CSMC 412Operating SystemsProf. Ashok K Agrawala© 2005 Ashok AgrawalaSet 3September 2006 1CMSC 412 Set 3Processes• Process Concept• Process Scheduling• Operations on Processes• Cooperating Processes• Interprocess Communication• Communication in Client-Server SystemsSeptember 2006 2CMSC 412 Set 3Process Concept• An operating system executes a variety of programs:– Batch system – jobs– Time-shared systems – user programs or tasks• Textbook uses the terms job and process almost interchangeably• Process – a program in execution; process execution must progress in sequential fashion• A process includes:– program counter – stack– data sectionSeptember 2006 3CMSC 412 Set 3Process in MemorySeptember 2006 4CMSC 412 Set 3Process State• 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 executionSeptember 2006 5CMSC 412 Set 3Diagram of Process StateSeptember 2006 6CMSC 412 Set 3Process Control Block (PCB)Information associated with each process• Process state• Program counter• CPU registers• CPU scheduling information• Memory-management information• Accounting information• I/O status informationSeptember 2006 7CMSC 412 Set 3Process Control Block (PCB)September 2006 8CMSC 412 Set 3CPU Switch From Process to ProcessSeptember 2006 9CMSC 412 Set 3Process 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 queuesSeptember 2006 10CMSC 412 Set 3Active Processes in LinuxSeptember 2006 11CMSC 412 Set 3Ready Queue And Various I/O Device QueuesSeptember 2006 12CMSC 412 Set 3Representation of Process SchedulingSeptember 2006 13CMSC 412 Set 3Schedulers• Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue• Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPUSeptember 2006 14CMSC 412 Set 3Addition of Medium Term SchedulingSeptember 2006 15CMSC 412 Set 3Schedulers (Cont.)• Short-term scheduler is invoked very frequently (milliseconds) (must be fast)• Long-term scheduler is invoked very infrequently (seconds, minutes) (may be slow)• The long-term scheduler controls the degree of multiprogramming• Processes can be described as either:– I/O-bound process – spends more time doing I/O than computations, many short CPU bursts– CPU-bound process – spends more time doing computations; few very long CPU burstsSeptember 2006 16CMSC 412 Set 3Context Switch• When CPU switches to another process, the system must save the state of the old process and load the saved state for the new process• Context-switch time is overhead; the system does no useful work while switching• Time dependent on hardware supportSeptember 2006 17CMSC 412 Set 3Process Creation• Parent process create children processes, which, in turn create other processes, forming a tree of processes• Resource sharing– Parent and children share all resources– Children share subset of parent’s resources– Parent and child share no resources• Execution– Parent and children execute concurrently– Parent waits until children terminateSeptember 2006 18CMSC 412 Set 3Process Creation (Cont.)• Address space– Child duplicate of parent– Child has a program loaded into it• UNIX examples– fork system call creates new process– exec system call used after a fork to replace the process’ m em ory space w ith a new programSeptember 2006 19CMSC 412 Set 3Process Tree for Solaris systemSeptember 2006 20CMSC 412 Set 3C Program Forking Separate Process#include <stdio.h>#include <unistd.h>int main(int argc, char *argv[]){int pid;/* fork another process */pid = fork();if (pid < 0) { /* error occurred */fprintf(stderr, "Fork Failed");exit(-1);}else if (pid == 0) { /* child process */execlp("/bin/ls","ls",NULL);}else { /* parent process *//* parent will wait for the child to complete */wait(NULL);printf("Child Complete");exit(0);}}September 2006 21CMSC 412 Set 3Parent and Child ProcessesSeptember 2006 22CMSC 412 Set 3Processes Tree on a UNIX SystemSeptember 2006 23CMSC 412 Set 3Process Termination• Process executes last statement and asks the operating system to decide it (exit)– Output data from child to parent (via wait)– Process’ resources are deallocated by operating system• Parent may terminate execution of children processes (abort)– Child has exceeded allocated resources– Task assigned to child is no longer required– If parent is exiting• Some operating system do not allow child to continue if its parent terminates– All children terminated - cascading terminationSeptember 2006 24CMSC 412 Set 3Cooperating Processes• Independent process cannot affect or be affected by the execution of another process• Cooperating process can affect or be affected by the execution of another process• Advantages of process cooperation– Information sharing – Computation speed-up– Modularity– ConvenienceSeptember 2006 25CMSC 412 Set 3Producer-Consumer Problem• Paradigm for cooperating processes, producerprocess produces information that is consumed by a consumer process– unbounded-buffer places no practical limit on the size of the buffer– bounded-buffer assumes that there is a fixed buffer sizeSeptember 2006 26CMSC 412 Set 3Bounded-Buffer – Shared-Memory Solutionpublic interface Buffer{// producers call this methodpublic abstract void insert(Object item);// consumers call this methodpublic abstract Object remove();}September 2006 27CMSC 412 Set 3Bounded-Buffer – Shared Memory Solution import java.util.*;public class BoundedBuffer implements Buffer{private static final int BUFFER SIZE = 5;private int count; // number of items in the bufferprivate int in; // points to the next free positionprivate int out; // points to the next full positionprivate Object[] buffer;public BoundedBuffer() { // buffer is initially emptycount = 0;in = 0;out = 0;buffer = new Object[BUFFER SIZE];}// producers calls this methodpublic void insert(Object item) { // Slide 4.24}// consumers calls this methodpublic
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