Chapter 6 Process Scheduling

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1 1 Chapter 6 Process Scheduling CPU scheduling deals with the problem of deciding which of the processes in the ready queue is to be allocated the CPU. There are many different CPU-scheduling algorithms. Different scheduling algorithms have different properties and may favor one class of processes over another. In choosing which algorithm to use in a particular situation, we must consider the properties of the various algorithms. Many criteria have been suggested for comparing CPU-scheduling algorithms. Which characteristics are used for comparison can make a substantial difference in the determination of the best algorithm. Criteria that are used include the following: Fairness, make sure each process gets its fair share of the CPU CPU utilization or Efficiency, keep CPU as busy as possible Throughput, the number of processes that are completed per time unit Turnaround time, the time of submission to the time of completion Waiting time (in the ready queue) Response time, the time from the submission of a request until the first response is produced. 6.1 First-Come, First-Served (FCFS) Scheduling The implementation of the FCFS is easily managed with a FIFO queue. When a process enters the ready queue, its PCB is linked onto the tail of the queue. When the CPU is free, it is allocated to the process at the head of the ready queue. Example, head Process Burst Time P1 24 P2 3 tail P3 3 P1 P2 P

2 2 The waiting time is 0 milliseconds for process P1, 24 milliseconds for process P2, and 27 milliseconds for process P3. Thus, the average waiting time is ( )/3 = 17 milliseconds. If the process arrive in the order P2, P3, P1, however, the results will be as follow: P2 P3 P The average waiting time is now (6+0+3)/3 = 3 milliseconds. Thus, the average waiting time under a FCFS is generally not minimal, and may vary substantially if the process CPU-burst times vary greatly. There is a convoy effect, as all the other processes wait for the one big process to get off the CPU. The FCFS scheduling algorithm is nonpreemptive. 6.2 Shortest-Job-First (SJF) Scheduling When the CPU is available, it is assigned to the process that has the smallest next CPU burst. If two processes have the same length next CPU burst, FCFS scheduling is used to break the tie. Example, for nonpreemptive, Process Burst Time P1 6 P2 8 P3 7 P4 3 P4 P1 P3 P The waiting time is 3 milliseconds for process P1, 16 milliseconds for process P2, 9 milliseconds for process P3, and 0 milliseconds for process P4. Thus, the average waiting time is ( )/4 = 7 milliseconds. The SJF scheduling algorithm is provably optimal, in that it gives the minimum average waiting time for a given set of processes. The real difficulty with the SJF algorithm is knowing the length of the next CPU request. For long-term (job) scheduling in batch system, we can use the process time limit. However, it is cannot be implemented at the level of short-term CPU scheduling. There is no way to know the length of the next CPU burst. One approach is to try to approximate SJF scheduling. The next CPU burst is generally predicted as an exponential average of the measured lengths of previous CPU bursts.

3 3 Let t n be the length of the n th CPU burst, and let τ n+1 be our predicted value for the next CPU burst. Then, for α, 0 α 1, define an exponential average, τ n+1 = αt n +(1-α)τ n. Example, α = 1/ burst length time CPU burst (t i ) guess (τ i ) The SJF algorithm may be either preemptive or nonpreemptive. The choice arises when a new process arrives at the ready queue while a previous process is executing. The new process may have a shorter next CPU burst than what is left of the currently executing process. A preemptive SJF algorithm will preempt the currently executing process, whereas a nonpreemptive SJF algorithm will allow the currently running process to finish its CPU burst. Preemptive SJF scheduling is sometimes called shortest-remaining-time-first scheduling. Example, for preemptive Process Arrival Time Burst Time P1 0 8 P2 1 4 P3 2 9 P4 3 5 P1 P2 P4 P1 P Process P1 is started at time 0, since it is the only process in the queue. Process P2 arrives at time 1. The remaining time for process P1 (7 milliseconds) is larger than the time required by process P2 (4 milliseconds), so process P1 is preempted, and process P2 is scheduled. The average waiting time for this example is ((10-1)+(1-1)+(17-2)+(5-3))/4 = 26/4 = 6.5 milliseconds. A nonpreemptive SJF scheduling would result in an average waiting time of 7.75 milliseconds.

4 4 6.3 Priority Scheduling A priority is associated with each process, and the CPU is allocated to the process with the highest priority. Equal-priority processes are scheduled in FCFS order. However, there is no general agreement on whether 0 is the highest or lowest priority. The SJF is a special case of the general priority scheduling algorithm, where the priority (p) is the inverse of the (predicted) next CPU burst (τ): p = 1/τ). The larger the CPU burst, the lower the priority, and vice versa. Example, Process Burst Time Priority P P2 1 1 P3 2 3 P4 1 4 P5 5 2 P2 P5 P1 P3 P The average waiting time is 8.2 milliseconds. A major problem with priority scheduling algorithms is indefinite blocking or starvation. It can leave some low-priority processes waiting indefinitely for the CPU. A solution to this problem is aging. Aging is a technique of gradually increasing the priority of processes that wait in the system for a long time. For example, if priorities range for 0 (low) to 127 (high), we could increment a waiting process s priority by 1 every 15 minutes. 6.4 Round-Robin (RR) Scheduling The round-robin scheduling algorithm is designed especially for time-sharing systems. A small unit of time, called a time quantum or time slice, is defined. A time quantum is generally from 10 to 100 milliseconds. The ready queue is treated as a circular queue. The CPU scheduler goes around the ready queue, allocating the CPU to each process for a time interval of up to 1 time quantum. The process may have a CPU burst less than 1 time quantum. In this case, the process itself will release the CPU voluntarily. Example, time quantum = 4 milliseconds, Process Burst Time next P1 24 P2 3 P3 3

5 5 P1 P2 P3 P1 P1 P1 P1 P The average waiting time is 17/3 = 5.66 milliseconds. The RR scheduling algorithm is preemptive. The performance of the RR algorithm depends heavily on the size of the time quantum. At one extreme, if the time quantum is very large (infinite), the RR is the same as the FCFS. If the time quantum is very small (say 1 microsecond), the RR is called processor sharing, and appears (in theory) to the users as though each of n processes has its own processor running at 1/n the speed of the real processor. In software, however, we need also to consider the effect of context switching on the performance of RR scheduling. For example, a small time quantum increases context switches, process time = 10 quantum context switches Thus, we want the time quantum to be large with respect to the context-switch time. Turnaround time also depends on the size of the time quantum. As we can see from the following figure: Process Time P1 6 P2 3 P3 1 P4 7 average turn around time time quantum

6 6 The average turnaround time of a set of process does not necessarily improve as the timequantum size increases. In general, the average turnaround time can be improved if most processes finish their next CPU burst in a single time quantum. For example, given three processes of 10 time units each and a quantum of 1 time unit, the average turnaround time is 29. If the time quantum is 10, however, the average turnaround time drops to Multilevel Queue Scheduling A multilevel queue scheduling algorithm partitions the ready queue into separate queues. For example, a common division is made between foreground (interactive) processes and background (batch) processes. Processes are permanently assigned to one queue. Each queue has its own scheduling algorithm. For example, foreground queue might be scheduled by a RR algorithm, while the background queue is scheduled by an FCFS algorithm. In addition, there must be scheduling between the queues. This is commonly a fixed-priority preemptive scheduling. For example, the foreground queue may have absolute priority over the background queue. Another possibility is to time slice between the queues. Each queue gets a certain portion of the CPU time, which it can then schedule among the various processes in its queue. For instance, in the foreground-background queue example, the foreground queue can be given 80 percent of the CPU time for RR among its processes, while the background queue receives 20 percent of the CPU to give its processes in a FCFS manner. Example, highest priority system processes interactive processes batch processes student processes lowest priority 6.6 Multilevel Feedback Queue Scheduling Multilevel feedback queue scheduling, allow a process to move between queues. If a process uses too much CPU time, it will be moved to a lower-priority queue.similarly, a process that

7 7 waits too long in a lower-priority queue may be moved to a higher priority queue. This is a form of aging that would prevent starvation. Example, quantum = 8 quantum = 16 FCFS 6.7 Multiple Processor Scheduling One major factor is the types of processors involved. The processors may be identical ( a homogeneous system) or different (a heterogeneous system). If the processors are different, the options are relatively limited. Each processor has its own queue and its own scheduling algorithm. If several identical processors are available, then load sharing can occur. It would be possible to provide a separate queue for each processor. In this case, however, one processor could be idle, with an empty queue, while another processor was very busy. To prevent this situation, we use a common ready queue. All processes go into one queue and are scheduled onto any available processor. Two scheduling approaches may be used: - Symmetric multiprocessing: each processor examines the common ready queue and selects a process to execute. We must ensure that two processors do not choose the same process, and that processes are not lost from the queue. - Asymmetric multiprocessing: appointing one processor as scheduler for the other processors, thus creating a master slave structure. 6.8 Real-Time Scheduling Real-time systems are generally categorized as hard real time, meaning there are absolute deadlines that must be met, or else, and soft real time, meaning that missing an occasional deadline is tolerable. While in theory it is possible to turn a general-purpose operating system into a real-time system by using one of the real-time scheduling (such as rate monotonic algorithm, earliest deadline first, see text book), in practice the context-switching overhead of general-purpose system is so large that real-time performance can only be archived for applications with easy time constraints. As a

8 8 consequence, most real-time work uses special real-time operating systems that have certain important properties. Typically these include a small size, fast interrupt time, rapid context switch, short interval during which interrupts are disabled, and the ability to manage multiple timers in the millisecond or microsecond range. 6.9 Algorithm Evaluation Analytic Evaluation - Deterministic Modeling Advantage: simple&fast Disadvantage: too specific & requires too much exact knowledge - Queueing Models We can use Little s formular n = λ x W where n = average queue length W = average waiting time λ = average arrival rate for the new processes in the queue Advantage: useful in comparing algorithm Disadvantage: accuracy depend on distribution which difficult to work with Simulations Advantage: more accurate Disadvantage: - very expensive - may be inaccurate in the real system

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