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Operating System Concepts – 9th^ Edition Silberschatz, Galvin and Gagne ©
Chapter 3: Processes
Operating System Concepts – 9th^ Edition 3.2 Silberschatz, Galvin and Gagne ©
Chapter 3: Processes
(^) Process Concept (^) Process Scheduling (^) Operations on Processes (^) Interprocess Communication (^) Examples of IPC Systems (^) Communication in Client-Server Systems
Objectives
(^) To introduce the notion of a process -- a program in execution, which forms the basis of all computation (^) To describe the various features of processes, including scheduling, creation and termination, and communication (^) To explore interprocess communication using shared memory and message passing (^) To describe communication in client-server systems
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 (^) Data section containing global variables (^) Heap containing memory dynamically allocated during run time
Operating System Concepts – 9th^ Edition 3.5 Silberschatz, Galvin and Gagne ©
Process Concept (Cont.)
(^) Program is passive entity stored on disk (executable file e.g. exe), process is active (^) Program becomes process when executable file loaded into memory (^) How Program Becomes a Process: (^) Execution of program started via GUI mouse clicks, command line entry of its name, etc (^) The loaded Program is now a process (^) One program –>multiple processes (^) Asdf Single program can creates a multiple processes (^) e.g. If two users open MS Word (^) Example: (^) Program: Chrome.exe (stored on disk) (^) Process: When open chrome it runs as process (chrome.exe in Task Manager) (^) Multiple Processes: Open multiple chrome tabs Operating System Concepts – 9th^ Edition 3.6 Silberschatz, Galvin and Gagne ©
Process in Memory
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
Diagram of Process State
Operating System Concepts – 9th^ Edition 3.13 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 – 9th^ Edition 3.14 Silberschatz, Galvin and Gagne ©
Representation of Process Scheduling
(^) Queueing diagram represents queues, resources, flows
Schedulers
(^) Short-term scheduler (or CPU scheduler) – Decide which process runs next on the CPU (^) Sometimes the only scheduler in a system (^) Runs very frequently (milliseconds) (must be fast, direct impact on system performance) (^) Example: switching between multiple applications running on your PC (browser , music player etc. ) (^) Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue (^) Invoked infrequently (seconds, minutes) (may be slow) (^) Controls the degree of multiprogramming (how many processes run simultaneously) (^) Processes can be described as either: (^) I/O-bound process – spends more time doing I/O than computations, Have short CPU burst followed by long waits. Example: web browser waits for a webpage to load (^) CPU-bound process – spends more time doing computations; Have long CPU bursts with fewer interruptions Example: Video Editing, rendering, gaming etc. (^) A well-optimized OS keeps a good mix of both process types for smooth performance!
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 3.17 Silberschatz, Galvin and Gagne ©
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 3.18 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
Operations on Processes
(^) System must provide mechanisms for: (^) process creation, (^) process termination, (^) and so on as detailed next
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 – 9th^ Edition 3.25 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 – 9th^ Edition 3.26 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 (google docs: multiple users edit the same doc.) (^) Computation speedup (video rendering: different frames processed in parallel by multiple cores) (^) Modularity (A web browser has separate processes for tabs, ensuring one crash doesn’t affect others.) (^) Convenience (Auto-save in Microsoft Word runs in the background while you continue typing) (^) Cooperating processes need interprocess communication (IPC) (^) Two models of IPC (^) Shared memory (^) Message passing Operating System Concepts – 9th^ Edition 3.28 Silberschatz, Galvin and Gagne ©
Producer-Consumer Problem
(^) Paradigm for cooperating processes, producer process produces information that is consumed by a consumer process (^) A producer generates data (e.g., a camera recording a video). (^) A consumer processes it (e.g., a video player displaying the footage). (^) They cooperate using shared memory or message passing. (^) 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 3.29 Silberschatz, Galvin and Gagne ©
Interprocess Communication – Shared Memory
(^) Shared Memory is an IPC mechanism where multiple processes can access a common memory region without OS intervention after it is set up. (^) 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 Chapter 5. Operating System Concepts – 9th^ Edition 3.30 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
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?
Message Passing (Cont.)
(^) Implementation of communication link (^) Physical: (^) Shared memory (^) Hardware bus (^) Network (^) Logical: (^) Direct or indirect (^) Synchronous or asynchronous (^) Automatic or explicit buffering
Operating System Concepts – 9th^ Edition 3.37 Silberschatz, Galvin and Gagne ©
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 3.38 Silberschatz, Galvin and Gagne ©
Buffering
(^) Queue of messages attached to the link. (^) implemented in one of three ways
- Zero capacity : no messages are queued on a link. Sender must wait for receiver (rendezvous)
- Bounded capacity : finite length of n messages Sender must wait if link full
- Unbounded capacity: Infinite length Sender never waits
Local Procedure Calls in Windows
Communications in Client-Server Systems
(^) Sockets (^) Remote Procedure Calls (^) Pipes (^) Remote Method Invocation (Java)
Operating System Concepts – 9th^ Edition 3.41 Silberschatz, Galvin and Gagne ©
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. (^) How Sockets Works?
- (^) Two devices establish a connection using sockets.
- (^) A socket needs both an IP address and a port to differentiate between services.
- (^) Communication happens between a pair of sockets (one on each device). (^) All ports below 1024 are well known, used for standard services Operating System Concepts – 9th^ Edition 3.42 Silberschatz, Galvin and Gagne ©
Socket Communication
Remote Procedure Calls
(^) RPC allows a program to call a function on another machine just like a local function call. This helps in network communication between processes without worrying about the underlying details. (^) How RPC Works? (^) Client calls a function
- The client calls a function that looks like local function (^) Client-Side Stub Handles Communication
- (^) The function is forwarded to the client-side stub, which:
- Locates the server
- Converts (marshalls) parameters into a format that can be sent over the network (^) Server-side stub receives & executes (Cont. Next Slide)
Remote Procedure Calls (Cont.)
(^) How RPC Works? (^) Server-side stub receives & executes
- (^) The server-side stub receives the request,
- (^) Unpacks (unmarshalls) the parameters
- (^) Calls the actual function on the server (^) Result Sent Back
- The result is sent back to the client via the stubs
- (^) The client receives the result as if it was a normal function call (^) Summary:
- (^) RPC makes remote function calls look like local calls
- (^) Stubs handle communication (client-side & server-side)
- (^) Marshalling converts data to a network-friendly format
- RPC uses ports for differentiation
- Operating System Concepts – 9th Edition Silberschatz, Galvin and Gagne ©
