Lecture 3. Questions? Friday, January 14 CS 470 Operating Systems - Lecture 3 1

Transcription

1 Lecture 3 Questions? Friday, January 14 CS 470 Operating Systems - Lecture 3 1

2 Outline CPU scheduling Comparison criteria Scheduling algorithms First Come, First Serviced (FCFS) Shortest Job First (SJF) Shortest Remaining Time First (SRTF) Priority Friday, January 14 CS 470 Operating Systems - Lecture 3 2

3 CPU Scheduling Most modern OS's multi-program, i.e., they "run" more than one process "simultaneously", even if there is only one CPU to maximize CPU utilization. Goal is to keep CPU busy. This works because programs typically alternate between doing computations (a CPU burst) and waiting for some I/O to be performed (an I/O burst). Friday, January 14 CS 470 Operating Systems - Lecture 3 3

4 CPU Scheduling For typical applications, I/O bursts are much longer than CPU bursts. Generally, CPU bursts are very short. Programs with long CPU bursts are said to be CPU-bound. Friday, January 14 CS 470 Operating Systems - Lecture 3 4

5 CPU scheduling Recall process state diagram. When does a scheduling decision need to be made? NEW dispatch TERMINATED admitted READY interrupt RUNNING exit I/O or event completion WAITING I/O or event wait Friday, January 14 CS 470 Operating Systems - Lecture 3 5

6 CPU Scheduling Scheduling decision must be made whenever the RUNNING state is vacated. Termination Transition to WAITING state - I/O request, explicit wait Also, can make a scheduling decision when the CPU is interrupted. Timer interrupt I/O or event completion interrupt. Friday, January 14 CS 470 Operating Systems - Lecture 3 6

7 CPU Scheduling If there is no scheduling done when the CPU is interrupted, the current running process continues in the RUNNING state. I.e., once a process starts to run, it holds the CPU until it terminates or waits. This is called nonpreemptive scheduling. When an interrupt can cause the currently running process to vacate the RUNNING state, it is called preemptive scheduling, and the preempted process transitions to the READY state. Friday, January 14 CS 470 Operating Systems - Lecture 3 7

8 CPU Scheduling Examples of non-preemptive OS's: Mac OS 9 and earlier, DOS/Win 3.1 and earlier. Both had programmer level "interrupts" that allowed cooperating programs to simulate preemptive scheduling. Most modern OS's are preemptive. They are generally better, but more complex/costly. Need to synchronize access to shared data (Ch. 6) Need to synchronize access to shared kernel data. Atomic system calls would be slow. Friday, January 14 CS 470 Operating Systems - Lecture 3 8

9 Comparison Criteria How do we compare scheduling algorithms? Some possible criteria: CPU utilization: How busy is the CPU? Throughput: Number of processes completed per unit time. A measure of work done. Turnaround time: Time from submission to completion. A measure of how fast individual processes complete. Friday, January 14 CS 470 Operating Systems - Lecture 3 9

10 Comparison Criteria Waiting time: Time process spends on the Ready Queue. A measure of "wasted" time. Response time: Time to first output, not necessarily completion. E.g., for interactive systems. Goal is to try to optimize the criteria deemed most important. Possibilities are the average, the maximum, possibly the variance. Friday, January 14 CS 470 Operating Systems - Lecture 3 10

11 Comparison Criteria To simplify comparison, we look as scenarios consisting of n processes, p 1,...,p n, where each process p i arrives at some time t i and has only one CPU burst of size s i. Often all of the arrival times will be 0. We primarily look at average waiting time and average turnaround time. Friday, January 14 CS 470 Operating Systems - Lecture 3 11

12 First Come, First Served (FCFS) Here is a scheduling scenario: Process CPU burst time Arrival time p p p with arrival order p 1, p 2, p 3 What would be the simplest scheduling algorithm? Friday, January 14 CS 470 Operating Systems - Lecture 3 12

13 First Come, First Served (FCFS) First process to arrive gets the CPU and holds it until it terminates or issues a wait. If the CPU is busy on arrival, processes enter the Ready Queue, which is serviced in FIFO order. Algorithm is called First Come, First Service (FCFS). Draw a Gantt chart showing CPU usage (not to scale): p1 p2 p Friday, January 14 CS 470 Operating Systems - Lecture 3 13

14 First Come, First Served (FCFS) Compute average waiting time Compute average turnaround time Suppose the arrival order is p 2, p 3, p 1? Compute average waiting and turnaround times again. Clearly, FCFS generally is not optimal for average waiting and turnaround times, and it has high variance. Can get a convoy effect where I/O jobs get stuck behind a CPU job. Friday, January 14 CS 470 Operating Systems - Lecture 3 14

15 Shortest Job First (SJF) As the second ordering for FCFS shows, reordering jobs can result in lower average waiting and turnaround times. In general, could allow shortest job to go first (with FCFS if there is a tie). Called Shortest Job First algoithm. Friday, January 14 CS 470 Operating Systems - Lecture 3 15

16 Shortest Job First (SJF) Here is another scenario: Process CPU burst time Arrival time p p p p Friday, January 14 CS 470 Operating Systems - Lecture 3 16

17 Shortest Job First (SJF) Draw a Gantt chart and compute average waiting time for the scenario using SJF. Draw a Gantt chart and ompute average waiting time for the same scenario using FCFS. SJF is provably optimal for a fixed set of processes. Unfortunately, generally cannot be used in a dynamic situation, since cannot not know burst size in advanced, though some systems try to predict using exponential averaging. Friday, January 14 CS 470 Operating Systems - Lecture 3 17

18 Shortest Remaining Time First (SRTF) SJF is non-preemptive. The currently running process holds the CPU even if a shorter job is admitted. Can interrupt the CPU when new job arrives and make a scheduling decision based on what the current job has left. Called Shortest Remaining Time First algorithm. Using the previous scenario with arrival times at 0, 1, 2, 3, draw Gantt charts and compute average waiting times for SJF and SRTF. Friday, January 14 CS 470 Operating Systems - Lecture 3 18

19 Priority SJF is a specifc example of a general priority scheduling algorithm. For SJF, job length is interpreted as the priority with shorter lengths being higher priority. (Some systems use higher numbers for higher priority. E.g., Unix) Can even say that FCFS is priority based on age, so that older processes have higher priority than newer processes. Priority scheduler can be either non-preemptive or preemptive. Friday, January 14 CS 470 Operating Systems - Lecture 3 19

20 Priority Priority can be set internally or externally Internal: measured quantities of the process. E.g., burst size, age, number of open files, etc. External: quantities set outside the OS. E.g., cost, department, importance to company, etc. Major problem for all priority algorithms is starvation. A continuous stream of high priority jobs can prevent low priority jobs from getting CPU. Solution is aging. Gradually increase priority as process waits. Friday, January 14 CS 470 Operating Systems - Lecture 3 20