Processes: Concept, Scheduling, Interprocess Communication and Cooperation, Slides of Computer Science

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Processes

Chapter 3

Processes

• Process Concept

• Process Scheduling

• Operations on Processes

• Cooperating Processes

• Interprocess Communication

• Communication in Client-Server Systems

Process in Memory

Process 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 processor
  • terminated : The process has finished execution

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

Representation of Process Scheduling

Schedulers (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 bursts

CPU Switch From Process to Process

Process 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 terminate
• Address space
  • Child duplicate of parent
  • Child has a program loaded into it

Process Creation (Cont.)

• UNIX examples
  • fork system call creates new process
  • exec system call used after a fork to replace the process’ memory space with a new program

C Program Forking Separate Process

int main() { pid_t 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); } }

Process synchronization

• The parent can wait for a child to terminate using the wait( ) function. int cpid_done, status; cpid_done = wait(&status);
• The caller sleeps until any child terminates.
• cpid_done gets the pid of the child that terminated.
• status gets the exit status of the terminated process.

int cpid_done, status, options = 0; cpid_done = waitpid(cpid, &status, options);

• caller sleeps until child with cpid terminates.
• returns the pid of the terminating child
• stores the status of the terminating child in status.

Basic Shell code

Pseudo-code a basic shell (not real C++ code):

while (1) { parse_command_line; (get command, args, redirect, etc.) if(cmd == exit) exit; p = fork( ); if (p == 0) { exec (cmd, args) } else { if (command doesn't end with &) wait( ); } }

Cooperating 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
• Convenience