Chapter 2: Process

• 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
    • Function parameters, return addresses, local variables
  • Data section containing global variables
  • Heap containing memory dynamically allocated during run time
• 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

https://byjus.com/gate/process-in-operating-system-notes/

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

https://byjus.com/gate/process-state-in-operating-system-notes/

Diagram of Process State

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

https://byjus.com/gate/process-control-block-notes/

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
• See next chapter

https://byjus.com/gate/what-is-thread-in-operating-system-notes/

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

https://byjus.com/gate/process-scheduling-in-operating-system-notes/

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

https://byjus.com/gate/process-scheduler-in-operating-system-notes/

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

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

https://byjus.com/gate/multitasking-operating-system-notes/

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

https://byjus.com/gate/context-switching-in-os-notes/

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

• 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

https://www.tutorialspoint.com/what-is-process-creation-in-operating-systems
https://byjus.com/gate/difference-between-fork-and-exec/

A Tree of Processes in Linux

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.
  • 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

https://www.tutorialspoint.com/what-is-process-termination
https://www.geeksforgeeks.org/difference-between-zombie-orphan-and-daemon-processes/

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

https://byjus.com/gate/multiprocessing-operating-system-notes/

Interprocess Communication

Interprocess communication (IPC) is a set of programming interfaces that allow a programmer to coordinate activities among different program processes that can run concurrently in an operating system. This allows a program to handle many user requests at the same time.

• 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

https://www.tutorialspoint.com/what-is-inter-process-communication
https://www.geeksforgeeks.org/inter-process-communication-ipc/
https://byjusexamprep.com/gate-cse/inter-process-communication

Communications Models

(a) Message passing. (b) shared memory.

Cooperating Processes

• 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
  • Computation speed-up
  • Modularity
  • Convenience

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.

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
• Implementation of communication link
  • Physical:
    • Shared memory
    • Hardware bus
    • Network
  • Logical:
    • Direct or indirect
    • Synchronous or asynchronous
    • Automatic or explicit buffering