Chapter 3: Process Concept

Transcription

1 Chapter 3: Process Concept By Worawut Srisukkham Updated By Dr. Varin Chouvatut, Silberschatz, Galvin and Gagne 2010

2 Chapter 3: Process-Concept Process Concept Process Scheduling Operations on Processes Interprocess Communication Examples of IPC Systems Communication in Client-Server Systems 3.2 Silberschatz, Galvin and Gagne 2010

3 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 3.3 Silberschatz, Galvin and Gagne 2010

4 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 3.4 Silberschatz, Galvin and Gagne 2010

5 Process in Memory 3.5 Silberschatz, Galvin and Gagne 2010

6 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 3.6 Silberschatz, Galvin and Gagne 2010

7 Diagram of Process State dispatch: ท าการส งข าวสาร 3.7 Silberschatz, Galvin and Gagne 2010

8 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 3.8 Silberschatz, Galvin and Gagne 2010

9 Process Control Block (PCB) 3.9 Silberschatz, Galvin and Gagne 2010

10 CPU Switch From Process to Process 3.10 Silberschatz, Galvin and Gagne 2010

11 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 queue set of processes waiting for an I/O device A process migrates among the various queues throughout its lifetime 3.11 Silberschatz, Galvin and Gagne 2010

12 Ready Queue And Various I/O Device Queues 3.12 Silberschatz, Galvin and Gagne 2010

13 Representation of Process Scheduling Arrows indicate the flow of processes in the system Circles represent the resources that serve the queues Queueing-diagram representation of Process scheduling Each regular box represents a queue 3.13 Silberschatz, Galvin and Gagne 2010

14 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 3.14 Silberschatz, Galvin and Gagne 2010

15 Addition of Medium-Term Scheduling Using in some OS with no long-term scheduler Such as UNIX and MS Windows Swapping Medium-term scheduler can removes process from memory and thus reduce the degree of multiprogramming Later, the process can be reintroduced into memory, and its execution can be continued where it left off Silberschatz, Galvin and Gagne 2010

16 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 (the number of processes in memory) Processes can be described as either: I/O-bound process spends more time doing I/O than computations; If all processes are I/O bound, the ready queue will almost always be empty, and the short-term scheduler will have little to do. CPU-bound process spends more time doing computations; If all processes are CPU bound, the I/O waiting queue will almost always be empty, devices will go unused. The system with the best performance will thus have a combination of CPU-bound and I/O bound processes Silberschatz, Galvin and Gagne 2010

17 Context Switch When CPU switches to another process, the system must save the state of the current process and load the saved state for a different process via a context switch Context of a process is represented in the PCB of the process Context-switch time is overhead; the system does no useful work while switching Time is dependent on hardware support 3.17 Silberschatz, Galvin and Gagne 2010

18 Process Creation Parent process creates its children processes, which, may in turn create other processes, forming a tree of processes Generally, a process is identified and managed via a unique process identifier (or pid), which is typically an integer number. Resource sharing (such as CPU time, memory, files, I/O devices) Parent and children share all resources Children share subset of parent s resources Parent and child share no resources Execution when a process creates a new process Parent and children execute concurrently Parent waits until some or all children terminate 3.18 Silberschatz, Galvin and Gagne 2010

19 A tree of processes on a typical Solaris 3.19 Silberschatz, Galvin and Gagne 2010

20 Process Creation (Cont) Address space of the new process Child is a duplicate of the parent Child has a new program loaded into it UNIX examples fork system call creates a new process exec system call is used after a fork to replace the process s memory space with a new program 3.20 Silberschatz, Galvin and Gagne 2010

21 Process Creation Using fork() System Call 3.21 Silberschatz, Galvin and Gagne 2010

22 C Program Forking a Separate Process int main() { pid_t pid; /* fork a child 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 to complete */ wait (NULL); printf ("Child Complete"); } return 0; } Fork() returns pid = 0 for child and nonzero for the parent 3.22 Silberschatz, Galvin and Gagne 2010

23 Process Termination Process finishes executing its last statement and asks the operating system to delete it (exit system call) Return a status value (an integer) to the process parent (via wait) Process resources are deallocated by operating system Parent may terminate execution of children processes (abort) Child has exceeded the allocated resources Task assigned to child is no longer required The parent is exiting Some operating systems do not allow child to continue if its parent terminates All children are terminated cascading termination 3.23 Silberschatz, Galvin and Gagne 2010

24 Interprocess Communication Processes within a system may be independent or cooperating Independent process cannot affect or be affected by the other processes executing in the system. Cooperating process can affect or be affected by the other processes executing in the system, including sharing data. Reasons for cooperating processes: Information sharing Computation speedup Modularity Convenience Cooperating processes need interprocess communication (IPC) that will allow them to exchange data and information. Two models of IPC Shared memory Message passing 3.24 Silberschatz, Galvin and Gagne 2010

25 Communication Models message passing shared memory 3.25 Silberschatz, Galvin and Gagne 2010

26 INTERPROCESS COMMUNICATION : SHARED-MEMORY SYSTEMS 3.26 Silberschatz, Galvin and Gagne 2010

27 Producer-Consumer Problem A common paradigm for cooperating processes, producer process produces information that is consumed by a consumer process Must have a buffer of items that can be filled by the producer and emptied by the consumer. This buffer will reside in a region of memory that is shared by the producer and consumer processes. Two types of buffers can be used: Unbounded buffer places no practical limit on the size of the buffer Bounded buffer assumes that there is a fixed buffer size 3.27 Silberschatz, Galvin and Gagne 2010

28 Bounded Buffer Shared-Memory Solution Shared data: Variables reside in a region of memory shared by the producer and consumer processes #define BUFFER_SIZE 10 typedefstruct {... } item; item buffer[buffer_size]; int in = 0; int out = 0; Solution is correct, but can only use BUFFER_SIZE-1 elements in :points to the next free position in the buffer out: points to the first full position in the buffer buffer empty when in==out buffer full when ((in+1) % BUFFER_SIZE)== out) % is modulus operator Ex. 3 %10 = Silberschatz, Galvin and Gagne 2010

29 Bounded Buffer Producer Process } item nextproduced; /*a local variable in which while (true) { /* Produce an item in nextproduced */ the new item to be produced is stored.*/ while (((in + 1) % BUFFER_SIZE) == out) ; /*do nothing -- no free buffers*/ buffer[in] = nextproduced; in = (in + 1) % BUFFER_SIZE; in :points to the next free position in the buffer out: points to the first full position in the buffer buffer empty when in==out buffer full when ((in+1) % BUFFER_SIZE)== out) % is modulus operator Ex. 3 %10 = Silberschatz, Galvin and Gagne 2010

30 Bounded Buffer Consumer Process item nextconsumed; // a local variable in which the item to be consumed is stored. while (true) { while (in == out) ; // do nothing -- nothing to consume } nextconsumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; // consume the item in nextconsumed in :points to the next free position in the buffer out: points to the first full position in the buffer buffer empty when in==out 3.30 Silberschatz, Galvin and Gagne 2010

31 INTERPROCESS COMMUNICATION : MESSAGE-PASSING SYSTEMS 3.31 Silberschatz, Galvin and Gagne 2010

32 Message-Passing Systems Provides a mechanism for processes to communicate and to synchronize their actions without sharing the same address space. Message system processes communicate with each other without resorting to the shared variables A message-passing facility provides at least two operations: send(message) message size can be either fixed or variable receive(message) If processes 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, network) logical (e.g., logical properties) What we are concerned with Silberschatz, Galvin and Gagne 2010

33 Logical Implementation Logical implementation of a link and the send()/receive() operations may be done by several methods: Direct or indirect communication Synchronous or asynchronous communication Automatic or explicit buffering 3.33 Silberschatz, Galvin and Gagne 2010

34 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? 3.34 Silberschatz, Galvin and Gagne 2010

35 Direct Communication Processes must explicitly name the recipient or sender: 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 3.35 Silberschatz, Galvin and Gagne 2010

36 Indirect Communication Messages are sent to 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 is established only if processes have a shared mailbox A link may be associated with more than 2 processes Each pair of processes may have several communication links, with each link corresponding to one mailbox. Link may be unidirectional or bi-directional 3.36 Silberschatz, Galvin and Gagne 2010

37 Indirect Communication Operations that OS must allow a process to do: create a new mailbox send and receive messages through mailbox delete a mailbox Primitives are defined as: send(a, message) send a message to mailbox A receive(a, message) receive a message from mailbox A 3.37 Silberschatz, Galvin and Gagne 2010

38 Indirect Communication 3.38 Silberschatz, Galvin and Gagne 2010

39 Synchronization Message passing may be either blocking or nonblocking Blocking is known as synchronous Blocking send. The sending process is blocked until the message is received by the receiver or by the mailbox. Blocking receive. The receiver blocks until a message is available. Nonblocking is known as asynchronous Nonblocking send. The sending process sends the message and continues. Nonblocking receive. The receiver retrieves either a valid message or a null Silberschatz, Galvin and Gagne 2010

40 Buffering A temporary queue of communicating messages attached to the link can be implemented in one of the three following ways: 1. Zero capacity The queue has a maximum length of zero; thus, the link cannot have any messages waiting in it. Sender must block until the receiver receives the message. 2. Bounded capacity The queue has finite length of n messages. Sender must wait (block) if the link is full. 3. Unbounded capacity The queue has infinite length. Sender never blocks. finite: จ าก ด infinite: ไม จ าก ด 3.40 Silberschatz, Galvin and Gagne 2010

41 Examples of IPC Systems - POSIX POSIX Shared Memory Process first creates a 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); 3.41 Silberschatz, Galvin and Gagne 2010

42 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() 3.42 Silberschatz, Galvin and Gagne 2010

43 Examples of IPC Systems Windows XP The message-passing facility in Windows XP is called the local procedure call (LPC) facility Only works between processes on the same system Uses ports (or mailboxes) to establish and maintain communication channels between 2 processes The 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 3.43 Silberschatz, Galvin and Gagne 2010

44 Local Procedure Calls in Windows XP 3.44 Silberschatz, Galvin and Gagne 2010

45 Communications in Client-Server Systems Sockets Remote Procedure Calls (RPCs) Remote Method Invocation (Java) 3.45 Silberschatz, Galvin and Gagne 2010

46 Sockets A socket is defined as an endpoint for communication Concatenation of an IP address and a port number Ex. ftp port: 21, telnet port: 23, http port: 80 Standard service port-numbers are below 1024 The socket :1625 refers to port 1625 on host Communication consists between a pair of sockets 3.46 Silberschatz, Galvin and Gagne 2010

47 Socket Communication port: 1625 port: Silberschatz, Galvin and Gagne 2010

48 Remote Procedure Calls Remote procedure call (RPC) abstracts procedure calls between processes on networked systems Stub 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 3.48 Silberschatz, Galvin and Gagne 2010

49 Execution of RPC 3.49 Silberschatz, Galvin and Gagne 2010

50 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 3.50 Silberschatz, Galvin and Gagne 2010

51 Marshalling Parameters 3.51 Silberschatz, Galvin and Gagne 2010

52 End of Chapter 3, Silberschatz, Galvin and Gagne 2010