CS 149 Operating Systems February 3 Class Meeting
CS 149: Operating Systems February 3 Class Meeting Department of Computer Science San Jose State University Spring 2015 Instructor: Ron Mak www. cs. sjsu. edu/~mak
Processes o A process is basically an abstraction of a running program. o The most central concept in any operating system. o Each process runs in its own address space. _ Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak 2
Context Switching A process’s state information is kept in its process control block (PCB). Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak Operating Systems: Design and Implementation, 3 rd ed. Andrew Tanenbaum & Albert Woodhull (c) 2006 Prentice-Hall, Inc. 0 -13 -142938 -8 3
Contents of a Process Control Block (PCB) Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak Operating Systems: Design and Implementation Tanenbaum & Woodhull (c) 2006 Prentice-Hall, Inc. All rights reserved. 0 -13 -142938 -8 4
Process States o Running o o Ready o o Actually using the CPU at that instant. Runnable, but temporarily stopped to let another process run. Blocked o Unable to run until some external event happens. Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak Operating Systems: Design and Implementation, 3 rd ed. Andrew Tanenbaum & Albert Woodhull (c) 2006 Prentice-Hall, Inc. 0 -13 -142938 -8 5
Process Scheduler o Transitions 2 and 3 are caused by the process scheduler. n n o o A part of the operating system. A process does not know about the transitions. Transition 2 can happen at the end of a time slice. Transition 3 can happen when it’s the process’s turn again to run. Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak 6
Process Scheduling Goals o The process scheduler decides which process can run, i. e. , which process can have the CPU. o The goal is to keep all parts of the computer system as busy as possible. o Which scheduling algorithm to use depends on n The type of system How it’s used What policies are in place Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak 7
Process Behavior o A process may be: n n o A compute-bound process spends most of its time computing. n o Computing: using the CPU Performing I/O Long CPU bursts and infrequent I/O waits. An I/O-bound process spends most of its time waiting for I/O operations to complete. n Long I/O waits punctuated by short CPU bursts. Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak 8
Process Behavior, cont’d o As CPUs become faster relative to disks: n n Programs are becoming more I/O bound. Scheduling I/O-bound processes becomes more important. Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak Operating Systems: Design and Implementation Tanenbaum & Woodhull (c) 2006 Prentice-Hall, Inc. All rights reserved. 0 -13 -142938 -8 9
Process Behavior, cont’d o Histogram of CPU burst durations. n n CPU-bound process: A few long bursts. I/O bound process: Many short bursts. Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak Silberschatz, Galvin, and Gagne Operating Systems Concepts with Java, 8 th edition (c) 2010 John Wiley & Sons. All rights reserved. 0 -13 -142938 -8 10
When to Schedule o When scheduling is absolutely required: n n o When a process exits. When a process blocks on I/O, or on a semaphore. When scheduling usually done (although not absolutely required): n n n When a new process is created. When an I/O interrupt occurs. When a clock interrupt occurs. Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak 11
Types of Scheduling Algorithms o Preemptive n n o Each process is assigned a time interval to run, called the quantum or time slice. At the end of a process’s time quantum, the scheduler suspends the process and schedules another process to run. Non-preemptive n The scheduler allows a process to run until it blocks: o o n Waiting for I/O. Waiting for another process. The process voluntarily releases the CPU. Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak 12
Scheduling Criteria o CPU utilization n o Throughput n o Keep the CPU as busy as possible. Number of processes that complete their execution per time unit. Turnaround time n The amount of elapsed time from when a process enters the ready queue to when it completes execution. Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak 13
Scheduling Criteria, cont’d o Waiting time n o Amount of time a process waits in the ready queue. Response time n In an interactive environment, the amount of elapsed time from when a request was submitted until the first response is produced. Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak 14
Scheduling Criteria, cont’d o Proportionality n n “Simple” tasks (as perceived by human users) should take little time. “Complex” tasks can take longer time. Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak 15
Scheduling Goals Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak Operating Systems: Design and Implementation Tanenbaum & Woodhull (c) 2006 Prentice-Hall, Inc. All rights reserved. 0 -13 -142938 -8 16
First-Come First-Served (FCFS) Process P 1 P 2 P 3 o o Burst Time 24 3 3 Suppose that the processes arrive in the order: P 1 , P 2 , P 3. The timeline for the schedule is: P 1 0 o o P 2 24 P 3 27 30 Waiting time for P 1 = 0; P 2 = 24; P 3 = 27 Average waiting time: (0 + 24 + 27)/3 = 17 Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak 17 Operating Systems Concepts with Java, 8 th edition Silberschatz, Galvin, and Gagne (c) 2010 John Wiley & Sons. All rights reserved. 0 -13 -142938 -8
Shortest Job First (SJF) o o Now suppose that the processes arrive in the order: Process Burst Time P 2 , P 3 , P 1 The timeline for the schedule is: P 2 0 o o P 1 P 2 P 3 3 24 3 3 P 1 6 30 Waiting time for P 1 = 6; P 2 = 0; P 3 = 3 Average waiting time: (6 + 0 + 3)/3 = 3 Much better than previous case. Schedule short processes ahead of long processes. Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak 18 Operating Systems Concepts with Java, 8 th edition Silberschatz, Galvin, and Gagne (c) 2010 John Wiley & Sons. All rights reserved. 0 -13 -142938 -8
Shortest Job First (SJF), cont’d o Shortest Job First (SJF) gives the optimal average waiting time for a given set of processes. o How can you know in advance each process’s expected run time? n It’s possible if you run the same jobs frequently. Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak 19
Practice Problem #1 o o Assume: Process Arrival Time Burst Time P 1 0. 0 8 P 2 0. 4 4 P 3 1. 0 1 What is the average turnaround time with the FCFS scheduling algorithm? P 1 P 2 P 3 o With the SJF scheduling algorithm? P 1 P 3 P 2 20
Shortest Remaining Time (SRT) o When a new job (process) arrives, its expected run time is compared to the expected remaining run time of the currently executing process. o If the new job needs less time to finish than the current process, the scheduler suspends the current process and starts the new job. o New short jobs get good service. o As with Shortest Job First, you must know in advance each process’s expected run time. Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak 21
Round Robin (RR) o Each process is assigned a time interval called a quantum during which it is allowed to run. o If the process is still running at the end of its quantum, the CPU is preempted and given to the next process. n o The preempted process moves to the end of the ready queue. Preemption can also occur if a process blocks. Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak 22
Round Robin (RR), cont’d Operating Systems: Design and Implementation Tanenbaum & Woodhull (c) 2006 Prentice-Hall, Inc. All rights reserved. 0 -13 -142938 -8 Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak 23
Round Robin (RR), cont’d o Round Robin example with time quantum 4: Process P 1 P 2 P 3 o The timeline is: P 1 0 o Burst Time 24 3 3 P 2 4 P 3 7 P 1 10 P 1 14 P 1 18 22 P 1 26 P 1 30 Typically, higher average turnaround than SJF, but with better response time. Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak 24 Operating Systems Concepts with Java, 8 th edition Silberschatz, Galvin, and Gagne (c) 2010 John Wiley & Sons. All rights reserved. 0 -13 -142938 -8
Time Quantum vs. Context Switch Time o A smaller time quantum increases the number of processor switches. n Recall each context switch takes time. Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak 25 Operating Systems Concepts with Java, 8 th edition Silberschatz, Galvin, and Gagne (c) 2010 John Wiley & Sons. All rights reserved. 0 -13 -142938 -8
Scheduling Interactive Systems o Turnaround time varies with the time quantum. o Each context switch incurs certain OS administrative overhead: n n n Save and load registers and memory maps. Switch memory maps. Update system tables and lists. Flush and reload the cache. etc. Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak 26
Scheduling Interactive Systems, cont’d o Increasing the time quantum can increase response time. o Example with 100 msec quantum: n n 10 users press carriage return simultaneously. The last user has to wait 1 second for a response Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak 27
Highest Priority First (HPF) Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak 28 Operating Systems Concepts with Java, 8 th edition Silberschatz, Galvin, and Gagne (c) 2010 John Wiley & Sons. All rights reserved. 0 -13 -142938 -8
Highest Priority First (HPF), cont’d o HPF scheduling with four priority classes: Use Round Robin scheduling within each priority class. Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak Operating Systems: Design and Implementation Tanenbaum & Woodhull (c) 2006 Prentice-Hall, Inc. All rights reserved. 0 -13 -142938 -8 29
Highest Priority First (HPF), cont’d o A serious problem with HPF is starvation. n o A solution is aging. n o The lowest priority processes may never get to execute. As time progresses, the process scheduler automatically raises the process’s priority. HPF can be preemptive. n A higher priority task that’s arriving can preempt a currently executing process with a lower priority. Computer Science Dept. Spring 2015: February 3 CS 149: Operating Systems © R. Mak 30
Practice Problem #2 o o o Assume: Process Burst Time Priority P 1 10 3 P 2 1 1 P 3 2 3 P 4 1 4 P 5 5 2 All arrived at time 0 in the order P 1, P 2, P 3, P 4, P 5. Highest priority = 1 Time quantum = 1 Draw timelines for the scheduling algorithms FCFS, SJF, HPF (nonpreemptive), and RR. What is the turnaround time of each process for each of these scheduling algorithms? What is the total waiting time for each process for each of these scheduling algorithms? Which of these scheduling algorithms results in the minimum average waiting time over all the processes? 31
Practice Problem #2 Timelines: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 FCFS P 1 P 1 P 1 P 2 P 3 P 4 P 5 P 5 P 5 SJF P 2 P 4 P 3 P 5 P 5 P 5 P 1 P 1 P 1 HPF P 2 P 5 P 5 P 5 P 1 P 1 P 1 P 3 P 4 RR P 1 P 2 P 3 P 4 P 5 P 1 P 3 P 5 P 1 P 1 P 1 Turnaround times: P 1 P 2 P 3 P 4 P 5 FCFS 10 11 13 14 19 SJF 19 1 4 2 9 HPF 16 1 18 19 6 RR 19 2 7 4 14 32
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