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CO – 2 : Processes

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CO – 2 : Processes

Process Concept
Process Scheduling
Operations on Processes

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Process Concept

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

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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

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Process in Memory

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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

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Diagram of Process State

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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

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CPU Switch From Process to Process

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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

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Process Representation in Linux

Represented by the C structure task_struct

�pid t_pid; /* process identifier */ �long state; /* state of the process */ �unsigned int time_slice /* scheduling information */ �struct task_struct *parent; /* this process’s parent */ �struct list_head children; /* this process’s children */ �struct files_struct *files; /* list of open files */ �struct mm_struct *mm; /* address space of this process */

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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

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Ready Queue And Various I/O Device Queues

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Representation of Process Scheduling

Queueing diagram represents queues, resources, flows

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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

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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

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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

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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

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Operations on Processes

System must provide mechanisms for:

process creation,
process termination,
and so on as detailed next

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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

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A Tree of Processes in Linux

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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

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C Program Forking Separate Process

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Creating a Separate Process via Windows API

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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

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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

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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

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