Chapter 3: Processes

1. Chapter 3: Processes

2. Chapter 3: Processes • Process Concept • Process Scheduling • Operations on Processes • Cooperating Processes • Interprocess Communication • Communication in Client-Server Systems

3. Process 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 section

4. Process in Memory

5. 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 process • terminated: The process has finished execution

6. Diagram of Process State

7. Process 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 information

8. Process Control Block (PCB)

9. CPU Switch From Process to Process

10. 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

11. Ready Queue And Various I/O Device Queues

12. Representation of Process Scheduling

13. Schedulers • 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 CPU

14. Addition of Medium Term Scheduling

15. 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

16. Context 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 support

17. 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

18. Process 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’ memory space with a new program

19. Process Creation

20. C Program Forking Separate Process #include <sys/wait.h> #include <stdio.h> #define NULL 0 int main(){ int y,x; printf("Process[%d]: Parent in execution ... \n", getpid() ); x = fork(); if (x == 0){ /* this is child */ fprintf(stderr,"Process[%d]: child in execution....\n",getpid() ); sleep(5); fprintf(stderr,"Process[%d]: child terminating....\n",getpid() ); exit(0); } /* Parent code here */ printf("Process[%d]: Parent in execution ... \n", getpid() ); sleep(2); if (wait(NULL) > 0) /* Child terminating */ printf("Process[%d]: Parent detects terminating child \n", getpi d() ); printf("Process[%d]: Parent terminating....\n", getpid() ); }

21. Two forks Only check for one of the children! Who executes the parent code?? complicated fork() example 6 int main(){ 7 int y,x; 8 printf("Process[%d]: Parent in execution ... \n", getpid() ); 9 x = fork(); 10 y = fork(); 11 if (x == 0){ /* this is child */ 12 fprintf(stderr,"Process[%d]: child in execution....\n",getpid() ); 13 sleep(5); 14 fprintf(stderr,"Process[%d]: child terminating....\n",getpid() ); 15 exit(0); 16 } 17 18 /* Parent code here */ 19 printf("Process[%d]: Parent in execution ... \n", getpid() ); 20 sleep(2); 21 if (wait(NULL) > 0) /* Child terminating */ 22 printf("Process[%d]: Parent detects terminating child \n", getpid() ); 23 printf("Process[%d]: Parent terminating....\n", getpid() ); 24 }

22. Why two children? Why two parents? Parent terminates, so get the prompt back. Later second and third children terminates. fork() example Note that we cannot predict the order of execution because the OS must schedule this bash-2.03$ fork1.2 Process[16750]: Parent in execution ... Process[16752]: child in execution.... Process[16751]: child in execution.... Process[16753]: Parent in execution ... Process[16750]: Parent in execution ... Process[16753]: Parent terminating.... Process[16750]: Parent detects terminating child Process[16750]: Parent terminating.... bash-2.03$ Process[16751]: child terminating.... Process[16752]: child terminating.... bash-2.03$

23. C Program Forking Separate Process • wait( ) #include <sys/types.h> #include <sys/wait.h> pid_t wait(int *stat_loc); • Parent process pauses until one of its children completes • Returns the pid of the child that terminated. • If stat_loc is not null, the exit status information from child is written into it. • Can interpret the exit status information by using macros defined in sys/wait.h (see next slide)

24. C Program Forking Separate Process • WIFEXITED(stat_val) • Non-zero if the child is terminated normally • WEXITSTATUS(stat_val) • If WIFEXITED is non-zero, this returns child exit code. • WIFSIGNALED(stat_val) • Non-zero if the child is terminated on an uncaught signal. • WTERMSIG(stat_val) • If WIFSIGNALED is non-zero this returns a signal number • WIFSTOPPED(stat_val) • Non-zero if the child has stopped. • WSTOPSIG(stat_val) • If WIFSTOPPED is non--zero, this returns a signal num

25. 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); } }

26. Overwriting a process with a new program #include <unistd.h> int execl (const char *path, const char *arg0, …, (char *)0); int execlp (const char *file, const char *arg0, …, (char *) 0); int execle(const char *path, const char *arg0, …, (char *) 0, const char *envp[]); int execv (const char *path, const char *argv[]); int execvp (const char *file, const char *argv[]); int execve (const char *path, const char *argv[], const char *envp[]); • Execl, execlp, and execle take a variable number of arugments ending with a null pointer. • Execv and execvp have as their second argument an array of strings • In both cases, the new program starts with the given arguments appearing in the argv array passed to the new process. • Functions suffixed with a p will search the PATH environmental variable to find the file name. • Functions suffixed with an e pass the environment to the new process.

27. Overwriting a process with a new program #include <unistd.h> /* example of an argument list */ /* Note that we need a program name for argv[0] */ Const char *ps_argv[] = {“ps”, “-ax”, 0}; /* example environment, not terribly useful */ Const char *ps_envp[] = {*PATH=/bin:/usr/bin”, “TERM=console”, 0}; /* possible calls to exec functions */ int execl (“/bin/ps”, “ps”, “-ax”, 0); /* assumes ps is in /bin */ int execlp (“ps”, “ps”, “-”ax”, 0); /* assumes /bin is in PATH */ int execle(“/bin/ps”, “ps”, “-ax”, 0, ps_envp); /* passes own environment*/ int execv (“/bin/ps”, ps_argv); int execvp (“ps”, ps_argv); int execve ((“/bin/ps”, ps_argv, ps_envp);

28. C Program Forking Separate Process 1 #include <sys/wait.h> 2 3 #define NULL 0 4 5 int main(){ 6 if (fork() == 0){ /* this is child */ 7 execl("child",0); 8 exit(1); /* should never get here; terminate */ 9 } 10 11 /* Parent code here */ 12 printf("Process[%d]: Parent in execution ... \n", getpid() ); 13 sleep(2); 14 if (wait(NULL) > 0) /* Child terminating */ 15 printf("Process[%d]: Parent detects terminating child \n", getpid() ); 16 printf("Process[%d]: Parent terminating....\n", getpid() ); 17 }

29. Child Program (Separate Process) /* child.c code */ 1 #include <stdio.h> 2 3 int main () { 4 /* The child process's new program 5 This program replaces the parent's program */ 6 7 fprintf(stderr,"Process[%d]: child in execution....\n",getpid() ); 8 sleep(5); 9 fprintf(stderr,"Process[%d]: child terminating....\n",getpid() ); 10 }

30. C Program Forking Separate Process (passing parameters to the child) 1 #include <sys/wait.h> 2 3 #define NULL 0 4 5 int main(){ 6 if (fork() == 0){ /* this is child */ 7 execl("child2","first","second","third","fourth",0); 8 exit(1); /* should never get here; terminate */ 9 } 10 11 /* Parent code here */ 12 printf("Process[%d]: Parent in execution ... \n", getpid() ); 13 sleep(2); 14 if (wait(NULL) > 0) /* Child terminating */ 15 printf("Process[%d]: Parent detects terminating child \n", getpid() ); 16 printf("Process[%d]: Parent terminating....\n", getpid() ); 17 }

31. C Program Forking Separate Process (passing parameters to the child) 1 /* child2.c code */ 2 #include <stdio.h> 3 int main (int argc, char *argv[]) { 4 int i; 5 6 7 /* The child process's new program 8 This program replaces the parent's program */ 9 10 fprintf(stderr,"Process[%d]: child in execution....\n",getpid() ); 11 for (i = 0; i < argc; i++) 12 fprintf(stderr,"Argument[%d]: %s\n",i, argv[i]); 13 14 fprintf(stderr, "\n"); 15 16 sleep(5); 17 fprintf(stderr,"Process[%d]: child terminating....\n",getpid() ); 18 }

32. A tree of processes on a typical Solaris

33. Process Termination • Process executes last statement and asks the operating system to delete 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 termination

34. 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

35. Producer-Consumer Problem • Paradigm for cooperating processes, producer process 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 size

36. Bounded-Buffer – Shared-Memory Solution • Shared data #define BUFFER_SIZE 10 Typedef struct { . . . } item; item buffer[BUFFER_SIZE]; int in = 0; int out = 0; • Solution is correct, but can only use BUFFER_SIZE-1 elements

37. Bounded-Buffer – Insert() Method while (true) { /* Produce an item */ while (((in = (in + 1) % BUFFER SIZE count) == out) ; /* do nothing -- no free buffers */ buffer[in] = item; in = (in + 1) % BUFFER SIZE; }

38. Bounded Buffer – Remove() Method while (true) { while (in == out) ; // do nothing -- nothing to consume // remove an item from the buffer item = buffer[out]; out = (out + 1) % BUFFER SIZE; return item; }

39. Interprocess Communication (IPC) • Mechanism for processes to communicate and to synchronize their actions • Message system – processes communicate with each other without resorting to shared variables • IPC facility provides two operations: • send(message) – message size fixed or variable • receive(message) • If P and Q wish to communicate, they need to: • establish a communicationlink between them • exchange messages via send/receive • Implementation of communication link • physical (e.g., shared memory, hardware bus) • logical (e.g., logical properties)

40. Implementation Questions • How are links established? • Can a link be associated with more than two processes? • How many links can there be between every pair of communicating processes? • What is the capacity of a link? • Is the size of a message that the link can accommodate fixed or variable? • Is a link unidirectional or bi-directional?

41. Communications Models

42. Direct Communication • Processes must name each other explicitly: • send (P, message) – send a message to process P • receive(Q, message) – receive a message from process Q • Properties of communication link • Links are established automatically • A link is associated with exactly one pair of communicating processes • Between each pair there exists exactly one link • The link may be unidirectional, but is usually bi-directional

43. Indirect Communication • Messages are directed and received from mailboxes (also referred to as ports) • Each mailbox has a unique id • Processes can communicate only if they share a mailbox • Properties of communication link • Link established only if processes share a common mailbox • A link may be associated with many processes • Each pair of processes may share several communication links • Link may be unidirectional or bi-directional

44. Indirect Communication • Operations • create a new mailbox • send and receive messages through mailbox • destroy a mailbox • Primitives are defined as: send(A, message) – send a message to mailbox A receive(A, message) – receive a message from mailbox A

45. Indirect Communication • Mailbox sharing • P1, P2, and P3 share mailbox A • P1, sends; P2and P3 receive • Who gets the message? • Solutions • Allow a link to be associated with at most two processes • Allow only one process at a time to execute a receive operation • Allow the system to select arbitrarily the receiver. Sender is notified who the receiver was.

46. Synchronization • Message passing may be either blocking or non-blocking • Blocking is considered synchronous • Blocking send has the sender block until the message is received • Blocking receive has the receiver block until a message is available • Non-blocking is considered asynchronous • Non-blocking send has the sender send the message and continue • Non-blocking receive has the receiver receive a valid message or null

47. Buffering • Queue of messages attached to the link; implemented in one of three ways 1. Zero capacity – 0 messagesSender must wait for receiver (rendezvous) 2. Bounded capacity – finite length of n messagesSender must wait if link full 3. Unbounded capacity – infinite length Sender never waits

48. Unix Pipes • Pipes are FIFO • Read bytes in the same order that they were written • Must use the read and write system calls to access data. • Pipes are buffered; will save data not read. • Read is blocking; • if a process does a read on a pipe, the process blocks until data becomes available • If the pipe is closed by the writing process, then a read will return a 0. • If the write pipe file descriptor is open in both the parent and child process, then both processes must close the write pipe file descriptor before the pipe will return a 0 on a read.

49. Indirect Communicationpipe1.c Pi#include <unistd.h> #include <stdlib.h> #include <stdio.h> #include <string.h> int main() { int data_processed; int file_pipes[2]; const char some_data[] = "123"; char buffer[BUFSIZ + 1]; memset(buffer, '\0', sizeof(buffer)); if (pipe(file_pipes) == 0) { data_processed = write(file_pipes[1], some_data, strlen(some_data)); printf("Wrote %d bytes\n", data_processed); data_processed = read(file_pipes[0], buffer, BUFSIZ); printf("Read %d bytes: %s\n", data_processed, buffer); exit(EXIT_SUCCESS); } exit(EXIT_FAILURE); } pes

50. Indirect Communicationpipe2.c #include <unistd.h> #include <stdlib.h> #include <stdio.h> #include <string.h> int main() { int data_processed; int file_pipes[2]; const char some_data[] = "456"; char buffer[BUFSIZ + 1]; int fork_result; memset(buffer, '\0', sizeof(buffer)); if (pipe(file_pipes) == 0) { fork_result = fork(); if (fork_result == -1) { fprintf(stderr, "Fork failure"); exit(EXIT_FAILURE); }