Chapter 3: Processes PDF

Chapter 3: Processes Operating System Concepts Silberschatz, Galvin and Gagne  Chapter 3: Processes Process Concept Process Scheduling Operations on Processes Inter-process Comm...

Chapter 3: Processes Operating System Concepts Silberschatz, Galvin and Gagne  Chapter 3: Processes Process Concept Process Scheduling Operations on Processes Inter-process Communication Example of IPC System Communication in Client-Server Systems Operating System Concepts 3.2 Silberschatz, Galvin and Gagne  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 3.3 Silberschatz, Galvin and Gagne  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 3.4 Silberschatz, Galvin and Gagne  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 3.5 Silberschatz, Galvin and Gagne  Diagram of Process State Operating System Concepts 3.6 Silberschatz, Galvin and Gagne  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 3.7 Silberschatz, Galvin and Gagne  CPU Switch From Process to Process Operating System Concepts 3.8 Silberschatz, Galvin and Gagne  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 3.9 Silberschatz, Galvin and Gagne  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 3.10 Silberschatz, Galvin and Gagne  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 3.11 Silberschatz, Galvin and Gagne  Operations on Processes System must provide mechanisms for: 1. process creation 2. process termination Operating System Concepts 3.12 Silberschatz, Galvin and Gagne  1. 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 3.13 Silberschatz, Galvin and Gagne  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 3.14 Silberschatz, Galvin and Gagne  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 3.15 Silberschatz, Galvin and Gagne  2. 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 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. Operating System Concepts 3.16 Silberschatz, Galvin and Gagne  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 3.17 Silberschatz, Galvin and Gagne  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 3.18 Silberschatz, Galvin and Gagne  Communications Models (a) Message passing. (b) shared memory. Operating System Concepts 3.19 Silberschatz, Galvin and Gagne  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 If processes P and Q wish to communicate, they need to: Establish a communication link between them Exchange messages via send/receive Operating System Concepts 3.20 Silberschatz, Galvin and Gagne  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. Operating System Concepts 3.21 Silberschatz, Galvin and Gagne  Example of IPC System – Windows Message-passing centric via advanced local procedure call (LPC) facility Only works between processes on the same system Uses ports (like mailboxes) to establish and maintain communication channels 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. Operating System Concepts 3.22 Silberschatz, Galvin and Gagne  Communications in Client-Server Systems 1. Sockets 2. Remote Procedure Calls 3. Pipes Operating System Concepts 3.23 Silberschatz, Galvin and Gagne  1. Sockets A socket is defined as an endpoint for communication Concatenation of IP address and port – a number included at start of message packet to differentiate network services on a host The socket 161.25.19.8:1625 refers to port 1625 on host 161.25.19.8 Communication consists between a pair of sockets All ports below 1024 are well known, used for standard services Special IP address 127.0.0.1 (loopback) to refer to system on which process is running Operating System Concepts 3.24 Silberschatz, Galvin and Gagne  Socket Communication Operating System Concepts 3.25 Silberschatz, Galvin and Gagne  2. Remote Procedure Calls Remote procedure call (RPC) abstracts procedure calls between processes on networked systems Again uses ports for service differentiation Stubs – 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 On Windows, stub code compile from specification written in Microsoft Interface Definition Language (MIDL) Operating System Concepts 3.26 Silberschatz, Galvin and Gagne  3. Pipes Acts as a conduit allowing two processes to communicate Issues: Is communication unidirectional or bidirectional? In the case of two-way communication, is it half or full-duplex? Must there exist a relationship (i.e., parent-child) between the communicating processes? Can the pipes be used over a network? Ordinary pipes Cannot be accessed from outside the process that created it. Typically, a parent process creates a pipe and uses it to communicate with a child process that it created. Named pipes Can be accessed without a parent-child relationship. Operating System Concepts 3.27 Silberschatz, Galvin and Gagne  Ordinary Pipes ▪ Unidirectional communication in standard producer-consumer style ▪ Producer writes to one end (the write-end of the pipe) ▪ Consumer reads from the other end (the read-end of the pipe) ▪ Require parent-child relationship between communicating processes ▪ Windows calls these anonymous pipes Named Pipes ▪ Named Pipes are more powerful than ordinary pipes ▪ Communication is bidirectional ▪ No parent-child relationship is necessary for communicating processes ▪ Provided on both UNIX and Windows systems Operating System Concepts 3.28 Silberschatz, Galvin and Gagne  Chapter 4: Threads Operating System Concepts Silberschatz, Galvin and Gagne  Chapter 4: Threads Introduction Multicore Programming Multithreading Models Thread Libraries Implicit Threading Threading Issues Examples Operating System Concepts 3.30 Silberschatz, Galvin and Gagne  Introduction Most modern applications are multithreaded Threads run within application Multiple tasks with the application can be implemented by separate threads Update display Fetch data Spell checking Answer a network request Process creation is heavy-weight while thread creation is light-weight Can simplify code, increase efficiency Kernels are generally multithreaded Operating System Concepts 3.31 Silberschatz, Galvin and Gagne  Multithreaded Server Architecture Operating System Concepts 3.32 Silberschatz, Galvin and Gagne  Benefits Responsiveness – may allow continued execution if part of process is blocked, especially important for user interfaces Resource Sharing – threads share resources of process, easier than shared memory or message passing Economy – cheaper than process creation, thread switching lower overhead than context switching Scalability – process can take advantage of multiprocessor architectures Operating System Concepts 3.33 Silberschatz, Galvin and Gagne  Multicore Programming Multicore or multiprocessor systems putting pressure on programmers, challenges include: Dividing activities Balance Data splitting Data dependency Testing and debugging Parallelism implies a system can perform more than one task simultaneously Concurrency supports more than one task making progress Single processor / core, scheduler providing concurrency Operating System Concepts 3.34 Silberschatz, Galvin and Gagne  Multicore Programming (Cont.) Types of parallelism Data parallelism – distributes subsets of the same data across multiple cores, same operation on each Task parallelism – distributing threads across cores, each thread performing unique operation As # of threads grows, so does architectural support for threading CPUs have cores as well as hardware threads Consider Oracle SPARC T4 with 8 cores, and 8 hardware threads per core Operating System Concepts 3.35 Silberschatz, Galvin and Gagne  Concurrency vs. Parallelism Concurrent execution on single-core system: Parallelism on a multi-core system: Operating System Concepts 3.36 Silberschatz, Galvin and Gagne  Single and Multithreaded Processes Operating System Concepts 3.37 Silberschatz, Galvin and Gagne  User Threads and Kernel Threads User threads - management done by user-level threads library Three primary thread libraries: POSIX Pthreads Windows threads Java threads Kernel threads - Supported by the Kernel Examples – virtually all general-purpose operating systems, including: Windows Solaris Linux Tru64 UNIX Mac OS X Operating System Concepts 3.38 Silberschatz, Galvin and Gagne  Multithreading Models Many-to-One One-to-One Many-to-Many Two-Level Operating System Concepts 3.39 Silberschatz, Galvin and Gagne  Many-to-One Many user-level threads mapped to single kernel thread One thread blocking causes all to block Multiple threads may not run in parallel on muticore system because only one may be in kernel at a time Few systems currently use this model Examples: Solaris Green Threads GNU Portable Threads Operating System Concepts 3.40 Silberschatz, Galvin and Gagne  One-to-One Each user-level thread maps to kernel thread Creating a user-level thread creates a kernel thread More concurrency than many-to-one Number of threads per process sometimes restricted due to overhead Examples: Windows Linux Solaris 9 and later Operating System Concepts 3.41 Silberschatz, Galvin and Gagne  Many-to-Many Model Allows many user level threads to be mapped to many kernel threads Allows the operating system to create a sufficient number of kernel threads Examples: Solaris prior to version 9 Windows with the ThreadFiber package Operating System Concepts 3.42 Silberschatz, Galvin and Gagne  Two-level Model Similar to M:M, except that it allows a user thread to be bound to kernel thread Examples: IRIX HP-UX Tru64 UNIX Solaris 8 and earlier Operating System Concepts 3.43 Silberschatz, Galvin and Gagne  Thread Libraries Thread library provides programmer with API for creating and managing threads Two primary ways of implementing Library entirely in user space Kernel-level library supported by the OS Pthreads May be provided either as user-level or kernel-level A POSIX standard (IEEE 1003.1c) API for thread creation and synchronization It is Specification, not implementation API specifies behavior of the thread library, implementation is up to development of the library Common in UNIX operating systems (Solaris, Linux, Mac OS X) Operating System Concepts 3.44 Silberschatz, Galvin and Gagne  Java Threads Java threads are managed by the JVM Typically implemented using the threads model provided by underlying OS Java threads may be created by: Extending Thread class Implementing the Runnable interface Operating System Concepts 3.45 Silberschatz, Galvin and Gagne  Implicit Threading Growing in popularity as numbers of threads increase, program correctness more difficult with explicit threads Creation and management of threads done by compilers and run-time libraries rather than programmers Two methods are explored Thread Pools Grand Central Dispatch Operating System Concepts 3.46 Silberschatz, Galvin and Gagne  Thread Pools Create a number of threads in a pool where they await work Advantages: Usually slightly faster to service a request with an existing thread than create a new thread Allows the number of threads in the application(s) to be bound to the size of the pool Separating task to be performed from mechanics of creating task allows different strategies for running task  i.e.Tasks could be scheduled to run periodically Windows API supports thread pools Operating System Concepts 3.47 Silberschatz, Galvin and Gagne  Grand Central Dispatch Apple technology for Mac OS X and iOS operating systems Extensions to C, C++ languages, API, and run-time library Allows identification of parallel sections Manages most of the details of threading Blocks placed in dispatch queue Assigned to available thread in thread pool when removed from queue Operating System Concepts 3.48 Silberschatz, Galvin and Gagne  Threading Issues Semantics of fork() and exec() system calls Signal handling Synchronous and asynchronous Thread cancellation of target thread Asynchronous or deferred Thread-local storage Scheduler Activations Operating System Concepts 3.49 Silberschatz, Galvin and Gagne  Signal Handling ▪ Signals are used in UNIX systems to notify a process that a particular event has occurred. ▪ A signal handler is used to process signals 1. Signal is generated by particular event 2. Signal is delivered to a process 3. Signal is handled by one of two signal handlers: 1. default 2. user-defined ▪ Every signal has default handler that kernel runs when handling signal ▪ User-defined signal handler can override default ▪ For single-threaded, signal delivered to process Operating System Concepts 3.50 Silberschatz, Galvin and Gagne  Signal Handling (Cont.) n Where should a signal be delivered for multi-threaded? l Deliver the signal to the thread to which the signal applies l Deliver the signal to every thread in the process l Deliver the signal to certain threads in the process l Assign a specific thread to receive all signals for the process Operating System Concepts 3.51 Silberschatz, Galvin and Gagne  Thread Cancellation Terminating a thread before it has finished Thread to be canceled is target thread Two general approaches: Asynchronous cancellation terminates the target thread immediately Deferred cancellation allows the target thread to periodically check if it should be cancelled Operating System Concepts 3.52 Silberschatz, Galvin and Gagne  Scheduler Activations Both M:M and Two-level models require communication to maintain the appropriate number of kernel threads allocated to the application Typically use an intermediate data structure between user and kernel threads – lightweight process (LWP) Appears to be a virtual processor on which process can schedule user thread to run Each LWP attached to kernel thread How many LWPs to create? Scheduler activations provide upcalls - a communication mechanism from the kernel to the upcall handler in the thread library This communication allows an application to maintain the correct number kernel threads Operating System Concepts 3.53 Silberschatz, Galvin and Gagne  Example 1: Windows Threads Windows implements the Windows API – primary API for Win 98, Win NT, Win 2000, Win XP, and Win 7 Implements the one-to-one mapping, kernel-level Each thread contains A thread id Register set representing state of processor Separate user and kernel stacks for when thread runs in user mode or kernel mode Private data storage area used by run-time libraries and dynamic link libraries (DLLs) The register set, stacks, and private storage area are known as the context of the thread Operating System Concepts 3.54 Silberschatz, Galvin and Gagne  Example 2: Linux Threads Linux refers to them as tasks rather than threads Thread creation is done through clone() system call clone() allows a child task to share the address space of the parent task (process) Flags control behavior Operating System Concepts 3.55 Silberschatz, Galvin and Gagne