Transcript Processes
OPERATING SYSTEMS PROCESSES TANNENBAUM, SECTION 2-1 DEFINITION: PROCESS • Program in execution • Current value of program counter • Values of registers • Values of variables THE PROCESS MODEL (1) Four Independently Scheduled Processes THE PROCESS MODEL (2) Only one program is active at once. STATES • Processes go through a series of discrete states • Running State • Process currently has cpu • Ready state • Could use cpu if one were available • Blocked state • Waiting for some event to happen, say i/o completion LISTS • Assume a single CPU system • 1 process can run at a time • Several process may be ready • Several processes may be blocked • Two Lists • Priority queue of ready processes • Unordered list of blocked processes • Unordered because processes become unblocked in the order in which the resources they require become available. EXAMPLE • Process A pty 1 waiting for keyboard input • Process B pty 2 waiting for printer • Process C pty 3 waiting for disk input Where 3 is highest If Process A gets input before C gets input, we shouldn’t hold process A STATES (2) Tanenbaum & Bo,Modern Operating Systems:4th ed., (c) 2013 Prentice-Hall, Inc. All rights reserved. READY TO RUNNING • Job is admitted to system • Process created and placed in ready queue • Gradually makes its way to the head of the queue • When CPU is available, scheduled (dispatched) RUNNING TO READY • Process exhausts quantum • Clock generates an interrupt • o/s dispatches highest priority ready process RUNNING TO BLOCKED • Running process does i/o op • Blocks itself BLOCKED TO READY • Event for which process was waiting completes • Process is added to priority queue PROCESS TABLE • Every O/S has a process table • • • • 1 entry per process, called the PCB PCB is a record (struct) Process table is an array of pointers to structs Each process is uniquely identified by its PCB PROCESS CONTROL BLOCK • PCB is what defines a process to the system • Current state of the process (running, ready, blocked) • Unique id of the process • Pointer to the process’ parent • Pointer to each child process • Process’ priority • Pointer to process’ memory • Pointer to allocated resources (printers, disk drives, unacknowledged messages) • Register save area • File descriptors • Processor that process is running on EXAMPLE: FORK int fork() • Returns • -1: no process was created • 0: returned to the child process • pid: returned to parent process • Creates • Copy of parent process but with its own pid and a different parent pid EXAMPLE: WAIT • int* status int wait(status) • Suspends calling process until it receives a signal to wakeup • Returns (in the case of a fork) • pid of child • -1 if childless • See example 10 on the website MODELING MULTIPROGRAMMING • Suppose a process spends a fraction, p, of its time waiting for i/o. • That is, at any randomly chosen time, the probability that the process is blocked waiting for i/o is p. • With n processes, the probability that all are waiting for i/o = pn • So, the probability that the cpu is used = 1 – pn MODELING MULTIPROGRAMMING (2) CPU use as a function of the number of processes in memory (i.e., n). As prob. of i/o blocking increases, cpu use increases with more processes. Of course, the more memory, the more processes.