Scheduling in Representative Operating Systems

1. Scheduling in Representative Operating Systems

2. Uniprocessor Scheduling CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

3. Uniprocessor Scheduling (cont.) CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

4. CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

5. CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

6. Scheduling Algorithms (cont.) • Short-term scheduling criteria • User oriented, performance related • Turnaround time • Response time • Deadlines • User oriented, other • Predictability • System oriented, performance related • Throughput • Processor utilization • System oriented, other • Fairness • Enforcing priorities • Balancing resources • Priorities CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

7. CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

8. Scheduling Algorithms (cont.) • Scheduling policies • First-Come-First Served (FCFS) or First-In-First-Out (FIFO) • Round Robin (RR) • Shortest Process Next (SPN) • Shortest Remaining Time (SRT) • Highest Response Ratio Next (SRRN) • Feedback • Fair-Share Scheduling CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

9. CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

10. Traditional UNIX Scheduling • SVR3 and 4.3 BSD UNIX: designed primarily for time-sharing interactive environment • Scheduling algorithm objectives • Provide good response time for interactive users • Ensure that low-priority background jobs do not starve • Scheduling algorithm implementation • Multilevel feedback using round robin within each of the priority queues • 1 second preemption • Priority based on process type and execution history • Priorities are recomputed once per second • Base priority divides all processes into fixed bands of priority levels • Bands used to optimize access to block devices (e.g., disk) and to allow the OS to respond quickly to system calls CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

11. Traditional UNIX Scheduling (cont.) • Priority calculation CPU j(i) = CPU j ( i – 1) 2 CPU j ( i – 1) P j (i) = Base j + 2 Where CPU j (i) = Measure of processor utilization by process j through interval i P j ( i ) = Priority of process j at beginning of interval i ; lower values equal higher priorities Base j= Base priority of process j CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

12. Traditional UNIX Scheduling (cont.) • Bands in decreasing order of priority • Swapper • Block I/O device control • File manipulation • Character I/O device control • User processes • Goals • Provide the most efficient use of I/O devices • Within the user process band, use the execution history to penalize processor-bound processes at the expense of I/O bound processes • Example of process scheduling • Processes A, B, and C are created at the same time with base priorities of 60 • Clock interrupts the system 60 times a second and increments counter for running process CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

13. Traditional UNIX Scheduling (Example CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

14. Multiprocessor and Real-Time Scheduling • Multiprocessor Thread Scheduling • Load sharing • Global queue of ready threads is maintained and each processor, when idle, selects a thread from the queue • Gang scheduling • Set of related threads is scheduled to run on a set of processors at the same time, on a one-to-one basis • Dedicated processor assignment • Each program is assigned a number of processors equal to the number of threads in the program, for the duration of program execution • Dynamic scheduling • Number of threads in a process can be altered during course of execution CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

15. Thread Scheduling • Load Sharing • Load is distributed evenly across the processors • No centralized scheduler required • The global queue can be organized and accessed using any scheduling policy • One of the most widely used schemes in multiprocessing • Gang Scheduling • Simultaneous scheduling of threads that make up a single process • Useful for applications where performance severely degrades when any part of the application is not running • Threads often need to synchronize with each other CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

16. Thread Scheduling • Dedicated Processor Assignment • When application is scheduled, each of its threads is assigned to a processor that remains dedicated to that thread through completion • Some processors may be idle • No multiprogramming of processors • Avoidance of process switching can improve speed of application • Dynamic Scheduling • The number of threads in the process are changed dynamically • Both the OS and the application are involved in scheduling: the OS assigns processors to jobs, the application decides which thread subset to run • For applications that can take advantage of dynamic scheduling, this approach is superior to both gang scheduling and dedicated processor assignment • The overhead may negate the apparent performance advantage CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

17. Real-Time Systems • Correctness of the system depends not only on the logical result of the computation but also on the time at which the results are produced • Tasks or processes attempt to control or react to events that take place in the outside world • The events being controlled occur in “real time”; the real-time task must be able to keep up with these events • Examples • Control of laboratory experiments • Process control plants • Robotics • Air traffic control • Telecommunications • Military command and control systems CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

18. Characteristics of Real-Time Operating Systems • Deterministic • Operations are performed at fixed, predetermined times or within predetermined time intervals • The OS must acknowledge interrupts in time and must have sufficient capacity to handle all requests within required time • Responsiveness • How long, after acknowledgment, it takes the operating system to service the interrupt • User control • User specifies task priority and may also specify the use of paging or process swapping, and the processes resident in main memory, • Reliability • Loss or degradation of performance may have catastrophic consequences • Attempt either to correct the problem or minimize its effects while continuing to run • Make sure that the most critical, high priority tasks execute CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

19. Features of Real-Time Operating Systems • Fast process or thread switch • Preemptive scheduling base on priority • Minimization of intervals during which interrupts are disabled • Ability to respond to external interrupts quickly • Use of special sequential files that can accumulate data at a fast rate • Special alarms and timeouts CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

20. Scheduling of a Real-Time Process CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

21. Scheduling of a Real-Time Process CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

22. Scheduling of a Real-Time Process CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

23. Scheduling of a Real-Time Process CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

24. Real-Time Scheduling • Classes of algorithms • Static table-driven scheduling • Applicable to tasks that are periodic • Scheduler develops a schedule that meet the requirements of all periodic tasks • Static priority-driven preemptive scheduling • Uses priority-driven preemptive scheduling mechanism • Priority assignment is related to the time constraints associated with each task • Dynamic planning-based scheduling • After the arrival of a new task, an attempt is made to revise the schedule such that the needs of the new task and the existing tasks are met • Dynamic best effort scheduling • On arrival, task assigned priority based on its characteristics • Deadline scheduling (e.g., earliest-deadline scheduling) is used CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

25. Deadline Scheduling • Real-time applications are concerned with completing (or starting) tasks on time • Scheduling tasks with the earliest deadline minimized the fraction of tasks that miss their deadlines • Recent proposals for real-time task scheduling are based on additional information about each task • Ready time • Starting deadline • Completion deadline • Processing time • Resource requirements • Priority • Subtask scheduler CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

26. Linux Scheduling • Enhances the traditional UNIX scheduling by adding two new scheduling classes for real-time processing • Scheduling classes • SCHED_FIFO: First-in-first-out real-time threads • SCHED_RR: Round-robin real-time threads • SCHED_OTHER: Other, non-real-time threads • Multiple priorities can be used within each class; priorities for real-time threads higher than for non-real-time threads CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

27. Linux Scheduling (cont.) • Scheduling for the FIFO threads is done according with the following rules • An executing FIFO thread can be interrupted only when • Another FIFO thread of higher priority becomes ready • The executing FIFO thread becomes blocked, waiting for an event • The executing FIFO thread voluntarily gives up the processor (sched_yield) • When an executing FIFO thread is interrupted, it is placed in a queue associated with its priority • When a FIFO thread becomes ready and it has a higher priority than the currently running thread, the running thread is pre-empted and the thread with the higher priority is executed. If several threads have the same, higher priority, the one that has been waiting the longest is assigned the processor • Scheduling for the Round-Robin threads is similar, except for a time quota associated with each thread • At the end of the time quota, the thread is suspended and a thread of equal or higher priority is assigned the processor CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

28. Linux Scheduling (cont.) • Example: The distinction between FIFO and RR scheduling • Assumptions (a) • A program has four threads with three relative priorities • All waiting threads are ready to execute when the current thread waits or terminates • No higher-priority thread is awakened while a thread is executing • Scheduling of FIFO threads (b) • Thread D executes until it waits or terminates • Thread B executes (it has been waiting longer than C) until waits or terminates • Thread C executes • Thread A executes • Scheduling of Round-Robin threads ( c ) • Thread D executes until it waits or terminates • Threads B and C execute time-sliced • Thread A executes • Scheduling of non-real-time threads • Execute only if no real-time threads are ready using the traditional UNIX scheduling algorithm CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

29. Linux Scheduling: FIFO vs. RR CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

30. UNIX SVR4 Scheduling • Complete redesign of the traditional UNIX scheduling algorithm • Goal: give preference in the following order • Highest preference to real-time processes • Next-highest to kernel-mode processes • Lowest preference to other user-mode processes (time-shared processes) • Major enhancements • The addition of a preemptable static priority scheduler • The introduction of 160 priority levels divided into three priority classes • The introduction of preemption points in the kernel (points where the kernel can safely interrupt and schedule a new process) • Priority classes and levels • Real-time (159-100) • Guaranteed execution before kernel or time-sharing processes • Preemption points can be used to preempt kernel or user processes • Kernel (99-60) • Guaranteed execution before time-sharing processes • Time-shared (59-0) • Lowest-priority processes: user applications other than real-time CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

31. UNIX SVR4 Scheduling (cont.) • Implementation • A dispatch queue is associated with each priority level; processes at that level are executed round-robin (dispq) • A bit-map vector, dqactmap, has one bit for each priority level; a 1 indicates a nonempty queue at that level • When a process leaves the Running state (block, time-slice expiration, or preemption), the dispatcher • Checks the dqactmap, and • Dispatches a process from the highest-priority non-empty queue • When a defined preemption point is reached, the kernel • Checks the flag kprunrun • If set (at least one real-time process is in the Ready state), preempts the current process if it is of lower priority than the highest-priority real-time ready process • A real-time process has a fixed priority and a fixed quantum • A time-sharing process has a variable priority and a variable time quantum • The priority is reduced every time the process uses up the time quantum and is raised when the process blocks on an event or resource • The time quantum changes with the priority (100ms for priority 0, 10ms for 59) CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

32. UNIX SVR4 Scheduling: Dispatch Queues CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

33. Windows 2000 Scheduling • Goal: optimize response for • Single user in highly interactive environment, or • Server • Implementation • Priority-driven preemptive scheduler with round-robin within a priority level • When a thread becomes ready and has higher priority than the currently running thread, the lower priority thread is preempted • Dynamic priority variation as function of current thread activity for some levels • Process and thread priorities organized into two bands (classes), each with 16 levels • Real-time priorities: • Real-time tasks or time-dependent functions (e.g., communications) • Have precedence over other threads • Variable priorities • Non-real-time tasks and functions CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

34. Windows 2000 Scheduling (cont.) CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

35. Windows 2000 Scheduling (cont.) • Real-time priority class • All threads have a fixed priority that never changes • All active threads at a given priority level are in a round-robin queue • Variable priority class • A thread’s priority begins at some initial assigned value and then may change, up or down, during the thread’s lifetime • There is a FIFO queue at each priority level, but a thread may migrate to other queues within the variable priority class • The initial priority of a thread is determined by • Process base priority: attribute of the process object, from 0 to 15 • Thread base priority: equal to that of its process or within two levels above or below that of the process • Dynamic priority • Thread starts with base priority and then fluctuates within given boundaries, never falling under base priority or exceeding 15 CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

36. Windows 2000 Scheduling (cont.) • Variable priority class: dynamic priorities • If thread is interrupted because it has used its current time quantum, executive lowers its priority: processor-bound threads move toward lower priorities • If thread is interrupted to wait on an I/O event, executive raises its priority: I/O-bound threads move toward higher priorities • Priority raised more for interactive waits (e.g., wait on keyboard or display) • Priority raised less for other I/O (e.g., disk I/O) • Example • A process object has a base priority of 4 • Each thread associated with this process can have an initial priority between 2 and 6 and dynamic priorities between 2 and 15 • The thread’s priority will change based on its execution characteristics CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

37. Windows 2000 Scheduling: Example of Dynamic Priorities CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]

38. Windows 2000 Scheduling (cont.) • Single processor vs. multiprocessor scheduling • Single processor • Highest-priority thread is always active unless it is waiting on an event • If there is more than one thread at the highest priority, the processor is shared, round-robin • Multiprocessor system with N processors • The (N – 1) highest-priority threads are always active, running on the (N – 1) processors • All other lower-priority threads share the remaining processor • Example: three processors, two highest-priority threads run on two processors, all other threads run on the remaining processor • Exception for threads with processor affinity attribute CS-550 (M.Soneru): Scheduling in Representative Operating Systems: [Sta’01]