ch3.pptx

Process Concept  An operating system executes a variety of programs that run as a process.  Process – a program in execution; process execution must progress in sequential fashion. No parallel execution of instructi...

Process Concept  An operating system executes a variety of programs that run as a process.  Process – a program in execution; process execution must progress in sequential fashion. No parallel execution of instructions of a single process  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 – 10th Edition 3.1 Silberschatz, Galvin and Gagne ©2018  Process Concept (Cont.)  Program is passive entity stored on disk (executable file); process is active Program becomes process when an executable file is 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 – 10th Edition 3.2 Silberschatz, Galvin and Gagne ©2018  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 – 10th Edition 3.3 Silberschatz, Galvin and Gagne ©2018  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 – 10th Edition 3.4 Silberschatz, Galvin and Gagne ©2018  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  Explore in detail in Chapter 4 Operating System Concepts – 10th Edition 3.5 Silberschatz, Galvin and Gagne ©2018  Process Scheduling  Process scheduler selects among available processes for next execution on CPU core  Goal -- Maximize CPU use, quickly switch processes onto CPU core  Maintains scheduling queues of processes Ready queue – set of all processes residing in main memory, ready and waiting to execute Wait queues – set of processes waiting for an event (i.e., I/O) Processes migrate among the various queues Operating System Concepts – 10th Edition 3.6 Silberschatz, Galvin and Gagne ©2018  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 pure 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 – 10th Edition 3.7 Silberschatz, Galvin and Gagne ©2018  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 – 10th Edition 3.8 Silberschatz, Galvin and Gagne ©2018  Operations on Processes  System must provide mechanisms for: Process creation Process termination Operating System Concepts – 10th Edition 3.9 Silberschatz, Galvin and Gagne ©2018  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 – 10th Edition 3.10 Silberschatz, Galvin and Gagne ©2018  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 Parent process calls wait()waiting for the child to terminate Operating System Concepts – 10th Edition 3.11 Silberschatz, Galvin and Gagne ©2018  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 – 10th Edition 3.12 Silberschatz, Galvin and Gagne ©2018  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 – 10th Edition 3.13 Silberschatz, Galvin and Gagne ©2018  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 – 10th Edition 3.14 Silberschatz, Galvin and Gagne ©2018  Producer-Consumer Problem  Paradigm for cooperating processes: producer process produces information that is consumed by a consumer process  Two variations: unbounded-buffer places no practical limit on the size of the buffer:  Producer never waits  Consumer waits if there is no buffer to consume bounded-buffer assumes that there is a fixed buffer size  Producer must wait if all buffers are full  Consumer waits if there is no buffer to consume Operating System Concepts – 10th Edition 3.15 Silberschatz, Galvin and Gagne ©2018  IPC – 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 Chapters 6 & 7. Operating System Concepts – 10th Edition 3.16 Silberschatz, Galvin and Gagne ©2018  IPC – Message Passing  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 – 10th Edition 3.17 Silberschatz, Galvin and Gagne ©2018  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 – 10th Edition 3.18 Silberschatz, Galvin and Gagne ©2018  Implementation of Communication Link  Physical: Shared memory Hardware bus Network  Logical: Direct or indirect Synchronous or asynchronous Automatic or explicit buffering Operating System Concepts – 10th Edition 3.19 Silberschatz, Galvin and Gagne ©2018  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 – 10th Edition 3.20 Silberschatz, Galvin and Gagne ©2018  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 – 10th Edition 3.21 Silberschatz, Galvin and Gagne ©2018  Indirect Communication (Cont.)  Operations Create a new mailbox (port) Send and receive messages through mailbox Delete a mailbox  Primitives are defined as: send(A, message) – send a message to mailbox A receive(A, message) – receive a message from mailbox A Operating System Concepts – 10th Edition 3.22 Silberschatz, Galvin and Gagne ©2018  Indirect Communication (Cont.)  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 – 10th Edition 3.23 Silberschatz, Galvin and Gagne ©2018  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 – 10th Edition 3.24 Silberschatz, Galvin and Gagne ©2018  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 – 10th Edition 3.25 Silberschatz, Galvin and Gagne ©2018  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 – 10th Edition 3.26 Silberschatz, Galvin and Gagne ©2018  Ordinary Pipes  Ordinary Pipes allow 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)  Ordinary pipes are therefore unidirectional  Require parent-child relationship between communicating processes  Windows calls these anonymous pipes Operating System Concepts – 10th Edition 3.27 Silberschatz, Galvin and Gagne ©2018  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 – 10th Edition 3.28 Silberschatz, Galvin and Gagne ©2018