ch3OS.pdf

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

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 i ni t pi d = 1 l ogi n kt hr e add s s hd pi d = 8415 pi d = 2 pi d = 3028 bas h khe l pe r pdf l us h s s hd pi d = 8416 pi d = 6 pi d = 200 pi d = 3610 e mac s t cs ch ps pi d = 9204 pi d = 4005 pi d = 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