CSC 271 Software I: Utilities and Internals

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1 CSC 271 Software I: Utilities and Internals Lecture 9 : What are Processes? The Process Concept Originally, computers ran only one program at a time, which had total access to all of the computer s resources. Modern systems run many programs concurrently, with the operating system running its own tasks to do its own work. The concept of the process allows us to compartmentalize everything being done on the computer.

2 What is a Process Harvey Deitel offers several definitions of a process: a program in execution an asynchronous activity the animated spirit of a procedure the locus of control of a procedure in execution that which is manifested by the existence of a process control block in the operating system that entity to which processors are assigned the dispatchable unit Process States Exit terminated Awake Timeout Running Block Dispatch new Admit Ready Wakeup Blocked Asleep

3 Process Control Block Pointer Process State Process Number Program Counter Registers Memory Limits Open Files List Misc. Accounting and Status Data Switching Processes Process A Process B Operating System Save state in PCB A Load state in PCB B Interrupt or System Call

4 Restoring Processes Process A Process B Operating System Save state in PCB B Load state in PCB A Interrupt or System Call Context Switch Context switch saving the state of one process and loading the saved state of another process. Context switch times varies depending on the hardware, with typically times of 1 to 1000 µsec.

5 Process Scheduling New Process Job Queue timeout Ready Queue CPU exit I/O I/O Device Queue CPU-Bound and I/O Bound Processes CPU-bound processes - Processes that spend more of their time doing computational work than input and output I/O-bound processes Processes that spend more of their time doing input and output than computational work.

6 Creating A Process Processes may be created by other processes with the parent process partitioning off resources for the child process and passing it data. The child process may also obtain resources directly from the operating system. A hierarchy can be created as child processes create their own child processes. Hierarchy of Processes A B C D E F G H I

7 Tasks In Process Creation Creating a process requires that the operating system: name the process insert it in the process table determine its initial priority create the process control block allocate its initial resources Parent and Child Processes Once a child process is created: both parent and child may execute concurrently or the parent become inactive until the child process terminates. Different possibilities exist for the child s core image: The child is a duplicate of the parent The child is a separate and distinct program

8 Process Creation in UNIX A simplified version of the shell while (TRUE) { /* repeat forever */ read_command(command, parameters); /* read terminal input */ } if (fork()!= 0) /* fork off child process */ /* Parent code */ waitpid(-1, &status, 0); else execve(command, parameters, 0); /* execute command */ Process Termination Normally, a process terminates at the end of program execution by using the exit system call. All its resources are either freed or returned to the parent s control. Data may also be returned to the parent. A parent may also terminate a child process by using an abort system call.

9 Why Kill A Child Process? A parent process might terminate a child process because: the child process used more resources than it is allowed. it is no longer needed. the parent process is terminating. The process of processes killing their descendent processes is called cascading. Concept of Multiprogramming Multiprogramming takes advantages of the fact that processes will spend a great deal of their time waiting for I/O operations to finish. While process #1 is waiting for I/O, the CPU will execute process #2.

10 CPU-I/O Burst Cycle A program running on a computer has main different bursts of activities: bursts of CPU activity and bursts of Input/Output activity. Since this is cyclic in nature, it is called the CPU-I/O Burst Cycle. A Process Is a Series of CPU and I/O Bursts Load Store Add Store Read from File Wait for I/O CPU burst I/O Burst Wait for I/O Load Store Add Write To File I/O Burst CPU burst Store Increment Index Write To File CPU burst

11 Process Spends Most Of Its Time Waiting For I/O I/O I/O I/O I/O CPU CPU CPU CPU CPU Three Processes Executing Without Multiprogramming

12 Three Processes Executing With Multiprogramming Scheduling Criteria Performance criteria - What do we want from our scheduling algorithm? Utilization - As near 100% CPU time as possible. Throughput - Highest possible number of finished processes per unit time. Turnaround time - As low as possible for jobs from start to finish. Response time - For interactive systems, this is more important than turnaround time. Waiting Time - minimize time in ready queue and device queues.

13 Preemptive vs. Nonpreemptive Scheduling Preemptive scheduling - processes using the CPU can be removed by the system. Nonpreemptive scheduling - processes using the CPU cannot be removed by the CPU. Starvation - A situation that arises when a process never gets to the CPU (or to perform an I/O operation, etc.) Scheduling Algorithms Scheduling Algorithms include: First-Come-First-Served Shortest Job First Round Robin Priority

14 First-Come-First-Served First-Come-First-Served Algorithm is the simplest CPU scheduling. Whichever process requests the CPU first gets it first. It is implemented using a standard FIFO single queue. Waiting time can be long and it depends heavily on the order in which processes request CPU time: An Example of First-Come-First- Served Imagine three processes with the following burst times: Process Burst time (in msec) P 1 24 P 2 3 P 3 4

15 Scenario #1 For FCFS Scheduling P 1 P 3 P Processing time = ( ) / 3 = 83/3 = 27.7 msec Scenario #2 For FCFS Scheduling P 2 P 3 P Waiting time = ( ) / 3 = 41/3 = 13.7 msec

16 Scenario #3 For FCFS Scheduling P 2 P 1 P Waiting time = ( ) / 3 = 61/3 = 20.3 msec Shortest Job First Most appropriately called Shortest Next CPU Burst First because it bases the order upon an approximation of how long what the next CPU burst will be. This can be proven to be the optimal scheduling algorithm with the shortest average processing (and waiting) time. The SJF algorithm can be preemptive or nonpreemptive, with the preemptive SJF algorithm more properly being called shortest- remainingtime-first scheduling.

17 Gantt Chart for Shortest Job First Example P 2 P 3 P Waiting time = ( ) / 3 = 41/3 = 13.7 msec Priority Scheduling Priority scheduling involves a priority assigned to each process, which is scheduled in accordance with its priority. Processes with equal priority are scheduled on a FCFS basis. A SJF algorithm is a special case of a priority scheduling algorithm with priority(p) being proportional to 1/p.

18 Priority Levels There is no general agreement on whether 0 is the highest or lowest priority (priority numbers are assumed to be positive). UNIX uses 0 as the highest priority IBM s MVS uses it as the default (lowest) priority. Setting Priorities Priorities can be set: internally (by some measurable quantity or quantities such as time limits, memory requirements, number of open files, I/O burstto-cpu burst ratio, etc.) or externally (by system policy, such as process importance, type or availability of funds, sponsoring department, etc.)

19 Starvation Starvation is a major problem of priority scheduling algorithms. On a busy system, a low-priority process may sit for extremely long periods of time. A solution to the problem is aging, where we increment the priority (make it a higher priority) for every 1-15 minutes of waiting. Scenario For Priority Scheduling Process Burst Time Priority P P P P P 5 5 2

20 Gantt Chart For Priority Scheduling Scenario P 2 P 5 P 1 P 3 P Processing time = ( ) / 5 = 8 msec Round Robin Scheduling Round-robin scheduling is designed for timesharing system. It is similar to the FCFS scheduling, but preemptive is added to switch between processes. A time quantum is typically 10 to 100 milliseconds. The ready queue is implemented in FIFO manner..

21 Round-Robin Scheduling and Preemption If a process needs less than a time quantum, it releases the CPU voluntarily. If a process needs more than a time quantum, it is preempted from the CPU and placed at the back of the ready queue. Scenario For Round-Robin Scheduling Process Burst Time P 1 24 P 2 3 P 3 3

22 Gantt Chart For Round Robin Scheduling Scenario P 1 P 2 P 3 P 1 P 1 P 1 P 1 P Processing time = ( ) / 3 = 15.7 msec

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