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Chapter 3: Processes Operating System Concepts – 9th Edition Silberschatz, Galvin and Gagne ©2013 Objectives To introduce the notion of a process To describe process scheduling, creation, 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 3.2 Silberschatz, Galvin and Gagne ©2013 OS Major Functions Interleave the execution of multiple processes, to maximize processor utilization while providing reasonable response time Allocate resources to processes Support interprocess communication and user creation of processes Operating System Concepts – 9th Edition 3.3 Silberschatz, Galvin and Gagne ©2013 Process A program in execution An instance of a program running on a computer The entity that can be assigned to and executed on a processor A unit of activity characterized by the execution of a sequence of instructions a current state an associated set of system resources Operating System Concepts – 9th Edition 3.4 Silberschatz, Galvin and Gagne ©2013 Process Concept Multiple parts The program code, also called text section Current activity context 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 3.5 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 How to execute a program? Execution of a program starts via GUI mouse click, command line entry of its name, etc. Can we have one program in several processes? Consider multiple users executing the same program on cse.unl.edu server Operating System Concepts – 9th Edition 3.6 Silberschatz, Galvin and Gagne ©2013 Process in Memory Operating System Concepts – 9th Edition 3.7 Silberschatz, Galvin and Gagne ©2013 Process Stack Parameter passing in a read procedure call: Figure (a) The stack before the call to read. (b) The stack while the called procedure is active. Operating System Concepts – 9th Edition 3.8 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 e.g., a a completion of an I/O operation or semaphore signal from another process ready: The process is waiting to be assigned to a processor terminated: The process has finished execution Operating System Concepts – 9th Edition 3.9 Silberschatz, Galvin and Gagne ©2013 Diagram of Process State Operating System Concepts – 9th Edition 3.10 Silberschatz, Galvin and Gagne ©2013 Process Elements (I) Identifier State Priority How to describe a process? What Memory pointers: to code and data included in elements are it? Program counter Operating System Concepts – 9th Edition 3.11 Silberschatz, Galvin and Gagne ©2013 Process Elements (II) Context data: value of CPU registers I/O status information Outstanding I/O requests Assigned I/O devices and used files Accounting information Amount of processor time & clock time used Time limits Operating System Concepts – 9th Edition 3.12 Silberschatz, Galvin and Gagne ©2013 Process Control Block (PCB) Each process is represented by a PCB Created and managed by the operating system Contains the process elements Operating System Concepts – 9th Edition 3.13 Silberschatz, Galvin and Gagne ©2013 Process Control Block • Allows support for multiple processes Operating System Concepts – 9th Edition 3.14 Silberschatz, Galvin and Gagne ©2013 OS Control Tables Tables are constructed for each entity (i.e., process, resource) the operating system manages Operating System Concepts – 9th Edition 3.15 Silberschatz, Galvin and Gagne ©2013 Process Tables Manage processes A process is composed of program, data, heap, stack, and attributes ---- process image User Data The modifiable part of the user space. May include program data, a heap area, and programs that may be modified. User Program The program to be executed. Stack Each process has one or more last-in-first-out (LIFO) stacks associated with it. A stack is used to store parameters and returning addresses for procedure and system calls. Process Control Block Data needed by the OS to control the process Operating System Concepts – 9th Edition 3.16 Silberschatz, Galvin and Gagne ©2013 Process Location Where are the processes located? To manage and execute a process, at least a small portion of its image must be maintained in main memory OS must know the location of each page of each process image, achieved by process tables Operating System Concepts – 9th Edition 3.17 Silberschatz, Galvin and Gagne ©2013 Process Images Operating System Concepts – 9th Edition 3.18 Silberschatz, Galvin and Gagne ©2013 PCB: Process Control Information Data structuring: a process may be linked to other process in a queue, ring, or some other structure. For example, all ready processes for a particular priority level may be linked in a queue. A process may exhibit a parent-child (creator-created) relationship with another process. The process control block may contain pointers to other processes to support these structures. Operating System Concepts – 9th Edition 3.19 Silberschatz, Galvin and Gagne ©2013 Process Creation Assign a unique process identifier Allocate space for the process Initialize process control block Set up appropriate linkages Create or expand other data structures Operating System Concepts – 9th Edition 3.20 Silberschatz, Galvin and Gagne ©2013 When to Switch Process Clock interrupt process has executed for the maximum allowable time slice I/O interrupt Memory fault memory address is in disk so it must be brought into main memory Operating System Concepts – 9th Edition 3.21 Silberschatz, Galvin and Gagne ©2013 When to Switch Process Trap error or exception occurred may cause process to be moved to Exit state System Call Inter-process communication I/O operation such as file open Lead to a transfer to an OS routine Operating System Concepts – 9th Edition 3.22 Silberschatz, Galvin and Gagne ©2013 How to Switch Process? Operating System Concepts – 9th Edition 3.23 Silberschatz, Galvin and Gagne ©2013 CPU Switch From Process to Process Operating System Concepts – 9th Edition 3.24 Silberschatz, Galvin and Gagne ©2013 Process (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 3.25 Silberschatz, Galvin and Gagne ©2013 Process (Context) Switch Save context of processor including program counter and other registers Update the process control block of the process that is currently in the Running state Move process control block to appropriate queue – e.g., ready; blocked Select another process for execution Update the process control block of the process selected Restore context of the selected process Update memory-management data structures (page tables, TLB, etc.) Operating System Concepts – 9th Edition 3.26 Silberschatz, Galvin and Gagne ©2013 Process Switch vs. Mode Switch Operating System Concepts – 9th Edition 3.27 Silberschatz, Galvin and Gagne ©2013 Execution of the Operating System Process-based operating system Implement the OS as a collection of system processes Process switch for OS service Operating System Concepts – 9th Edition 3.28 Silberschatz, Galvin and Gagne ©2013 Execution of the Operating System Execution Within User Processes Operating Mode system software within context of a user process switch for OS service Operating System Concepts – 9th Edition 3.29 Silberschatz, Galvin and Gagne ©2013 OS Executes in User Space • Mode switch for OS service Operating System Concepts – 9th Edition 3.30 Silberschatz, Galvin and Gagne ©2013 Process vs. Mode Switch Save context of processor including program counter and other registers Update the process control block of the process that is currently in the Running state Move process control block to appropriate queue – ready; blocked; ready/suspend Select another process for execution Update the process control block of the process selected Update memory-management data structures Restore context of the selected process Operating System Concepts – 9th Edition 3.31 Silberschatz, Galvin and Gagne ©2013 Process vs. Mode Switch Save context of processor including program counter and other registers Update the process control block of the process that is currently in the Running state Move process control block to appropriate queue – ready; blocked; ready/suspend Select another process for execution Update the process control block of the process selected Restore context of the selected process Update memory-management data structures Operating System Concepts – 9th Edition 3.32 Silberschatz, Galvin and Gagne ©2013 Mode Switch Save context of processor including program counter and other registers Update the process control block of the running process Mode changes to kernel mode, finishes the OS routine Mode changes back, restore the context, and continue the running process in user mode Operating System Concepts – 9th Edition 3.33 Silberschatz, Galvin and Gagne ©2013 Process Scheduling (I) What does process scheduling do? What is the objective of doing process scheduling? Operating System Concepts – 9th Edition 3.34 Silberschatz, Galvin and Gagne ©2013 Process Scheduling (II) To maximize CPU use and for time sharing, quickly switch processes onto CPU Process scheduler selects among available processes for next execution on CPU Maintains scheduling queues of processes 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 3.35 Silberschatz, Galvin and Gagne ©2013 Process Scheduling (III) Three different categories of process scheduler Short-term scheduler Long-term scheduler Medium-term scheduler Operating System Concepts – 9th Edition 3.36 Silberschatz, Galvin and Gagne ©2013 Short-term Scheduler Short-term scheduler (or CPU scheduler or Dispatcher) – 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) Trace of the Process Sequence of instructions that execute for a process Dispatcher switches the processor from one process to another Interleaving process traces Operating System Concepts – 9th Edition 3.37 Silberschatz, Galvin and Gagne ©2013 Example Execution Operating System Concepts – 9th Edition 3.38 Silberschatz, Galvin and Gagne ©2013 Trace of Process Operating System Concepts – 9th Edition 3.39 Silberschatz, Galvin and Gagne ©2013 Combined Trace of Process Operating System Concepts – 9th Edition 3.40 Silberschatz, Galvin and Gagne ©2013 Long-term Scheduler Long-term scheduler (or job scheduler) – selects which processes should be admitted (i.e., brought into ready queue) The long-term scheduler controls the (maximum) degree of multiprogramming Long-term scheduler is invoked infrequently (seconds, minutes) (may be slow) 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 3.41 Silberschatz, Galvin and Gagne ©2013 Addition of Medium Term Scheduling Medium-term scheduler can be added if (actual) degree of multiprogramming needs to change Remove process from memory, store on disk, bring back in from disk to continue execution: swapping Present in all systems with virtual memory (Chap 9) Operating System Concepts – 9th Edition 3.42 Silberschatz, Galvin and Gagne ©2013 Suspended Processes Processor is faster than I/O so all executable processes could be waiting for I/O, while there are some new processes waiting to be admitted Swap these processes to disk to free up more memory to admit new processes Blocked state becomes suspend state when swapped to disk Two new states Blocked/Suspend Ready/Suspend Operating System Concepts – 9th Edition 3.43 Silberschatz, Galvin and Gagne ©2013 Two Suspend States Operating System Concepts – 9th Edition 3.44 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 3.45 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 For example, rfork in FreeBSD permits fine-grained sharing of resources between parent and child processes Execution options Parent and children execute concurrently (e.g., fork) Parent waits until children terminate (e.g., vfork) Operating System Concepts – 9th Edition 3.46 Silberschatz, Galvin and Gagne ©2013 Process Creation (Cont.) Address space Child duplicate of parent UNIX example fork() system call creates new process Operating System Concepts – 9th Edition 3.47 Silberschatz, Galvin and Gagne ©2013 fork() Process creation in Unix could be made by means of the kernel system call, fork() When a process issues a fork request, the OS: Allocates a slot in the process table for the new process; Assigns a unique process ID to the child process; Makes a copy of the process image of the parent, with the exception of any shared memory; Increments counters for any files owned by the parent, to reflect that an additional process now also owns those files; Assigns the child process to the Ready to Run state; Returns the ID number of the child to the parent process, and a 0 value to the child process. Operating System Concepts – 9th Edition 3.48 Silberschatz, Galvin and Gagne ©2013 Operating System Concepts – 9th Edition 3.49 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 3.50 Silberschatz, Galvin and Gagne ©2013 C Program Forking Separate Process Operating System Concepts – 9th Edition 3.51 Silberschatz, Galvin and Gagne ©2013 A Tree of Processes in Linux init pid = 1 login pid = 8415 khelper pid = 6 bash pid = 8416 ps pid = 9298 Operating System Concepts – 9th Edition sshd pid = 3028 kthreadd pid = 2 pdflush pid = 200 sshd pid = 3610 tcsch pid = 4005 emacs pid = 9204 3.52 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. Operating System Concepts – 9th Edition 3.53 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); Operating System Concepts – 9th Edition 3.54 Silberschatz, Galvin and Gagne ©2013 Process Termination If no parent waiting (did not invoke wait()), the terminated child process is a zombie, exit status kept in process table. Some operating systems allow child to exists if its parent has terminated. parent terminated without invoking wait , process is an orphan, Linux and UNIX assign the init process as the new parent to orphaned processes, who invokes wait periodically to collect the exit status of any orphaned process and release its pid and process-table entry. If Operating System Concepts – 9th Edition 3.55 Silberschatz, Galvin and Gagne ©2013 Multiprocess Application What are the reasons that we sometimes want to develop multiprocess applications? Operating System Concepts – 9th Edition 3.56 Silberschatz, Galvin and Gagne ©2013 Cooperating Processes 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: video surveillance Computation speed-up: merge sort Modularity multiprocess browser; concurrent server system Convenience: a user’s multiple processes Operating System Concepts – 9th Edition 3.57 Silberschatz, Galvin and Gagne ©2013 Multiprocess Architecture – Chrome Browser 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 3.58 Silberschatz, Galvin and Gagne ©2013 Example: Concurrent Server Dispatcher process Request dispatched to Server a worker process Worker process Operating System Concepts – 9th Edition 3.59 Silberschatz, Galvin and Gagne ©2013 Interprocess Communication Cooperating processes need interprocess communication (IPC) Two models of IPC Shared memory Message passing Operating System Concepts – 9th Edition 3.60 Silberschatz, Galvin and Gagne ©2013 Communications Models (a) Message Passing Operating System Concepts – 9th Edition (b) Shared Memory 3.61 Silberschatz, Galvin and Gagne ©2013 Producer-Consumer Problem Paradigm for cooperating processes: producer process produces information that is consumed by a consumer process Operating System Concepts – 9th Edition 3.62 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 user processes, not the operating system Major issue is to provide mechanism that will allow the user processes to synchronize their actions when they access shared memory Synchronization is discussed in great detail in Chapter 5 Operating System Concepts – 9th Edition 3.63 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 3.64 Silberschatz, Galvin and Gagne ©2013 IPC POSIX Producer Operating System Concepts – 9th Edition 3.65 Silberschatz, Galvin and Gagne ©2013 IPC POSIX Consumer Operating System Concepts – 9th Edition 3.66 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 Self-read Chap 3.4 on message passing and understand how to solve the producer and consumer problem using message passing Operating System Concepts – 9th Edition 3.67 Silberschatz, Galvin and Gagne ©2013 Reading Assignment By next Wednesday, finish reading Chap 4 Operating System Concepts – 9th Edition 3.68 Silberschatz, Galvin and Gagne ©2013