1. Silberschatz, Galvin and Gagne  2002 4.1 Operating System Concepts Chapter 4: Processes Process Concept Process Scheduling Operations on Processes Cooperating Processes Interprocess Communication Communication in Client-Server Systems

2. Silberschatz, Galvin and Gagne  2002 4.2 Operating System Concepts Process Concept An operating system executes a variety of programs:  Batch system ≈ jobs  Time-shared systems ≈ processes or tasks we use the terms job and process interchangeably Process  a program in execution  process execution must progress in sequential fashion A process includes:  code section  data section  program counter  stack

3. Silberschatz, Galvin and Gagne  2002 4.3 Operating System Concepts 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

4. Silberschatz, Galvin and Gagne  2002 4.4 Operating System Concepts Diagram of Process State

5. Silberschatz, Galvin and Gagne  2002 4.5 Operating System Concepts Process Control Block Information associated with each process: Process state Program counter CPU registers CPU scheduling information Memory-management information Accounting information I/O status information

6. Silberschatz, Galvin and Gagne  2002 4.6 Operating System Concepts Process Control Block (PCB)

7. Silberschatz, Galvin and Gagne  2002 4.7 Operating System Concepts CPU Switch From Process to Process

8. Silberschatz, Galvin and Gagne  2002 4.8 Operating System Concepts 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 A Process moves between the various queues

9. Silberschatz, Galvin and Gagne  2002 4.9 Operating System Concepts Ready And Device Queues

10. Silberschatz, Galvin and Gagne  2002 4.10 Operating System Concepts Representation of Process Scheduling

11. Silberschatz, Galvin and Gagne  2002 4.11 Operating System Concepts Schedulers Long-term scheduler (or job scheduler):  selects which processes should be moved to ready queue Short-term scheduler (or CPU scheduler):  selects which process should execute next  allocates CPU

12. Silberschatz, Galvin and Gagne  2002 4.12 Operating System Concepts Also: Medium-term Scheduling

13. Silberschatz, Galvin and Gagne  2002 4.13 Operating System Concepts Scheduler Issues 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

14. Silberschatz, Galvin and Gagne  2002 4.14 Operating System Concepts Scheduler Issues 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

15. Silberschatz, Galvin and Gagne  2002 4.15 Operating System Concepts 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 depends on hardware support

16. Silberschatz, Galvin and Gagne  2002 4.16 Operating System Concepts Operations on Processes Parent process creates children processes, which, in turn create other processes, forming a tree of processes Resource sharing choices:  Parent and children share all resources  Children share subset of parent’s resources  Parent and child share no resources Execution choices:  Parent and children execute concurrently  Parent waits until children terminate

17. Silberschatz, Galvin and Gagne  2002 4.17 Operating System Concepts Process Creation (Cont.) Address space choices:  Child duplicate of parent  Child has a new 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

18. Silberschatz, Galvin and Gagne  2002 4.18 Operating System Concepts Unix example: create new process void main(int argc, char *argv[]) { int pid=fork(); if (pid < 0) { fprintf(stderr, "Fork failed"); exit(-1); } else if (pid == 0) { // child process execlp("/bin/ls", "ls", NULL); } else {// parent process wait(NULL); printf("Child complete"); exit(0); }

19. Silberschatz, Galvin and Gagne  2002 4.19 Operating System Concepts Processes Tree on a UNIX System

20. Silberschatz, Galvin and Gagne  2002 4.20 Operating System Concepts Process Termination process executes last statement or, asks OS for explicit exit  return data from child to parent (via wait)  process’ resources are deallocated by operating system parent may abort execution of children process  child has exceeded allocated resources  task assigned to child is no longer required  parent is exiting  OS does not allow child to continue if its parent terminates –Term: Cascading termination

21. Silberschatz, Galvin and Gagne  2002 4.21 Operating System Concepts 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

22. Silberschatz, Galvin and Gagne  2002 4.22 Operating System Concepts Producer-Consumer Problem Paradigm for cooperating processes: producer process produces information that is consumed by a consumer process  unbounded-buffer: no practical limit on the size of the buffer  bounded-buffer: assumes that there is a fixed buffer size

23. Silberschatz, Galvin and Gagne  2002 4.23 Operating System Concepts Bounded-Buffer – Shared-Memory Solution #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

24. Silberschatz, Galvin and Gagne  2002 4.24 Operating System Concepts Bounded-Buffer – Producer Process item nextProduced; while (1) { while (((in + 1) % BUFFER_SIZE) == out) ; /* do nothing */ buffer[in] = nextProduced; in = (in + 1) % BUFFER_SIZE; }

25. Silberschatz, Galvin and Gagne  2002 4.25 Operating System Concepts Bounded-Buffer – Consumer Process item nextConsumed; while (1) { while (in == out) ; /* do nothing */ nextConsumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; }

26. Silberschatz, Galvin and Gagne  2002 4.26 Operating System Concepts Inter-process 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 communication link between them  exchange messages via send/receive Implementation of communication link  physical (e.g., shared memory, hardware bus)  logical (e.g., logical operations)

27. Silberschatz, Galvin and Gagne  2002 4.27 Operating System Concepts 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?

28. Silberschatz, Galvin and Gagne  2002 4.28 Operating System Concepts 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  link is associated with exactly one pair of processes  between each pair there exists exactly one link  link may be unidirectional, but is usually bi-directional

29. Silberschatz, Galvin and Gagne  2002 4.29 Operating System Concepts 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  Link may be associated with many processes  Each pair of processes may share several communication links  Link may be unidirectional or bi-directional

30. Silberschatz, Galvin and Gagne  2002 4.30 Operating System Concepts 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

31. Silberschatz, Galvin and Gagne  2002 4.31 Operating System Concepts Indirect Communication Mailbox sharing  P 1, P 2, and P 3 share mailbox A  P 1, sends; P 2 and P 3 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)

32. Silberschatz, Galvin and Gagne  2002 4.32 Operating System Concepts Synchronization Message passing may be either blocking or non-blocking Blocking is considered synchronous Non-blocking is considered asynchronous send and receive primitives may be either blocking or non-blocking

33. Silberschatz, Galvin and Gagne  2002 4.33 Operating System Concepts 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

34. Silberschatz, Galvin and Gagne  2002 4.34 Operating System Concepts Example: Mach Message-based micro-kernel OS Task = process with multiple threads Port = mailbox  has port rights: receive, send, send-once  each task has Kernel and Notify port Message  data structure: typed collection of objects  contains port rights Message transfer primitives:  msg_send, msg_receive, msg_rpc Port set  group of ports (with receive right) that share a queue  thread can serve a port set

35. Silberschatz, Galvin and Gagne  2002 4.35 Operating System Concepts Example: Mach Port functionality:  allocate a new port with port rights  only one process has receive right  de-allocate a port  includes notification of port rights holders  get port status  create backup port  to receive messages if port is de-allocated Message content:  destination port  reply port  message length  port rights  pointers to parameters Message delivery via memory mapping

36. Silberschatz, Galvin and Gagne  2002 4.36 Operating System Concepts Example: Windows XP LPC: local procedure call Provides communication among Windows subsystems Port for  Connection  Communication Message passing techniques  Message copy (< 256 bytes)  Shared memory  Direct read/write (only to/from Win32 subsystem)

37. Silberschatz, Galvin and Gagne  2002 4.37 Operating System Concepts Client-Server Communication Sockets Remote Procedure Calls Remote Method Invocation (Java)

38. Silberschatz, Galvin and Gagne  2002 4.38 Operating System Concepts 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 occurs between a pair of sockets

39. Silberschatz, Galvin and Gagne  2002 4.39 Operating System Concepts Socket Communication

40. Silberschatz, Galvin and Gagne  2002 4.40 Operating System Concepts Java Server example import java.net.*; import java.io.*; import java.util.*; class Server { public static void main(String [] args) { try { ServerSocket server = new ServerSocket(5155); while (true) { Socket s = server.accept(); PrintWriter out = new PrintWriter(s.getOutputStream(), true); out.println(new Date().toString()); out.close(); s.close(); } } catch (IOException ioe) { System.err.println(ioe); }

41. Silberschatz, Galvin and Gagne  2002 4.41 Operating System Concepts Java Client example import java.net.*; import java.io.*; class Client { public static void main(String [] args) { try { Socket s = new Socket("127.0.0.1", 5155); BufferedReader bin = new BufferedReader( new InputStreamReader(s.getInputStream()); String line; while ((line = bin.readLine()) != null) System.out.println(line); } catch (IOException ioe) { System.out.println(ioe); }

42. Silberschatz, Galvin and Gagne  2002 4.42 Operating System Concepts Remote Procedure Calls Remote procedure call (RPC) abstracts procedure calls between processes on networked systems Stubs  proxies for the actual procedure  client-side stub  locates the server  marshals the parameters  server-side stub  receives this message,  unpacks the marshaled parameters  performs the procedure on the server

43. Silberschatz, Galvin and Gagne  2002 4.43 Operating System Concepts Remote Method Invocation Remote Method Invocation (RMI) is a Java mechanism similar to RPC RMI allows a Java program on one machine to invoke a method on a remote object

44. Silberschatz, Galvin and Gagne  2002 4.44 Operating System Concepts Marshalling Parameters

45. Silberschatz, Galvin and Gagne  2002 4.45 Operating System Concepts RMI example

46. Silberschatz, Galvin and Gagne  2002 4.46 Operating System Concepts Server Interface import java.rmi.*; public interface QuoteServerI extends Remote { String getQuote() throws RemoteException; }

47. Silberschatz, Galvin and Gagne  2002 4.47 Operating System Concepts Server Implementation (1/2) import java.rmi.*; import java.rmi.server.*; public class QuoteServer extends UnicastRemoteObject implements QuoteServerI { public QuoteServer() throws RemoteException { super(); } public String getQuote() throws RemoteException { return "hello here is your quote"; }

48. Silberschatz, Galvin and Gagne  2002 4.48 Operating System Concepts Server Implementation (2/2) public static void main(String[] args) { System.setSecurityManager(new RMISecurityManager()); try { Naming.rebind("/quoter", new QuoteServer()); } catch (Exception x) { x.printStackTrace(); }

49. Silberschatz, Galvin and Gagne  2002 4.49 Operating System Concepts Client Implementation import java.rmi.*; public class QuoteClient { public static void main(java.lang.String[] args) { try { QuoteServerI server; server = (QuoteServerI) Naming.lookup("quoter"); System.out.println(server.getQuote()); } catch (Exception x) { x.printStackTrace(); }

50. Silberschatz, Galvin and Gagne  2002 4.50 Operating System Concepts Steps to run Generate stubs and skeleton classes: rmic Compile all Java code Run rmiregistry Provide security policy Run QuoteServer Run QuoteClient

51. Silberschatz, Galvin and Gagne  2002 4.51 Operating System Concepts Run RMI application rmiregistry java –Djava.security.policy=policy QuoteServer java –Djava.security.policy=policy QuoteClient policy example: grant { permission java.security.AllPermission; };