1. Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Silberschatz, Galvin and Gagne ©2007 Processes and Their Scheduling

2. 5.2 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Topics for the Day: Processes, Scheduling and Synchronization We will cover:  Process Concept  Process Scheduling  Process Synchronization (with focus on mutual exclusion and semaphores)  Some Classic Problems in Concurrency

3. 5.3 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Process Concept  An operating system executes a variety of programs: Batch system – jobs Time-shared systems – user programs or tasks  Process – a program in execution; process execution must progress in sequential fashion  Note: process (active entity) is different from program (passive entity). Several processes may be instances of the same program.  A process includes, e.g.: program counter Stack (what do you think this is used for?) data section

4. 5.4 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Process State  A process state describes the current status of its execution.  As a process executes, it changes state new: The process is being created running: Instructions are being executed waiting: The process is waiting for some event to occur ready: The process is waiting to be assigned to a process terminated: The process has finished execution How do processes change state and why?

5. 5.5 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Diagram of Process State

6. 5.6 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Process Control Block (PCB) Information associated with each process by the OS  Process state  Program counter  CPU registers  CPU scheduling information  Memory-management information  Accounting information  I/O status information A snapshot of the process in execution. To be saved at CPU context switches.

7. 5.7 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Process Control Block (PCB): Pictorially

8. 5.8 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 CPU Switch From Process to Process

9. 5.9 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Where do PCBs of Idle Processes Go?  PCBs come in and out of process scheduling queues Remind me:  What is a queue?  What is a priority queue?  Which types of queues are common in an OS?

10. 5.10 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Process Scheduling Queues  Job queue – set of all processes in the system  Ready queue – set of all processes residing in main memory, ready and waiting to execute  Device queues – set of processes waiting for an I/O device  Processes migrate among the various queues

11. 5.11 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Flow of Processes in the OS

12. 5.12 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Overview: CPU Scheduling  Basic Concepts  Scheduling Criteria  Scheduling Algorithms

13. 5.13 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Basic Concepts and Assumptions  Fact of Life: Maximum CPU utilization obtained with multiprogramming  scheduling!  CPU–I/O Burst Cycle Assumption: Process execution consists of a cycle of CPU execution and I/O wait  CPU burst distribution

14. 5.14 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 CPU Scheduler  Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them  CPU scheduling decisions may take place when a process: 1.Switches from running to waiting state 2.Switches from running to ready state 3.Switches from waiting to ready 4.Terminates  Scheduling under 1 and 4 is nonpreemptive  All other scheduling is preemptive. (Pretty much the rule today, especially for real-time systems.)

15. 5.15 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Typical Scheduling Criteria  CPU utilization – keep the CPU as busy as possible  Throughput – # of processes that complete their execution per time unit  Turnaround time – amount of time to execute a particular process  Waiting time – amount of time a process has been waiting in the ready queue  Response time – amount of time it takes from when a request was submitted until the first response is produced (important for time-sharing environment)  Question: Which one do we want to optimize? How? General framework: Combinatorial optimization.

16. 5.16 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Optimization Criteria  Max CPU utilization  Max throughput  Min turnaround time  Min waiting time (our sample criterion)  Min response time  Fact of life 1: Selecting an appropriate scheduling algorithm is a major design decision. Often one needs to consider several criteria.  Fact of life 2: Proper mathematical analysis and experimental evaluation are needed.

17. 5.17 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 First-Come, First-Served (FCFS) Scheduling ProcessBurst Time P 1 24 P 2 3 P 3 3  Suppose that the processes arrive in the order: P 1, P 2, P 3 The Gantt Chart for the schedule is:  Average waiting time?  Can we do better?  Pros and cons? P1P1 P2P2 P3P3 2427300

18. 5.18 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Shortest-Job-First (SJF) Scheduling  Idea: Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time  Two schemes: nonpreemptive – once CPU given to the process it cannot be preempted until completes its CPU burst preemptive – if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the Shortest-Remaining-Time-First (SRTF)  SJF is optimal – gives minimum average waiting time for a given set of processes

19. 5.19 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 ProcessArrival TimeBurst Time P 1 0.07 P 2 2.04 P 3 4.01 P 4 5.04  SJF (non-preemptive)  Average waiting time?  What if we used preemptive scheduling? Example of Non-Preemptive SJF P1P1 P3P3 P2P2 73160 P4P4 812

20. 5.20 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Example of Preemptive SJF ProcessArrival TimeBurst Time P 1 0.07 P 2 2.04 P 3 4.01 P 4 5.04  SJF (preemptive)  Average waiting time?  Recall that SJF is optimal. Should we look any further? P1P1 P3P3 P2P2 42 11 0 P4P4 57 P2P2 P1P1 16

21. 5.21 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Priority Scheduling  A priority number (integer) is associated with each process  The CPU is allocated to the ready process with the highest priority (typically, smallest integer  highest priority) Preemptive nonpreemptive  SJF is a priority scheduling where priority is the predicted next CPU burst time  Problem  Starvation – low priority processes may never execute (example: LegOS scheduler)LegOS scheduler  Solution  Aging – as time progresses increase the priority of the process in the ready queue

22. 5.22 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Round Robin (RR)  Idea: Each process gets a small unit of CPU time (time quantum), usually 10-100 milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue.  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 q large  FIFO q small  q must be large with respect to context switch, otherwise overhead is too high

23. 5.23 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Example of RR with Time Quantum = 20 ProcessBurst Time P 1 53 P 2 17 P 3 68 P 4 24  The Gantt chart is: Is avg waiting time good for RR?  Typically, higher average turnaround than SJF, but better response P1P1 P2P2 P3P3 P4P4 P1P1 P3P3 P4P4 P1P1 P3P3 P3P3 02037577797117121134154162

24. 5.24 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Algorithm Evaluation  Deterministic modeling – takes a particular predetermined workload and defines the performance of each algorithm for that workload  Queueing models  Implementation

25. 5.25 Silberschatz, Galvin and Gagne ©2007 Operating System Concepts with Java – 7 th Edition, Nov 15, 2006 Algorithm Evaluation Evaluation of CPU schedulers by simulation