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Processes

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A process is a program in execution; process execution must progress in sequential fashion. Can be characterized by its trace. The OS interleaves the execution of several processes to maximize processor utilization A process requires resources, which are managed by the operating system OS supports Inter-Process Communication (IPC) and user creation of processes 2

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Trace of Process Sequence of instructions that execute for a process Dispatcher switches the processor from one process to another 3

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4 Time slice ~= 1/10 th of a second

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Process state models The OS is responsible for interleaving the execution of the processes. It is important to understand the possible behaviors of all the processes and how to react to them. The behavior of processes is described by means of the process state model. 5

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Two-State Process Model Process may be in one of two states Running Not-running 6

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Queuing Diagram Dispatcher Program that assigns the processor to one process or another Prevents a single process from monopolizing processor time 7 Processor Enter Queue Dispatch Exit Pause (a) Queuing diagram

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Reason for Process Creation 8

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Reasons for Process Termination 9

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5 Process States As a process executes, it changes state New: The process is being created. Running: Instructions are being executed. Waiting or blocked: The process is waiting for some event to occur. Ready: The process is waiting to be assigned to a process. Terminate or Exit: The process has finished execution. 11

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Diagram of Process State 12

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Example for 3 Processes 13

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OS Control Structures

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Process Control Block (PCB) Information associated with each process. Process ID Process state Program counter (PC) CPU registers (Context data) CPU scheduling information (Priority) Memory-management information (Memory pointer) Accounting information (Time, acct. #) I/O status information (requests, devices assigned) 15

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Process Control Block Contains the process attributes Created and manage by the operating system Allows support for multiple processes Process = Program code + Data + PCB 16

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Suspending Processes 17

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Two Suspended States 18

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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. Process migration between the various queues. 19

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Ready Queue, I/O Device Queues 20

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Process Scheduling 21

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Schedulers Long-term scheduler: job scheduler selects which processes should be brought into the ready queue. invoked infrequently (seconds, minutes) controls the degree of multiprogramming Medium-term scheduler allocates memory for process. invoked periodically or as needed. Short-term scheduler: CPU scheduler selects which process should be executed next and allocates CPU. invoked frequently (ms) 22

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Process Switching Definition The activity that occurs when the OS kernel switches between processes in an effort to share the CPU among competing, runable processes Actions Save contents of hardware registers (PC, SP,...) Load PC with location of resume point Load hardware registers with new context if full context switch 23

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Process Switching Process-switch time is overhead CPU utilization affected by quantum and process-switch time Quantum: Time-slice (max CPU interval) given to a process Time dependent on hardware support Types Voluntary: Process blocks Involuntary: End of time quantum or higher-priority, runable process gets control of CPU 24

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CPU Process Switch 25

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Mode switch vs Process switch When a mode switch occurs, the PCB of the process that was running is still marked Running and is not placed in the Blocked or Ready queue However, depending on what caused the mode switch, a process switch could follow 26

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Modes of execution User mode: Some parts of virtual address space can not be accessed Some instructions (e.g., memory management) can not be executed Kernel mode: Can access kernel address space Is a fixed part of the virtual address space of every process System call puts user into kernel mode 27

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Mode, Space and Context 28 Application (user space) System calls Exceptions (user + system space) X not allowed Interrupts, System tasks (user space) Kernel Mode More privileges User Mode Process Context Kernel Context System call Interrupt occurs

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Execution of the OS 29

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Execution within user processes 30

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Unix Process Creation Parents create children results in a tree of processes Resource sharing Parent and children share all resources Children share subset of parents resources Parent and child share no resources Execution Parent and children execute concurrently Parent waits until children terminate 31

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Unix Process Creation Address space Child duplicate of parent Child has a program loaded into it UNIX examples fork system call creates new process exec family of system calls used after a fork to replace the process memory space with a new program 32

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Unix Process Creation (fork) Allocate new PID and proc structure Init childs process control block (PCB) Copy parents registers to childs hardware context Copy parents process image to childs Mark child runnable and give to scheduler Return PID = 0 to child Return child PID to parent 33

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Unix Process Creation (fork) Effects of fork Child gets a copy of its parents memory BUT changes made by child are not reflected in parent EXCEPT a child can affect the I/O state of parent File descriptors can point to same file table entry WARNING: Parent should fflush(stdout) before fork if using printf() 34

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exec Operations Use to overlay process image with new program Find executable file (argv[0]) Copy exec args and env variables to kernel space Free memory currently allocated to process image Allocate new memory for process image Copy exec args and env variables into new space Begin executing in main() 35

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exec Functions Six exec functions: execl execv execle execve execlp execvp 36 l list of arguments v argv[] vector p file name e envp[] vector instead of inheriting environment of parent

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waitpid pid_t waitpid(pid_t pid, int *status, int options); Unlike wait(2), waitpid doesnt have to block wait(2) blocks until one of its children terminates 37

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waitpid pid parameter pid == -1: Wait for any child to terminate pid > 0: Wait for child whose PID equals pid pid == 0: Wait for any child whose PGID equals the PGID of process pid pid < -1: Wait for any child whose PGID = abs(pid) 38

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waitpid status parameter Contains exit status, signal number, and flags WIFEXITED(status) True if child terminated normally Ex: printf(exit = %d\n, WEXITSTATUS(status)); WIFSIGNALED(status) True if child terminated and didnt catch signal Ex: printf(signo = %d\n, WTERMSIG(s)); WIFSTOPPED(status) True if child is STOPPED Ex: printf(signo = %d\n, WSTOPSIG(s)); 39

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waitpid options parameter options = 0 No special handling options = WNOHANG Dont block if child is not available and return 0 Ex: rc = waitpid(pid, &status, WNOHANG); if (rc == -1) {... error... } else if (rc == 0) {... no child available...} else {... rc should equal pid... } 40

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waitpid options options = WUNTRACED Return status of child if child stopped and status has not already been returned (assumes job control support) 41