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CHAPTER 6 CPU SCHEDULING

CHAPTER 6 CPU SCHEDULING. CGS 3763 - Operating System Concepts UCF, Spring 2004. OVERVIEW. CPU/IO Burst Cycle Scheduling Criteria Scheduling Algorithms First Come, First Serve (FCFS) Priority Scheduling Shortest Job First Round Robin Multi-Level Queuing Scheduling for Multi-Processors

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CHAPTER 6 CPU SCHEDULING

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  1. CHAPTER 6CPU SCHEDULING CGS 3763 - Operating System Concepts UCF, Spring 2004

  2. OVERVIEW • CPU/IO Burst Cycle • Scheduling Criteria • Scheduling Algorithms • First Come, First Serve (FCFS) • Priority Scheduling • Shortest Job First • Round Robin • Multi-Level Queuing • Scheduling for Multi-Processors • Scheduling for Real-Time Systems • Evaluating Scheduling Algorithms • Deterministic Modeling • Queuing Models • Simulations

  3. BASIC CONCEPTS • OS scheduling occurs in several places: • Long Term: When can jobs enter a system • Short Term: Determining which process gets the CPU • Medium Term: Used to balance process mix • In this chapter, focus on short term or CPU scheduling • Object is to maximum CPU utilization • Primarily applies to multiprogramming systems • Also increases memory and I/O device utilization • Multiprogramming requires that the CPU be shared among numerous processes • This sharing in part based on concept of CPU–I/O Burst Cycle • Process execution consists of a cycle of CPU execution followed by an I/O operation and waiting • This fact allows us to share the CPU during I/O operations

  4. ALTERNATING CPU AND I/O BURSTS

  5. HISTOGRAM OF CPU-BURST TIMES • Most CPU bursts are very short in duration

  6. CPU SCHEDULING • The process whereby the OS selects from among processes in the ready queue, and allocates the CPU to one of them. • CPU scheduling decisions (selecting a new process to execute) may take place when a process: • 1. Switches from running to waiting state. • 2. Currently running process terminates • 3. Switches from running to ready state (timer/RR). • 4. Switches from waiting to ready (I/O completed). • 5. New job enters ready queue. • 6. Process moves from suspend to ready queue (MTS) • Scheduling under 1 and 2 is nonpreemptive. • Currently executing process leaves running state voluntarily • Scheduling under 2 thru 6 can be preemptive.

  7. DISPATCHER • Dispatcher is term given to OS module which assigns control of the CPU to the process selected by the short-term scheduler; • The Dispatcher performs the following: • switching context from one process to another • switching to user mode after switch is made • 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. • The longer the dispatch latency, the less time available for the CPU to perform useful work.

  8. SCHEDULING CRITERIA • A good scheduling algorithm is one which: • Maximizes CPU utilization • keep the CPU as busy as possible • Maximizes Throughput • # of processes that complete their execution per time unit • Minimizes Turnaround Time • amount of time to execute a particular process • Minimizes Waiting Time • amount of time process has been waiting in ready queue • Minimizes Response Time • amount of time it takes from when a request was submitted until the first response is produced (for time-sharing environment) • Eliminates Starvation • when a process never or rarely executes due to the algorithm

  9. FIRST-COME, FIRST-SERVED (FCFS) • Processes are assign the CPU in the same order in which they entered the ready queue. • Sometimes referred to as First-In, First-Out (FIFIO) • Non-preemptive algorithm by its very nature • Avoids starvation (not possible with FCFS) • Not usable in time sharing systems • Average waiting time is generally not minimal • Can result in convoy affect

  10. P1 P2 P3 0 24 27 30 FIRST-COME, FIRST-SERVED EXAMPLE ProcessBurst Time P1 24 P2 3 P3 3 • Assume that processes arrive at time=0 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 • Illustrates “convoy” effect • short processes get stuck behind processes with long bursts

  11. P2 P3 P1 0 3 6 30 FIRST-COME, FIRST-SERVED EXAMPLE • 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, no “convoy” effect

  12. LAST-IN, FIRST-OUT (LIFO) • Not used in CPU scheduling. • But for purposes of illustration….. • Last process to enter ready queue gets CPU first • Highly susceptible to starvation • Not used for that very reason

  13. PRIORITY SCHEDULING • Processes assigned to CPU based on some priority value determined by system administrator. Higher priority process gets CPU. • If two or more processes have same priority, then choose from among them based on FCFS (the tie breaker) • Priority scheduling is a “general” algorithm. Can apply to • External Priorities: • Uses criteria that are external to OS such as importance of process, job type (student vs. faculty job), amount of funds (pay more for higher priority), etc. • can be set by user when job first submitted • Internal Priorities: • Priority based on some measurable computational quality such as time limit, memory requirements, number of files needed, past I/O to CPU burst ratio, etc. • can be set by user when job first submitted or can vary over time as process executes.

  14. PRIORITY SCHEDULING (cont.) • Regardless of priority type, a single value is associated with each process & saved in each process’ PCB • often an integer • smallest integer = highest priority • Priority scheduling can be preemptive or nonpreemptive. • Preemptive priority scheduling can reduce turnaround time on higher priority jobs but only at the expense of • lower priority jobs taking longer and • additional context switches

  15. PRIORITY SCHEDULING (cont.) • With priority scheduling: • Average waiting time generally not minimal • Assigning higher priority to shorter jobs can reduce the overall average wait time. Concept is applied more explicitly using burst sizes with SJF scheduling algorithm. • Still not good for time sharing • Could assign higher priorities to time share processes and use preemption – sort of a work around but possible starvation of batch processes • Biggest problem with priority scheduling: • Starvation, low priority processes may never or rarely executes • Can use aging to prevent starvation. As time progresses increase the priority of each waiting process in the ready queue. Over time, the priority will increase to a point where the process will be assigned the ready queue.

  16. P2 P4 P1 0 1 5 9 16 NON-PREMPTIVE PRIORITY SCHEDULING • Assume processes arrive at time=0 in the order: P1 , P2 , P3, P4 • Lower number indicates higher priority ProcessBurst TimePriority P1 7 3 P2 4 2 P3 1 1 P4 4 2 • Waiting time for P1 = 9; P2 = 1; P3 = 0; P4 = 5 • Average waiting time: (9 + 1 + 0 + 5) / 4 = 3.75 P3

  17. PREEMPTIVE PRIORITY EXAMPLE • Assume processes arrive at times indicated • Lower number indicates higher priority ProcessArrivalBurst TimePriority P1 0 7 3 P2 5 4 2 P3 3 1 1 P4 2 4 2 • Waiting time for P1 = ___; P2 = ___; P3 = ___; P4 = ___ • Average waiting time:

  18. SHORTEST JOB FIRST (SJF) • Associate with each process the length of its next CPU burst. Use these lengths to select the process with the shortest time. • Use FCFS to break tie if two or more processes have the same next burst length • SJF can be proven to yield the shortest minimum average waiting time for a given set of processes. • However, can starve processes with long CPU bursts • Two schemes: • Nonpreemptive – once CPU given to the process it cannot be preempted until completes its CPU burst. • Preemptive – if a new process arrives in the ready queue with CPU burst length less than remaining time of current executing process, preempt. Known as • Shortest-Remaining-Time-First (SRTF) or • Shortest-Remaining-Burst-First (SRBF)

  19. P1 P3 P2 P4 0 3 7 8 12 16 NON-PREMPTIVE SJF EXAMPLE ProcessArr. TimeBurst Time P1 0 7 P2 2 4 P3 4 1 P4 5 4 • Average waiting time = (0 + 6 + 3 + 7)/4 = 4

  20. P1 P2 P3 P2 P4 P1 11 16 0 2 4 5 7 PREMPTIVE SJF EXAMPLE ProcessArr. TimeBurst Time P1 0 7 P2 2 4 P3 4 1 P4 5 4 • Average waiting time = (9 + 1 + 0 +2)/4 = 3

  21. ESTIMATING LENGTH OF NEXT CPU BURST • Don’t actually know length of next CPU burst. • Can only estimate the length. • Can be done by using the length of previous CPU bursts and process known as exponential averaging. •  is constant that determines weight of factors • If  = 0, past history does not count. • If  = 1, only the actual last CPU burst counts (next = last) • If 0 >  < 1, tacks into account last burst and burst history

  22. PREDICTING NEXT CPU BURST

  23. ROUND ROBIN SCHEDULING • Each process gets a small unit or slice of CPU time • referred to as a time quantum • usually 10-100 milliseconds • After time has elapsed (timer interrupt), the process is preempted and added to the end of the ready queue • Assumes process has not already terminated or moved to waiting state • The process at front of ready queue assigned CPU next • Operates on a FCFS basis when selecting next process

  24. ROUND ROBIN (cont.) • 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 • If time quantum (q) larger than largest CPU burst, round robin effectively becomes FCFS scheduling • If time quantum (q) small we get processor sharing. Looks like each process has its own CPU running at 1/n the speed of the real CPU. • If time quantum (q) very very small, too many context switches will be required, overhead too high. Should be large with respect to context switching time.

  25. P1 P2 P3 P4 P1 P3 P4 P1 P3 P3 0 20 37 57 77 97 117 121 134 154 162 EXAMPLE OF RR w/ TIME QUANTUM = 20 Process Burst Time P1 53 P2 17 P3 68 P4 24 • The Gantt chart is: • Typically, higher average turnaround than SJF, but better response.

  26. MULTILEVEL QUEUING • Ready queue is partitioned into separate queues. • Each queue can have its own scheduling algorithm. • For example a two-queue system may be as follows: • foreground queue (interactive) with RR Scheduling • background queue (batch) with FCFS Scheduling • In addition, scheduling must be done between the queues. • Fixed priority scheduling • e.g., serve all processes in foreground queue then those in background (possibility of starvation) • Time slice scheduling • each queue gets a certain amount of CPU time which it can schedule amongst its processes • e.g., 80% to foreground queue, 20% to background queue • Can result in starvation for processes in lower level queues

  27. MULTILEVEL QUEUING (cont.) • Number of queues can vary by system.

  28. MULTILEVEL FEEDBACK QUEUING • A process can move between the various queues using an aging algorithm • Multilevel-feedback-queue scheduler defined by the following parameters: • number of queues • scheduling algorithms for each queue • method used to determine which queue a process will enter when that process needs service • method used to determine when to upgrade a process • (move to a higher queue) • method used to determine when to demote a process • (move to a lower queue)

  29. MULTILEVEL FEEDBACK QUEUING EXAMPLE • Three queues: Q0 – time quantum 8 milliseconds Q1 – time quantum 16 milliseconds Q2 – FCFS • Scheduling • A new job enters queue Q0 which is served FCFS. • 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.

  30. MULTILEVEL FEEDBACK QUEUING (cont.)

  31. MUTLI-PROCESSOR SCHEDULING • CPU scheduling more complex when multiple CPUs (processors) are available. • Homogeneous: All processors alike and have access to same resources. • Heterogeneous: Processors can be different (e.g., speed, capability) or have access to different resources • Requires additional effort to determine which processes can run on which processors/CPUs • Individual vs. Common Ready Queues • Does each processor have its own queue or are all jobs placed in single queue (allows load sharing) • What if process already allocated memory? • Processor scheduling options: • centralized scheduler, can be dedicated CPU • each processor is self-scheduling • must have access to shared OS data structures if a single queue.

  32. REAL-TIME SCHEDULING • Hard real-time systems: • Guaranteed completion times for all critical tasks • Can’t use secondary storage or virtual memory • Generally special purpose processors • Soft real-time computing: • Approximation of hard real-time system • Very small dispatch latencies • Uses priority scheduling • critical processes receive priority over lower priority (non-real-time) processes. • must not age non-real time processes to a level equal to or greater than that of real time processes.

  33. SCHEDULING ALGORITHM EVALUATION • How do we decide which scheduling algorithm(s) to use with an operating system? • Determine what your evaluation criteria will be: • Utilization, turn around time, response time, etc. • Evaluate each algorithm with respect to your criteria • Deterministic Modeling • Queuing Models • Simulations

  34. DETERMINISTIC MODELING • Take a particular predetermined workload and define the performance of each algorithm for that workload. • Gantt chart problems are examples of this method • Simple, fast (if small workload) • Must have exact data • Gives answer for very specific situation • Used primarily for illustration purposes

  35. QUEUING MODELS • Treat OS and processes as system or network of queues and servers • Use queuing network analysis to determine characteristics of the system • Various software packages on the market to perform the analysis • Generally must make assumptions about the distribution of CPU bursts and arrival rates of processes • At best, queuing models provide an approximation of a real world system

  36. LITTLE’S FORMULA • Little’s formula gives us an example of how queuing analysis works n = L * W • Where n = average queue length L = average arrival rate into the queue W = average wait time in queue • Assume system is in steady state • # processes arriving in ready queue equals number of processes leaving ready queue. • If average queue length = 14 and one process arrives in the queue every 7 seconds then: 14 processes in queue = 1 process / 7 seconds * W W = 98 seconds (average wait time in queue)

  37. SIMULATIONS • Build a computer model of the operating system and its scheduling algorithms. Run the model using statistical data from a real system. Compare results. • Data for the model can come from many sources: • Randomly generated • Generated based on predetermined distributions (e.g., uniform, exponential, Poisson) • Captured using trace tapes from existing system. • The more detailed the simulation, the greater the cost • Programming time (writing the simulator) • Storage of trace data • Running time (varies with level of complexity and size of data set)

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