Chapter 3: Processes PDF

Chapter 3: Processes Operating System Concepts – 9th Edition Silberschatz, Galvin and Gagne ©2013  Chapter 3: Processes Process Concept Process Scheduling Operations on Processes Interprocess Communi...

Chapter 3: Processes Operating System Concepts – 9th Edition Silberschatz, Galvin and Gagne ©2013  Chapter 3: Processes Process Concept Process Scheduling Operations on Processes Interprocess Communication Examples of IPC Systems Communication in Client-Server Systems Operating System Concepts – 9th Edition 3.2 Silberschatz, Galvin and Gagne ©2013  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 explore interprocess communication using shared memory and message passing To describe communication in client-server systems Operating System Concepts – 9th Edition 3.3 Silberschatz, Galvin and Gagne ©2013  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 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 Operating System Concepts – 9th Edition 3.4 Silberschatz, Galvin and Gagne ©2013  Process Concept (Cont.) Program is passive entity stored on disk (executable file), 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 Operating System Concepts – 9th Edition 3.5 Silberschatz, Galvin and Gagne ©2013  Process in Memory Operating System Concepts – 9th Edition 3.6 Silberschatz, Galvin and Gagne ©2013  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 Operating System Concepts – 9th Edition 3.7 Silberschatz, Galvin and Gagne ©2013  Diagram of Process State Operating System Concepts – 9th Edition 3.8 Silberschatz, Galvin and Gagne ©2013  Process Control Block (PCB) Information associated with each process (also called task control block) Process state – running, waiting, etc Program counter – location of instruction to next execute CPU registers – contents of all process- centric registers CPU scheduling information- priorities, scheduling queue pointers Memory-management information – memory allocated to the process Accounting information – CPU used, clock time elapsed since start, time limits I/O status information – I/O devices allocated to process, list of open files Operating System Concepts – 9th Edition 3.9 Silberschatz, Galvin and Gagne ©2013  CPU Switch From Process to Process Operating System Concepts – 9th Edition 3.10 Silberschatz, Galvin and Gagne ©2013  Quiz Why : 1-Tertiary storage need not be fast. 2- Program is a passive entity while process is an activity entity. What are the properties of storage media? Fill: The cache memory managed by ________ 1- Compiler 2- Operating System 3- Hardware 4- CPU To execute any program all (or part)of the _________ and ________ must be in memory 1- CPU, registers 2- HD, SSD 3- Instructions, Data 4- Process Control Block Operating System Concepts – 9th Edition 3.11 Silberschatz, Galvin and Gagne ©2013  Quiz When is the execution of the program started? What are the three possibilities that the process can turn into after it is in a running state? Explain the CPU Switch From Process(P0) to Process (P1) Fill in the Planck: Stack containing temporary data. Example are : 1- -------------------- 2- ------------------- 3- -------------------- Which one of the following is not a process states: 1- New 2- Running 3- Task 4- Waiting Operating System Concepts – 9th Edition 3.12 Silberschatz, Galvin and Gagne ©2013  Threads So far, process has a single thread of execution Consider having multiple program counters per process Multiple locations can execute at once  Multiple threads of control -> threads Must then have storage for thread details, multiple program counters in PCB See next chapter Operating System Concepts – 9th Edition 3.13 Silberschatz, Galvin and Gagne ©2013  Process Representation in Linux Represented by the C structure task_struct pid t_pid; long state; unsigned int time_slice struct task_struct *parent; struct list_head children; struct files_struct *files; struct mm_struct *mm; Operating System Concepts – 9th Edition 3.14 Silberschatz, Galvin and Gagne ©2013  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 Operating System Concepts – 9th Edition 3.15 Silberschatz, Galvin and Gagne ©2013  Ready Queue And Various I/O Device Queues Operating System Concepts – 9th Edition 3.16 Silberschatz, Galvin and Gagne ©2013  Representation of Process Scheduling Queueing diagram represents queues, resources, flows Operating System Concepts – 9th Edition 3.17 Silberschatz, Galvin and Gagne ©2013  Schedulers Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU Sometimes the only scheduler in a system Short-term scheduler is invoked frequently (milliseconds)  (must be fast) Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue Long-term scheduler is invoked 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 Long-term scheduler strives for good process mix Operating System Concepts – 9th Edition 3.18 Silberschatz, Galvin and Gagne ©2013  Addition of Medium Term Scheduling Medium-term scheduler can be added if degree of multiple programming needs to decrease Remove process from memory, store on disk, bring back in from disk to continue execution: swapping Operating System Concepts – 9th Edition 3.19 Silberschatz, Galvin and Gagne ©2013  Multitasking in Mobile Systems Some mobile systems (e.g., early version of iOS) allow only one process to run, others suspended Due to screen real estate, user interface limits iOS provides for a Single foreground process- controlled via user interface Multiple background processes– in memory, running, but not on the display, and with limits Limits include single, short task, receiving notification of events, specific long-running tasks like audio playback Android runs foreground and background, with fewer limits Background process uses a service to perform tasks Service can keep running even if background process is suspended Service has no user interface, small memory use Operating System Concepts – 9th Edition 3.20 Silberschatz, Galvin and Gagne ©2013  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 ➔ the longer the context switch Time dependent on hardware support Some hardware provides multiple sets of registers per CPU ➔ multiple contexts loaded at once Operating System Concepts – 9th Edition 3.21 Silberschatz, Galvin and Gagne ©2013  Operations on Processes System must provide mechanisms for: process creation, process termination, and so on as detailed next Operating System Concepts – 9th Edition 3.22 Silberschatz, Galvin and Gagne ©2013  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 options Parent and children share all resources Children share subset of parent’s resources Parent and child share no resources Execution options Parent and children execute concurrently Parent waits until children terminate Operating System Concepts – 9th Edition 3.23 Silberschatz, Galvin and Gagne ©2013  A Tree of Processes in Linux init pid = 1 login kthreadd sshd pid = 8415 pid = 2 pid = 3028 bash khelper pdflush sshd pid = 8416 pid = 6 pid = 200 pid = 3610 emacs tcsch ps pid = 9204 pid = 4005 pid = 9298 Operating System Concepts – 9th Edition 3.24 Silberschatz, Galvin and Gagne ©2013  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 Operating System Concepts – 9th Edition 3.25 Silberschatz, Galvin and Gagne ©2013  C Program Forking Separate Process Operating System Concepts – 9th Edition 3.26 Silberschatz, Galvin and Gagne ©2013  Creating a Separate Process via Windows API Operating System Concepts – 9th Edition 3.27 Silberschatz, Galvin and Gagne ©2013  Process Termination Process executes last statement and then asks the operating system to delete it using the exit() system call. Returns status data from child to parent (via wait()) Process’ resources are deallocated by operating system Parent may terminate the execution of children processes using the abort() system call. Some reasons for doing so: Child has exceeded allocated resources Task assigned to child is no longer required The parent is exiting and the operating systems does not allow a child to continue if its parent terminates Operating System Concepts – 9th Edition 3.28 Silberschatz, Galvin and Gagne ©2013  Process Termination Some operating systems do not allow child to exists if its parent has terminated. If a process terminates, then all its children must also be terminated. cascading termination. All children, grandchildren, etc. are terminated. The termination is initiated by the operating system. The parent process may wait for termination of a child process by using the wait()system call. The call returns status information and the pid of the terminated process pid = wait(&status); If no parent waiting (did not invoke wait()) process is a zombie If parent terminated without invoking wait , process is an orphan Operating System Concepts – 9th Edition 3.29 Silberschatz, Galvin and Gagne ©2013  Multiprocess Architecture – Chrome Browser Many web browsers ran as single process (some still do) If one web site causes trouble, entire browser can hang or crash Google Chrome Browser is multiprocess with 3 different types of processes: Browser process manages user interface, disk and network I/O Renderer process renders web pages, deals with HTML, Javascript. A new renderer created for each website opened  Runs in sandbox restricting disk and network I/O, minimizing effect of security exploits Plug-in process for each type of plug-in Operating System Concepts – 9th Edition 3.30 Silberschatz, Galvin and Gagne ©2013  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 Operating System Concepts – 9th Edition 3.31 Silberschatz, Galvin and Gagne ©2013  Communications Models (a) Message passing. (b) shared memory. Operating System Concepts – 9th Edition 3.32 Silberschatz, Galvin and Gagne ©2013  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 Operating System Concepts – 9th Edition 3.33 Silberschatz, Galvin and Gagne ©2013  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 Operating System Concepts – 9th Edition 3.34 Silberschatz, Galvin and Gagne ©2013  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 Operating System Concepts – 9th Edition 3.35 Silberschatz, Galvin and Gagne ©2013  Bounded-Buffer – Producer item next_produced; while (true) { while (((in + 1) % BUFFER_SIZE) == out) ; buffer[in] = next_produced; in = (in + 1) % BUFFER_SIZE; } Operating System Concepts – 9th Edition 3.36 Silberschatz, Galvin and Gagne ©2013  Bounded Buffer – Consumer item next_consumed; while (true) { while (in == out) ; next_consumed = buffer[out]; out = (out + 1) % BUFFER_SIZE; } Operating System Concepts – 9th Edition 3.37 Silberschatz, Galvin and Gagne ©2013  Interprocess Communication – Shared Memory An area of memory shared among the processes that wish to communicate The communication is under the control of the users processes not the operating system. Major issues is to provide mechanism that will allow the user processes to synchronize their actions when they access shared memory. Synchronization is discussed in great details in Chapter 5. Operating System Concepts – 9th Edition 3.38 Silberschatz, Galvin and Gagne ©2013  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) receive(message) The message size is either fixed or variable Operating System Concepts – 9th Edition 3.39 Silberschatz, Galvin and Gagne ©2013  Message Passing (Cont.) If processes P and Q wish to communicate, they need to: Establish a communication link between them Exchange messages via send/receive Implementation issues: 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? Operating System Concepts – 9th Edition 3.40 Silberschatz, Galvin and Gagne ©2013  Message Passing (Cont.) Implementation of communication link Physical:  Shared memory  Hardware bus  Network Logical:  Direct or indirect  Synchronous or asynchronous  Automatic or explicit buffering Operating System Concepts – 9th Edition 3.41 Silberschatz, Galvin and Gagne ©2013  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 Operating System Concepts – 9th Edition 3.42 Silberschatz, Galvin and Gagne ©2013  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 Operating System Concepts – 9th Edition 3.43 Silberschatz, Galvin and Gagne ©2013  Indirect Communication Operations create a new mailbox (port) 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 Operating System Concepts – 9th Edition 3.44 Silberschatz, Galvin and Gagne ©2013  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. Operating System Concepts – 9th Edition 3.45 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9th Edition 3.46 Silberschatz, Galvin and Gagne ©2013  Synchronization Message passing may be either blocking or non-blocking Blocking is considered synchronous Blocking send -- the sender is blocked until the message is received Blocking receive -- the receiver is blocked until a message is available Non-blocking is considered asynchronous Non-blocking send -- the sender sends the message and continue Non-blocking receive -- the receiver receives: A valid message, or Null message Different combinations possible If both send and receive are blocking, we have a rendezvous Operating System Concepts – 9th Edition 3.47 Silberschatz, Galvin and Gagne ©2013  Synchronization (Cont.) Producer-consumer becomes trivial message next_produced; while (true) { send(next_produced); } message next_consumed; while (true) { receive(next_consumed); } Operating System Concepts – 9th Edition 3.48 Silberschatz, Galvin and Gagne ©2013  Buffering Queue of messages attached to the link. implemented in one of three ways 1. Zero capacity – no messages are queued on a link. 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 Operating System Concepts – 9th Edition 3.49 Silberschatz, Galvin and Gagne ©2013  Examples of IPC Systems - POSIX POSIX Shared Memory Process first creates shared memory segment shm_fd = shm_open(name, O CREAT | O RDWR, 0666); Also used to open an existing segment to share it Set the size of the object ftruncate(shm fd, 4096); Now the process could write to the shared memory sprintf(shared memory, "Writing to shared memory"); Operating System Concepts – 9th Edition 3.50 Silberschatz, Galvin and Gagne ©2013