Processes OS PDF

Chapter 3: Processes Operating System Concepts – 9th Edition 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 4 Function parameters, return addresses, local variables Data section containing global variables Heap containing memory dynamically allocated during run time Operating System Concepts – 9th Edition 3.2 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 3.3 Silberschatz, Galvin and Gagne ©2013 Process in Memory Operating System Concepts – 9th Edition 3.4 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 Certain operating systems also more ﬁnely delineate process states. It is important to realize that only one process can be running on any processor at any instant. Many processes may be ready and waiting, Operating System Concepts – 9th Edition 3.5 Silberschatz, Galvin and Gagne ©2013 Diagram of Process State Operating System Concepts – 9th Edition 3.6 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 3.7 Silberschatz, Galvin and Gagne ©2013 CPU Switch From Process to Process Operating System Concepts – 9th Edition 3.8 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 4 Multiple threads of control -> threads Must then have storage for thread details, multiple program counters in PCB. Most modern operating systems have extended the process concept to allow a process to have multiple threads of execution and thus to perform more than one task at a time. This feature is especially beneﬁcial on multicore systems, where multiple threads can run in parallel. On a system that supports threads, the PCB is expanded to include information for each thread. Other changes throughout the system are also needed to support threads. Threads can be created both in user and kernel modes, but process can be created only in kennel mode. Operating System Concepts – 9th Edition 3.9 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 3.10 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 3.11 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 3.12 Silberschatz, Galvin and Gagne ©2013 Ready Queue And Various I/O Device Queues A new process is initially put in the ready queue. It waits there until it is selected for execution, or Once the process is allocated the CPU and is executing, one of several events could occur: The process could issue an I/O request and then be placed in an I/O queue. The process could create a new child process and wait for the child’s termination. The process could be removed forcibly from the CPU, as a result of an interrupt, and be put back in the ready queue. Operating System Concepts – 9th Edition 3.13 Silberschatz, Galvin and Gagne ©2013 Representation of Process Scheduling Queueing diagram represents queues, resources, flows Operating System Concepts – 9th Edition 3.14 Silberschatz, Galvin and Gagne ©2013 Schedulers A process migrates among the various scheduling queues throughout its lifetime. The operating system must select, for scheduling purposes, processes from these queues in some fashion. The selection process is carried out by the appropriate scheduler. Often, in a batch system, more processes are submitted than can be executed immediately. These processes are spooled to a mass-storage device (typically a disk), where they are kept for later execution. Operating System Concepts – 9th Edition 3.15 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 The primary distinction between these two schedulers lies in frequency of execution. The short-term scheduler must select a new process for the CPU frequently. A process may execute for only a few milliseconds before waiting for an I/O request. Often, the short-term scheduler executes at least once every 100 milliseconds. Because of the short time between executions, the short-term scheduler must be fast. If it takes 10 milliseconds to decide to execute a process for 100 milliseconds, then 10/(100 + 10) = 9 percent of the CPU is being used (wasted) simply for scheduling the work. Operating System Concepts – 9th Edition 3.16 Silberschatz, Galvin and Gagne ©2013 Schedulers 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. On some systems, the long-term scheduler may be absent or minimal. For example, time-sharing systems such as UNIX and Microsoft Windows systems often have no long-term scheduler but simply put every new process in memory for the short-term scheduler. The stability of these systems depends either on a physical limitation (such as the number of available terminals) or on the self-adjusting nature of human users. If performance declines to unacceptable levels on a multiuser system, some users will simply quit. Operating System Concepts – 9th Edition 3.17 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 3.18 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 3.19 Silberschatz, Galvin and Gagne ©2013 Context Switch Interrupts cause the operating system to change a CPU from its current task and to run a kernel routine. Such operations happen frequently on general-purpose systems. When an interrupt occurs, the system needs to save the current context of the process running on the CPU so that it can restore that context when its processing is done, essentially suspending the process and then resuming it. The context is represented in the PCB of the process. It includes the value of the CPU registers, the process and memory-management information state 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 During the course of execution, a process may create several new processes. As mentioned earlier, the creating process is called a parent process, and the new processes are called the children of that process. Most operating systems (including UNIX, Linux, and Windows) identify processes according to a unique process identiﬁer (or pid), which is typically an integer number. The pid provides a unique value for each process in the system, and it can be used as an index to access various attributes of a process within the kernel. Each of these new processes may in turn create other processes, forming a tree of 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 A Tree of Processes in Linux Operating System Concepts – 9th Edition 3.23 Silberschatz, Galvin and Gagne ©2013 Operations on Processes The init process (which always has a pid of 1) serves as the root parent process for all user processes. Once the system has booted, the init process can also create various user processes, such as a web or print server, an ssh server, and the like. The kthreadd process is responsible for creating additional processes that perform tasks on behalf of the kernel (in this situation, khelper and pdflush). The sshd process is responsible for managing clients that connect to the system by using ssh (which is short for secure shell). The login process is responsible for managing clients that directly log onto the system. In this example, a client has logged on and is using the bash shell, which has been assigned pid 8416. Using the bash command-line interface, this user has created the process ps as well as the emacs editor. On UNIX and Linux systems, we can obtain a listing of processes by using the ps command. For example, the command ps -el Operating System Concepts – 9th Edition 3.24 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 the parent process may pass along initialization data (input) to the child process. Execution options Parent and children execute concurrently Parent waits until children terminate Operating System Concepts – 9th Edition 3.25 Silberschatz, Galvin and Gagne ©2013 Process Creation In general, when a process creates a child process, that child process will need certain resources (CPU time, memory, ﬁles, I/O devices) to accomplish its task. A child process may be able to obtain its resources directly from the operating system, or it may be constrained to a subset of the resources of the parent process. The parent may have to partition its resources among its children, or it may be able to share some resources (such as memory or ﬁles) among several of its children. Restricting a child process to a subset of the parent’s resources prevents any process from overloading the system by creating too many child processes. Operating System Concepts – 9th Edition 3.26 Silberschatz, Galvin and Gagne ©2013 Process Creation When a process creates a new process, two possibilities for execution exist: 1. The parent continues to execute concurrently with its children. 2. The parent waits until some or all of its children have terminated. There are also two address-space possibilities for the new process: 1. The child process is a duplicate of the parent process (it has the same program and data as the parent). 2. The child process has a new program loaded into it. Both processes (the parent and the child) continue execution at the instruction after the fork(),with one difference: the return code for the fork() is zero for the new (child) process, whereas the (nonzero) process identiﬁer of the child is returned to the parent. Operating System Concepts – 9th Edition 3.27 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 3.28 Silberschatz, Galvin and Gagne ©2013 C Program Forking Separate Process Operating System Concepts – 9th Edition 3.29 Silberschatz, Galvin and Gagne ©2013 Process Creation (Cont.) After a fork() system call, one of the two processes typically uses the exec() system call to replace the process’s memory space with a new program. The exec() system call loads a binary ﬁle into memory (destroying the memory image of the program containing the exec() system call) and starts its execution. In this manner, the two processes are able to communicate and then go their separate ways. The parent can then create more children; or, if it has nothing else to do while the child runs, it can issue a wait() system call to move itself off the ready queue until the termination of the child. Operating System Concepts – 9th Edition 3.30 Silberschatz, Galvin and Gagne ©2013 Creating a Separate Process via Windows API Operating System Concepts – 9th Edition 3.31 Silberschatz, Galvin and Gagne ©2013 Process Processes are created in the Windows API using the CreateProcess() function, which is similar to fork() in that a parent creates a new child process. However, whereas fork() has the child process inheriting the address space of its parent, CreateProcess() requires loading a speciﬁed program into the address space of the child process at process creation. Furthermore, whereas fork() is passed no parameters, CreateProcess() expects no fewer than ten parameters. CreateProcess() function, which creates a child process that loads the application mspaint.exe. We opt for many of the default values of the ten par ameters passed to CreateProcess(). The two parameters passed to the CreateProcess() function are instances of the STARTUPINFO and PROCESS INFORMATION structures. STARTUPINFO speciﬁes many properties of the new process, such as window size and appearance and handles to standard input and output ﬁles. The PROCESS INFORMATION structure contains a handle and the identiﬁers to the newly created process and its thread. We invoke the ZeroMemory() function to allocate memory for each of these structures before proceeding with CreateProcess(). Operating System Concepts – 9th Edition 3.32 Silberschatz, Galvin and Gagne ©2013 Process Termination A process terminates when it ﬁnishes executing its ﬁnal statement and asks the operating system to delete it by using the exit() system call. At that point, the process may return a status value (typically an integer) to its parent process (via the wait() system call). All the resources of the process—including physical and virtual memory, open ﬁles, and I/O buffers—are deallocated by the operating system. 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 3.33 Silberschatz, Galvin and Gagne ©2013 Process Termination A process can cause the termination of another process via an appropriate system call (for example, TerminateProcess() in Windows). 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 3.34 Silberschatz, Galvin and Gagne ©2013 Interprocess Communication Processes within a system may be independent or cooperating 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 Cooperating processes need interprocess communication (IPC) Two models of IPC Shared memory Message passing Operating System Concepts – 9th Edition 3.35 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 4 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 3.36 Silberschatz, Galvin and Gagne ©2013 Communications Models (a) Message passing. (b) shared memory. Operating System Concepts – 9th Edition 3.37 Silberschatz, Galvin and Gagne ©2013 IPC Both of the models just mentioned are common in operating systems, and many systems implement both. Message passing is useful for exchanging smaller amounts of data, because no conﬂicts need be avoided. Message passing is also easier to implement in a distributed system than shared memory. Shared memory can be faster than message passing, since message-passing systems are typically implemented using system calls and thus require the more time-consuming task of kernel intervention. In shared-memory systems, system calls are required only to establish shared-memory regions Once shared memory is established, all accesses are treated as routine memory accesses, and no assistance from the kernel is required. Operating System Concepts – 9th Edition 3.38 Silberschatz, Galvin and Gagne ©2013 IPC – Shared Memory System Establish a region of shared memory. Typically, a shared-memory region resides in the address space of the process creating the shared-memory segment. Other processes that wish to communicate using this shared-memory segment must attach it to their address space. Recall that, normally, the operating system tries to prevent one process from accessing another process’s memory. Shared memory requires that two or more processes agree to remove this restriction. They can then exchange information by reading and writing data in the shared areas. The form of the data and the location are determined by these processes and are not under the operating system’s control. The processes are also responsible for ensuring that they are not writing to the same location simultaneously. Major issues is to provide mechanism that will allow the user processes to synchronize their actions when they access shared memory. Operating System Concepts – 9th Edition 3.39 Silberschatz, Galvin and Gagne ©2013 IPC – Shared Memory System What the Problem? Producer Process should not produce any data when the shared buffer is full. Consumer Process should not consume any data when the shared buffer is empty. The access to the shared buffer should be mutually exclusive i.e at a time only one process should be able to access the shared buffer and make changes to it. Operating System Concepts – 9th Edition 3.40 Silberschatz, Galvin and Gagne ©2013 Producer-Consumer Problem One solution to the producer–consumer problem uses shared memory. To allow producer and consumer processes to run concurrently, we must have available 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. A producer can produce one item while the consumer is consuming another item. The producer and consumer must be synchronized, so that the consumer does not try to consume an item that has not yet been produced. 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 3.41 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 3.42 Silberschatz, Galvin and Gagne ©2013 Producer-Consumer Problem The shared buffer is implemented as a circular array with two logical pointers: in and out. The variable in points to the next free position in the buffer; Out points to the ﬁrst full position in the buffer. The buffer is empty when in == out; The buffer is full when ((in +1)%BUFFERSIZE)==out One issue this illustration does not address concerns the situation in which both the producer process and the consumer process attempt to access the shared buffer concurrently. Operating System Concepts – 9th Edition 3.43 Silberschatz, Galvin and Gagne ©2013 Bounded-Buffer – Producer item next_produced; while (true) { // check if there is no space // for production. // if so keep waiting. while (((in + 1) % BUFFER_SIZE) == out) ; buffer[in] = next_produced; in = (in + 1) % BUFFER_SIZE; } Operating System Concepts – 9th Edition 3.44 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 3.45 Silberschatz, Galvin and Gagne ©2013 Interprocess Communication – Message Passing Message passing provides a mechanism to allow processes to communicate and to synchronize their actions without sharing the same address space. It is particularly useful in a distributed environment, where the communicating processes may reside on different computers connected by a network. 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 If only ﬁxed-sized messages can be sent, the system-level implementation is straight-forward. This restriction, however, makes the task of programming more difﬁcult. Conversely, variable-sized messages require a more complex system- level implementation, but the programming task becomes simpler. This is a common kind of tradeoff seen throughout operating-system design. Operating System Concepts – 9th Edition 3.46 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 3.47 Silberschatz, Galvin and Gagne ©2013 Message Passing (Cont.) Implementation of communication link Physical: 4 Shared memory 4 Hardware bus 4 Network Logical: 4 Direct or indirect 4 Synchronous or asynchronous 4 Automatic or explicit buffering Operating System Concepts – 9th Edition 3.48 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 4 This approach is called Symmetry in addressing 4 In Asymmetry, only the sender names the recipient; the recipient is not required to name the sender 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 3.49 Silberschatz, Galvin and Gagne ©2013 Direct Communication The disadvantage in both of these schemes (symmetric and asymmetric) is the limited modularity of the resulting process deﬁnitions. Changing the identiﬁer of a process may necessitate examining all other process deﬁnitions. All references to the old identiﬁer must be found, so that they can be modiﬁed to the new identiﬁer. In general, any such hard-coding techniques, where identiﬁers must be explicitly stated, are less desirable than techniques involving indirection Operating System Concepts – 9th Edition 3.50 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 3.51 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 3.52 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. A mailbox may be owned either by a process or by the operating system Process: distinguish between owner and user Operating System: Not attached to any processes 4 create a new mailbox (port) 4 send and receive messages through mailbox 4 destroy a mailbox Operating System Concepts – 9th Edition 3.53 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 3.54 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 3.55 Silberschatz, Galvin and Gagne ©2013 Buffering Whether communication is direct or indirect, messages exchanged by communicating processes reside in a temporary queue. Queue of messages attached to the link. Basically, such queues can be implemented in 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 The zero-capacity case is sometimes referred to as a message system with no buffering. The other cases are referred to as systems with automatic buffering. Operating System Concepts – 9th Edition 3.56 Silberschatz, Galvin and Gagne ©2013

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