minimize the max response time so all users get good service

  9. P1 P2 P3 0 24 27 30 First-Come, First-Served (FCFS) Scheduling • Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is: • Waiting time for P1 = 0; P2 = 24; P3 = 27 • Average waiting time: (0 + 24 + 27)/3 = 17 • Response time in FCFS is same as Waiting Time so Average Response time: 17

  10. P1 P2 P3 0 24 27 30 First-Come, First-Served (FCFS) Scheduling • Suppose that the processes arrive in the order: P1 , P2 , P3 The Gantt Chart for the schedule is: • Turnaround time for P1 = 24; P2 = 27; P3 = 30 • Average turnaround time: (24 + 27+ 30)/3 = 27 • Throughput: 3/30 = 0.1

  11. P2 P3 P1 0 3 6 30 FCFS Scheduling (Cont) Suppose that the processes arrive in the order P2 , P3 , P1 • The Gantt chart for the schedule is: • Waiting time for P1 = 6;P2 = 0; P3 = 3 • Average waiting time: (6 + 0 + 3)/3 = 3 • Much better than previous case • Convoy effect : short process wait for one long CPU-bound process to complete a CPU burst before they get a turn

  12. Shortest-Job-First (SJF) Scheduling • Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time • SJF is optimal – gives minimum average waiting time for a given set of processes • The difficulty is knowing the length of the next CPU request

  13. P3 P2 P4 P1 3 9 16 24 0 Example of Non-Preemptive SJF • SJF scheduling chart • Throughput = 4/24 • Average waiting time = (3 + 16 + 9 + 0) / 4 = 7 • Average response time = (3 + 16 + 9 + 0) / 4 = 7 • Average turnaround time = (9 + 24 + 16 + 3) / 4 = 13

  14. Determining Length of Next CPU Burst • Can only estimate the length • Can be done by using the length of previous CPU bursts, using exponential averaging

  15. Prediction of the Length of the Next CPU Burst

  16. Examples of Exponential Averaging •  =0 • n+1 = n • Recent history does not count •  =1 • n+1 = tn • Only the actual last CPU burst counts • If we expand the formula, we get: n+1 =  tn+(1 - ) tn-1+ … +(1 -  )j tn-j+ … +(1 -  )n +1 0 • Since both  and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor

  17. P1 P2 P3 P2 P4 P1 11 16 0 2 4 5 7 Example of Preemptive SJF • The Gantt chart for preemptive SJF is: • Throughput = 4 /16 = 0.25 • Average waiting time = (9 + 1 + 0 +2)/4 = 3 • Average response time = ( 0 + 0 + 0 + 2) / 4 = 0.5 • Average turnaround time = ( 16 + 5 + 1 + 6) / 4 = 7

  18. Priority Scheduling A priority number (integer) is associated with each process The CPU is allocated to the process with the highest priority (smallest integer  highest priority) Preemptive nonpreemptive SJF is a priority scheduling where priority is the predicted next CPU burst time Problem  Starvation – low priority processes may never execute Solution  Aging – as time progresses increase the priority of the process

  19. Example of Priority Scheduling (nonpreemptive) • The Gantt chart is (all processes arrive at time 0): • Average waiting time = (6 + 0 + 16 +18+1)/5 = 8.2 P2 P5 P1 P3 P4 16 18 19 0 1 6

  20. Round Robin (RR) • Each process 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. • If there 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. • Performance • q large  FIFO • q small  q must be large with respect to context switch, otherwise overhead is too high • TA = Completion time – arrival time • Waiting Time = TA – Burst time

  21. P1 P2 P3 P1 P1 P1 P1 P1 0 10 14 18 22 26 30 4 7 Example of RR with Time Quantum = 4 • The Gantt chart is: • Typically, higher average turnaround than SJF, but better response

  22. Time Quantum and Context Switch Time

  23. Turnaround Time Varies With The Time Quantum

  24. Multilevel Queue • Ready queue is partitioned into separate queues, for example: foreground (interactive) background (batch) • Each queue has its own scheduling algorithm • foreground – RR • background – FCFS • Scheduling must be done between the queues • Commonly a fixed priority preemptive scheduling; (i.e., serve all from foreground then from background). Possibility of starvation. • Another solution: 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 20% to background in FCFS

  25. Multilevel Queue Scheduling

  26. Multilevel Feedback Queue • A process can move between the various queues; aging can be implemented this way • Example, 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 servedRR. 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 RR and receives 16 additional milliseconds. If it still does not complete (in 24ms) , it is preempted and moved to queue Q2. • At Q2 job is served FCFS

  27. Multilevel Feedback Queues Q0 Q1 Q2

  28. Multilevel Feedback Queue • 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

  29. Solaris Scheduling • Priority based scheduling and where, each thread belongs to a class (from 6) • Time sharing (TS) • Interactive (IA) • Real time (RT) • System (SYS) • Fair share (FSS) • Fixed priority (FP)

  30. Solaris Scheduling

  31. Solaris Dispatch Table for TS and IA Priority if:

  32. Windows XP Priorities • Priority based, preemptive scheduling algorithm • 32 priorities are divided into two classes • Variable class – 1 through 15 and Real time class – 16 through 31 • Priority is based on both the priority class it belongs to (realtime, high, etc) and the relative priority within each class • When a threads quantum runs out, the thread is interrupted. If it is in the variable priority class its priority is lowered (never below the base priority for that class) • Gives good response to interactive threads (mouse, windows) and enables I/O bound threads to keep the I/O devices busy while compute bound use spare CPU cycles in the background

  33. Linux Scheduling • Constant order O(1) scheduling time • Preemptive, priority-based with two priority ranges: time-sharing and real-time • Real-time range from 0 to 99 and time sharing value from 100 to 140 • Higher-priority tasks got longer time quanta

  34. Priorities and Time-slice length

  35. List of Tasks Indexed According to Priorities • When all tasks have exhausted their time slices, the two priority arrays are exchanged

  36. End of Chapter 5

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