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1. Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Chapter 3: Processes 2. 3.2 Silberschatz, Galvin and Gagne ©2009Operating System…
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  • 1. Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Chapter 3: Processes
  • 2. 3.2 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Chapter 3: Processes  Process Concept  Process Scheduling  Operations on Processes  Interprocess Communication  Examples of IPC Systems  Communication in Client-Server Systems
  • 3. 3.3 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Objectives  To introduce the notion of a process -- a program in execution, which forms the basis of all computation  To describe the various features of processes, including scheduling, creation and termination, and communication  To describe communication in client-server systems
  • 4. 3.4 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition 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
  • 5. 3.5 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition The Process  Multiple parts  The program code, also called text section  Current activity including program counter, processor registers  Stack containing temporary data  Function parameters, return addresses, local variables  Data section containing global variables  Heap containing memory dynamically allocated during run time  Program is passive entity, process is active  Program becomes process when executable file loaded into memory  Execution of program started via GUI mouse clicks, command line entry of its name, etc  One program can be several processes  Consider multiple users executing the same program
  • 6. 3.6 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Process in Memory
  • 7. 3.7 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition 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
  • 8. 3.8 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Diagram of Process State
  • 9. 3.9 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition 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
  • 10. 3.10 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Process Control Block (PCB)
  • 11. 3.11 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition CPU Switch From Process to Process
  • 12. 3.12 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Process Scheduling  Maximize CPU use, quickly switch processes onto CPU for time sharing  Process scheduler selects among available processes for next execution on CPU  Maintains scheduling queues of processes  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
  • 13. 3.13 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Process Representation in Linux  Represented by the C structure task_struct pid t pid; /* process identifier */ long state; /* state of the process */ unsigned int time slice /* scheduling information */ struct task struct *parent; /* this process’s parent */ struct list head children; /* this process’s children */ struct files struct *files; /* list of open files */ struct mm struct *mm; /* address space of this pro */
  • 14. 3.14 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Ready Queue And Various I/O Device Queues
  • 15. 3.15 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Representation of Process Scheduling
  • 16. 3.16 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition 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  Sometimes the only scheduler in a system
  • 17. 3.17 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition 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
  • 18. 3.18 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Addition of Medium Term Scheduling
  • 19. 3.19 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition 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 via a context switch.  Context of a process represented in the PCB  Context-switch time is overhead; the system does no useful work while switching  The more complex the OS and the PCB -> longer the context switch  Time dependent on hardware support  Some hardware provides multiple sets of registers per CPU -> multiple contexts loaded at once
  • 20. 3.20 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Process Creation  Parent process create children processes, which, in turn create other processes, forming a tree of processes  Generally, process identified and managed via a process identifier (pid)  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
  • 21. 3.21 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition 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
  • 22. 3.22 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Process Creation
  • 23. 3.23 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition C Program Forking Separate Process #include <sys/types.h> #include <studio.h> #include <unistd.h> int main() { pid_t pid; /* fork another process */ pid = fork(); if (pid < 0) { /* error occurred */ fprintf(stderr, "Fork Failed"); return 1; } else if (pid == 0) { /* child process */ execlp("/bin/ls", "ls", NULL); } else { /* parent process */ /* parent will wait for the child */ wait (NULL); printf ("Child Complete"); } return 0; }
  • 24. 3.24 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition A Tree of Processes on Solaris
  • 25. 3.25 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition 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 systems do not allow child to continue if its parent terminates – All children terminated - cascading termination
  • 26. 3.26 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Interprocess Communication  Processes within a system may be independent or cooperating  Cooperating process can affect or be affected by other processes, including sharing data  Reasons for cooperating processes:  Information sharing  Computation speedup  Modularity  Convenience  Cooperating processes need interprocess communication (IPC)  Two models of IPC  Shared memory  Message passing
  • 27. 3.27 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Communications Models
  • 28. 3.28 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition 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
  • 29. 3.29 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition 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
  • 30. 3.30 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition 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
  • 31. 3.31 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Bounded-Buffer – Producer 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; }
  • 32. 3.32 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Bounded Buffer – Consumer 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; }
  • 33. 3.33 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Interprocess Communication – Message Passing  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 communication link between them  exchange messages via send/receive  Implementation of communication link  physical (e.g., shared memory, hardware bus)  logical (e.g., logical properties)
  • 34. 3.34 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition 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?
  • 35. 3.35 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition 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
  • 36. 3.36 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition 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
  • 37. 3.37 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition 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
  • 38. 3.38 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Indirect Communication  Mailbox sharing  P1, P2, and P3 share mailbox A  P1, sends; P2 and 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.
  • 39. 3.39 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition 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
  • 40. 3.40 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Buffering  Queue of messages attached to the link; implemented in one of three ways 1. Zero capacity – 0 messages Sender must wait for receiver (rendezvous) 2. Bounded capacity – finite length of n messages Sender must wait if link full 3. Unbounded capacity – infinite length Sender never waits
  • 41. 3.41 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Examples of IPC Systems - POSIX  POSIX Shared Memory  Process first creates shared memory segment segment id = shmget(IPC PRIVATE, size, S IRUSR | S IWUSR);  Process wanting access to that shared memory must attach to it shared memory = (char *) shmat(id, NULL, 0);  Now the process could write to the shared memory sprintf(shared memory, "Writing to shared memory");  When done a process can detach the shared memory from its address space shmdt(shared memory);
  • 42. 3.42 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Examples of IPC Systems - Mach  Mach communication is message based  Even system calls are messages  Each task gets two mailboxes at creation- Kernel and Notify  Only three system calls needed for message transfer msg_send(), msg_receive(), msg_rpc()  Mailboxes needed for commuication, created via port_allocate()
  • 43. 3.43 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Examples of IPC Systems – Windows XP  Message-passing centric via local procedure call (LPC) facility  Only works between processes on the same system  Uses ports (like mailboxes) to establish and maintain communication channels  Communication works as follows:  The client opens a handle to the subsystem’s connection port object.  The client sends a connection request.  The server creates two private communication ports and returns the handle to one of them to the client.  The client and server use the corresponding port handle to send messages or callbacks and to listen for replies.
  • 44. 3.44 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Local Procedure Calls in Windows XP
  • 45. 3.45 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Communications in Client-Server Systems  Sockets  Remote Procedure Calls  Pipes  Remote Method Invocation (Java)
  • 46. 3.46 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Sockets  A socket is defined as an endpoint for communication  Concatenation of IP address and port  The socket 161.25.19.8:1625 refers to port 1625 on host 161.25.19.8  Communication consists between a pair of sockets
  • 47. 3.47 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Socket Communication
  • 48. 3.48 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Remote Procedure Calls  Remote procedure call (RPC) abstracts procedure calls between processes on networked systems  Stubs – client-side proxy for the actual procedure on the server  The client-side stub locates the server and marshalls the parameters  The server-side stub receives this message, unpacks the marshalled parameters, and performs the procedure on the server
  • 49. 3.49 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Execution of RPC
  • 50. 3.50 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Pipes  Acts as a conduit allowing two processes to communicate  Issues  Is communication unidirectional or bidirectional?  In the case of two-way communication, is it half or full-duplex?  Must there exist a relationship (i.e. parent-child) between the communicating processes?  Can the pipes be used over a network?
  • 51. 3.51 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Ordinary Pipes  Ordinary Pipes allow communication in standard producer-consumer style  Producer writes to one end (the write-end of the pipe)  Consumer reads from the other end (the read-end of the pipe)  Ordinary pipes are therefore unidirectional  Require parent-child relationship between communicating processes
  • 52. 3.52 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Ordinary Pipes
  • 53. 3.53 Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition Named Pipes  Named Pipes are more powerful than ordinary pipes  Communication is bidirectional  No parent-child relationship is necessary between the communicating processes  Several processes can use the named pipe for communication  Provided on both UNIX and Windows systems
  • 54. Silberschatz, Galvin and Gagne ©2009Operating System Concepts – 8th Edition End of Chapter 3
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