Process Scheduling in CPU - Optimization and Algorithms

1. Cosc 4740 Chapter 5 Process Scheduling

2. CPU Scheduling • Short-term Scheduler • Selects a process from the ready queue when current process releases the CPU • Very Fast to avoid wasting CPU time • Very efficient • the problem: selecting a CPU algorithm for a particular system • Also updates PCBs and controls the queues.

3. Basic concept of a Process • CPU–I/O Burst Cycle • Process execution consists of a cycle of CPU execution and I/O wait

4. How to decide which ready process to put on the CPU next?

5. Goals • Fairness or Waiting time • amount of time a process has been waiting in the ready queue • – No process will wait “too” long • CPU utilization – keep it busy most of the time • normally about 40-90% of the time • Response time – for interactive users • Turnaround time • the time of completion from submission • Throughput: • # of processes that complete their execution per time unit Consider if it is possible to Max all 5 goals

6. We want to optimize all the goals • This is not possible: • Fairness vs CPU utilization • All schedulers favor a couple of goals. • Normally those goals that help the kind of system it written for.

7. 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 4. Terminates • Scheduling under 1 and 4 is nonpreemptive • All other scheduling is preemptive

8. 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

9. Types of Scheduling Algorithms • Non-preemptive Scheduling • A running process is only suspended when it blocks or terminates • Pros: • Decreases turnaround time, high CPU utilization • Does not require special HW (like a timer) • Cons: • Poor performance for response time and fairness • Limited choice of scheduling Algorithms

10. Preemptive scheduling • A Running process can be taken off the CPU for any (or no) reason. Suspended by the scheduler • Pros: • No limitations on the choice of scheduling algorithms • Increased Fairness and response time • Cons • Addition overheads (frequent context switching) • deceased CPU utilization and longer turnaround time

11. FCFS scheduling • First-Come First Served (non-preemptive) • CPU is allocated to the process that requests it first • Pros: • Simple to understand and implement • Very fast selection for scheduling • Cons: • Avg. waiting time is long and varies • Not suitable for time-sharing OS

12. P1 P2 P3 0 24 27 30 FCFS Example 1 ProcessBurst Time P1 24 P2 3 P3 3 • 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

13. P2 P3 P1 0 3 6 30 FCFS Example 2 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 behind long process

14. SJF Scheduling • Shortest Job Next: Preemptive or Non-Preemptive • Pros: • Yields minimum avg. waiting time for a given set of processes • Cons: • Difficult to estimate CPU burst time • Starvation (indefinite blocking)

15. P1 P3 P2 P4 0 3 7 8 12 16 Example of Non-Preemptive SJF Process Arrival TimeBurst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 • SJF (non-preemptive) • Average waiting time = (0 + 6 + 3 + 7)/4 = 4

16. P1 P2 P3 P2 P4 P1 11 16 0 2 4 5 7 Example of Preemptive SJF Process Arrival TimeBurst Time P1 0.0 7 P2 2.0 4 P3 4.0 1 P4 5.0 4 • SJF (preemptive) • Average waiting time = (9 + 1 + 0 +2)/4 = 3

17. 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

18. Round Robin Scheduling • Preemptive scheduling • Most widely used algorithm • Very simple – CPU isn’t taxed by scheduler • Fair • Each Process is allowed a time interval call a “quantum” or “time slice” • Max time process can run on CPU

19. If a process is still running at the end of a quantum, the scheduler preempts it. • If a process blocks or terminates before it quantum expires, another is allocated immediately CPUReady queue O PQRS (Then quantum expiries) P QRSO

20. P1 P2 P3 P4 P1 P3 P4 P1 P3 P3 0 20 37 57 77 97 117 121 134 154 162 Example of RR Time Quantum = 20 ProcessBurst Time P1 53 P2 17 P3 68 P4 24 • The Gantt chart is: • Typically, higher average turnaround than SJF, but better response time

21. Quantum • How long should a quantum be? • Problems: • quantum is too small (20 ms) • CPU is under utilized – to much context switching • quantum is too large (1 second) • Not fair, poor response time

22. Typical quantum between 100ms and 200ms • When O/S is installed it is tested and adjusted for specific environment • OS could monitor itself, but it is complex • scheduler would not be simple • So normally done by “hand”.

23. Priority Scheduling • Not all processes are equally important • Each process is assigned a priority (integer value) • Process with highest priority runs • If more than 1 with same highest priority, then Round Robin among those processes

24. Priority Scheduling (2) • 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

25. Priority Scheduling (3) • Array of priority queues Highest 0 PQR 1 OS 2 3 A Lowest 4  C • Priority must change over time, or low priority processes will suffer (starve) • Solution  Aging – as time progresses increase the priority of the process

26. Multiple Queue with Dynamic Priority • Priority of processes changes dynamically • Processes are divided into 2 classes • CPU bound – computation intensive • I/O bound – most of time I/O is used • Who should be giving higher priority?

27. If you know which the process is: • I/O bound uses processor for short time, then I/O operations • So I/O bound will leave the CPU sooner, then CPU bound processes • So give I/O bound higher priority.

28. Should quantum size be the same for I/O and CPU bound processes?

29. Vary quantum size w/ dynamic priority • Higher priority will get lower quantum, since it is I/O bound and won’t need it for long Priority Quantum 0 1 Unit Highest priority/lowest quantum 1 2 Units 2 4 Units 3 8 Units Lowest priority/highest quantum

30. How do you know if the process is I/O or CPU bound?? • It can even change as a processes runs!

31. We figure it dynamically • If a running process uses it’s quantum, then lower the priority by 1 • If a running process blocks, the raise priority by 1 • Start new process with priority of 0 (highest) • If I/O bound – we did it right • If CPU bound – quantum is small anyway.

32. Interactive process respond quickly and are usually I/O bound. • So good response time • Fair, since CPU bound process get a longer quantum as they run. • Works well when a process changes from CPU to I/O bound or vise versa.

33. Multi-Processor Scheduling • Very complex • Depends on the underlying HW structure • Heterogeneous processors: Difficult! • Process may have code compiled for only 1 CPU. • Homogeneous processors with separated ready queues • load balancing problem

34. Multi-Processor Scheduling (2) • Homogeneous processors with shared ready queue • Symmetric: Processors are self-scheduling • Need to be synchronized to access the shared ready queue to prevent multiple processors trying to load the same process in the queue. • Asymmetric: There is one master processor who distributes next processes to the other processors. • Simpler than symmetric approach

35. NUMA and CPU Scheduling Note that memory-placement algorithms can also consider affinity

36. Multi-Processor Scheduling (3) • Processor affinity – process has affinity for processor on which it is currently running • soft affinity • hard affinity • Variations including processor sets

37. Real Time Systems • An absolute deadline for process execution must be met. • these take the additional parameter of deadline into account in scheduling ie: P1 P2 P3 P4 P5 deadline 10:00 10:05 9:00 11:00 11:10 • Assign CPU to the process that is in the greatest danger of missing it’s deadline.

38. Hard real-time systems • guaranteed amount of time. Special purpose hardware/software machines • Soft real-time systems • Based on priority based scheduling. • Even kernel must be pre-emptive so a critical process can run. • Resource allocation is “pre-emptive”. If a critical process (A) needs a resourced owned by another process (B), that process B gets process A priority, so it can release the resource. Once processes B released the resource it priority is returned to “normal”.

39. Windows Scheduling • Windows uses priority-based preemptive scheduling • Highest-priority thread runs next • Thread runs until (1) blocks, (2) uses time slice, (3) preempted by higher-priority thread • Real-time threads can preempt non-real-time • 32-level priority scheme • Variable class is 1-15, real-time class is16-31 • Priority 0 is memory-management thread • Queue for each priority • If no run-able thread, runs idle thread

40. Windows Priority Classes • Win32 API identifies several priority classes to which a process can belong • REALTIME_PRIORITY_CLASS, HIGH_PRIORITY_CLASS, ABOVE_NORMAL_PRIORITY_CLASS,NORMAL_PRIORITY_CLASS, BELOW_NORMAL_PRIORITY_CLASS, IDLE_PRIORITY_CLASS • All are variable except REALTIME • A thread within a given priority class has a relative priority • TIME_CRITICAL, HIGHEST, ABOVE_NORMAL, NORMAL, BELOW_NORMAL, LOWEST, IDLE • Priority class and relative priority combine to give numeric priority • Base priority is NORMAL within the class • If quantum expires, priority lowered, but never below base • If wait occurs, priority boosted depending on what was waited for • Foreground window given 3x priority boost

41. Windows XP Priorities

42. Linux Scheduling • Constant order O(1) scheduling time • Preemptive, priority based • Two priority ranges: time-sharing and real-time • Real-time range from 0 to 99 and nice value from 100 to 140 • Map into global priority with numerically lower values indicating higher priority • Higher priority gets larger q • Task run-able as long as time left in time slice (active) • If no time left (expired), not run-able until all other tasks use their slices • All run-able tasks tracked in per-CPU runqueuedata structure • Two priority arrays (active, expired) • Tasks indexed by priority • When no more active, arrays are exchanged

43. Linux Scheduling (Cont.) • Real-time scheduling according to POSIX.1b • Real-time tasks have static priorities • All other tasks dynamic based on nice value plus or minus 5 • Interactivity of task determines plus or minus • More interactive -> more minus • Priority recalculated when task expired • This exchanging arrays implements adjusted priorities

44. Priorities and Time-slice length

45. List of Tasks Indexed According to Priorities

46. Choosing an Algorithm • There is no “best” algorithm, just better suited. • Need to choose the goals that are more important to the environment.

47. Memory and Processes • When main memory is insufficient, some processes will be “swapped out” and ready to run. • Short-term schedulers works with medium-term scheduler to bring these process back into memory. • Why not just have the Short-term Scheduler do the work?

48. Q A &