1. H.-H. S. Lee 1 ECE3055 Computer Architecture and Operating Systems Lecture 11 Processes Prof. Hsien-Hsin Sean Lee School of Electrical and Computer Engineering Georgia Institute of Technology

2. H.-H. S. Lee 2 Overview of Processes  Process Concept  Process Scheduling  Operations on Processes  Cooperating Processes  Interprocess Communication  Communication in Client-Server Systems

3. H.-H. S. Lee 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. H.-H. S. Lee 4 Process State state  As a process executes, it changes state new new: The process is being created ready ready: The process is waiting to be assigned to a process running running: Instructions are being executed waiting waiting: The process is waiting for some event (e.g. I/O) to occur terminated terminated: The process has finished execution

5. H.-H. S. Lee 5 Diagram of Process State

6. H.-H. S. Lee 6 Process Control Block (PCB) Information associated with each process  Process state  Program counter  CPU registers (for context switch)  CPU scheduling information (e.g. priority)  Memory-management information (e.g. page table, segment table)  Accounting information (PID, user time, constraint)  I/O status information (list of I/O devices allocated, list of open files etc.)

7. H.-H. S. Lee 7 Process Control Block

8. H.-H. S. Lee 8 CPU Switch From Process to Process

9. H.-H. S. Lee 9 Process Scheduling Queues  Job queue  Job queue – set of all processes in the system  Ready queue main memory  Ready queue – set of all processes residing in main memory, ready and waiting to execute  Device queues  Device queues – set of processes waiting for an I/O device  Process migration between the various queues

10. H.-H. S. Lee 10 Ready Queue And Various I/O Device Queues

11. H.-H. S. Lee 11 Representation of Process Scheduling

12. H.-H. S. Lee 12 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

13. H.-H. S. Lee 13 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 (# of processes in memory)  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  If all processes are I/O bound, the short-term scheduler will have little to do  If all processes are CPU-bound, the I/O queue is empty, devices are not used, leading to a unbalanced system

14. H.-H. S. Lee 14 Addition of Medium Term Scheduling

15. H.-H. S. Lee 15 Short-term Scheduling Example Timeline Process A Process B Process C Dispatcher = Running = Ready= Waiting (Blocked)

16. H.-H. S. Lee 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. H.-H. S. Lee 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. H.-H. S. Lee 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. H.-H. S. Lee 19 C Program Forking Separate Process #include 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); }

20. H.-H. S. Lee 20 Processes Tree on a UNIX System

21. H.-H. S. Lee 21 Process 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 termination

22. H.-H. S. Lee 22 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 (e.g. a shared file) Computation speed-up  Divide a task to subtasks  Execute in parallel (e.g. CPU, I/O channels) Modularity  Divide a system function into threads Convenience  User work on many tasks at the same time  Editing, printing, compiling in parallel

23. H.-H. S. Lee 23 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

24. H.-H. S. Lee 24 Bounded-Buffer – Shared-Memory Solution public interface Buffer { // producers call this method public abstract void insert(Object item); // consumers call this method public abstract Object remove(); }

25. H.-H. S. Lee 25 Bounded-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 buffer private int in; // points to the next free position private int out; // points to the next full position private Object[] buffer; public BoundedBuffer() { // buffer is initially empty count = 0; in = 0; out = 0; buffer = new Object[BUFFER SIZE]; } // producers calls this method public void insert(Object item) { // Slide 4.24 } // consumers calls this method public Object remove() { // Figure 4.25 }

26. H.-H. S. Lee 26 Bounded-Buffer – Insert() Method public void insert(Object item) { while (count == BUFFER SIZE) ; // do nothing -- no free buffers // add an item to the buffer ++count; buffer[in] = item; in = (in + 1) % BUFFER SIZE; }

27. H.-H. S. Lee 27 Bounded Buffer – Remove() Method public Object remove() { Object item; while (count == 0) ; // do nothing -- nothing to consume // remove an item from the buffer --count; item = buffer[out]; out = (out + 1) % BUFFER SIZE; return item; }

28. H.-H. S. Lee 28 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 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)

29. H.-H. S. Lee 29 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?

30. H.-H. S. Lee 30 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

31. H.-H. S. Lee 31 Indirect Communication  Messages are directed and received from mailboxes (also referred to as ports) send(A, message) – send a message to mailbox A receive(A, message) – receive a message from mailbox A 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 link correspond to one mailbox Each pair of processes may share several communication links Link may be unidirectional or bi-directional

32. H.-H. S. Lee 32 Indirect Communication  If mailbox owned by the OS  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

33. H.-H. S. Lee 33 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.

34. H.-H. S. Lee 34 Synchronization  Message passing may be either blocking or non- blocking  Blocking is considered synchronous Blocking send Blocking send has the sender block until the message is received Blocking receive Blocking receive has the receiver block until a message is available  Non-blocking is considered asynchronous Non-blocking send Non-blocking send has the sender send the message and continue Non-blocking receive Non-blocking receive has the receiver receive a valid message or null

35. H.-H. S. Lee 35 Buffering  Queue of messages attached to the link; implemented in one of three ways 1.Zero capacity – 0 space for queuing messages Sender must block until recipient receives the msg (rendezvous) since there is no queue 2.Bounded capacity – finite queue length of n messages Sender must block if link full 3.Unbounded capacity – infinite length Sender never waits

36. H.-H. S. Lee 36 Client-Server Communication  Sockets  Remote Procedure Calls  Remote Method Invocation (Java)

37. H.-H. S. Lee 37 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  Well known ports ftp server listens to port 21 Web/http server listens to port 80  Communication consists between a pair of sockets

38. H.-H. S. Lee 38 Socket Communication

39. H.-H. S. Lee 39 Java socket : TCP Data Server ServerSocket sock = new ServerSocket(6013) // listen for connections While (true) { Socket client = sock.accept(); PrintWriter pout = new PrintWriter(client.getOutputStream(), true); // write the data to the socket pout.println(new java.util.Date().toString()); // close the socket and resume listening for connections client.close(); }

40. H.-H. S. Lee 40 Java socket : TCP Data Client Socket sock = new Socket(“127.0.0.1”, 6013) InputStream in = sock.getInputStream(); BufferReader bin = new BufferedReader(new InputStreamReader(in)); //read the date from the socket String line; While ( (line = bin.readLine()) ! NULL) System.out.println(line); // close the socket connection sock.close();

41. H.-H. S. Lee 41 Remote Procedure Calls  Remote procedure call (RPC) abstracts procedure calls between processes on networked systems.  Stubs proxy for the actual procedure on the server. Hiding all the network code inside stub  The client-side stub locates the server Package up the parameters into some standard format and build network messages (called marshaling)  The server-side stub receives this message unmarshals the parameters performs the procedure on the server  Returning value follows the same procedure by reversing the direction

42. H.-H. S. Lee 42 Remote Procedure Call (RPC) Model client routines client stub network routines server routines server stub network routines Local kernelRemote kernel Client processSever process Local procedure call Stub marshals msg (1) System call (2) Network Communications (3) Unmarshal msg (4) Local procedure call (5) Return values for marshalling (6) (7) (8) (9) Value return (10)

43. H.-H. S. Lee 43 Execution of RPC

44. H.-H. S. Lee 44 Remote Method Invocation  Remote Method Invocation (RMI) is a Java mechanism similar to RPCs.  RMI allows a Java program on one machine to invoke a method on a remote object.

45. H.-H. S. Lee 45 Marshalling Parameters

46. H.-H. S. Lee 46 Threads  A thread (or lightweight process) is a basic unit of CPU utilization; it consists of: program counter register set stack space  A thread shares with its peer threads its: code section data section operating-system resources collectively know as a task.  A traditional or heavyweight process is equal to a task with one thread

47. H.-H. S. Lee 47 Threads (Cont.)  In a multiple threaded task, while one server thread is blocked and waiting, a second thread in the same task can run. Cooperation of multiple threads in same job confers higher throughput and improved performance. Applications that require sharing a common buffer (i.e., producer-consumer) benefit from thread utilization.  Threads provide a mechanism that allows sequential processes to make blocking system calls while also achieving parallelism.  Kernel-supported threads (Mach and OS/2).  User-level threads; supported above the kernel, via a set of library calls at the user level (Project Andrew from CMU).  Hybrid approach implements both user-level and kernel- supported threads (Solaris 2).

48. H.-H. S. Lee 48 Multiple Threads within a Task

49. H.-H. S. Lee 49 Threads Support in Solaris 2  Solaris 2 is a version of UNIX with support for threads at the kernel and user levels, symmetric multiprocessing, and real-time scheduling.  LWP – intermediate level between user-level threads and kernel-level threads.  Resource needs of thread types: Kernel thread: small data structure and a stack; thread switching does not require changing memory access information – relatively fast. LWP: PCB with register data, accounting and memory information,; switching between LWPs is relatively slow. User-level thread: only need stack and program counter; no kernel involvement means fast switching. Kernel only sees the LWPs that support user-level threads.

50. H.-H. S. Lee 50 Solaris 2 Threads