Processes

Process Concept
 An operating system executes a variety of programs that run as a
process.
 Process – a program (Job) in execution; process execution must
progress in sequential fashion. No parallel execution of instructions of
a single process
 This entity contains at least two types of elements:
• Program code and a set of data

 Process (also called task) is an executed program code on the

processor, that is, a part of an active entity
 A Thread is a portion of a process that can be run independently
 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
 Program vs Process vs Thread

 A program  functionProgram.py
 A process  Execution of this program (run the program)
 A thread  a portion (sub-task) of this process

Thread 1: Taking
fName value from user
Thread 2: Taking
sName value from user

Thread 3: Calling a
function (greeting)
Thread 4: Printing message
on computer screen
Difference between program & process

PROGRAM PROCESS

It consists of instructions in any It consists of instructions in

programming language. machine code.

It resides in the secondary storage

It resides in the main memory
device

The program only needs memory The resource requirement is quite

for storage high in the case of process

Its life span is unlimited. Its life span is limited

It is a passive entity. It is an active entity.
 Difference between process & thread

PROCESS THREAD

Process means any program is in Thread means a segment (part) of

execution. a process.

The process is isolated. Threads share memory.

If one process is blocked then it will If a user-level thread is blocked,

not affect the execution of other then all other user-level threads
processes are blocked.
The process does not share data Threads share data with each
with each other. other.
It takes more time for execution. It takes less time for execution.
 Process in Memory

Multiple parts of Process

• The program code, also called text

section
• Current activity including program
counter, processor registers
• Data section containing global
variables
• Stack containing temporary data
• Function parameters, return
addresses, local variables
• Heap containing memory
dynamically allocated during run
time
Memory Layout of a C Program
 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
 Process Control Block (PCB)
Information associated with each process(also called
Process(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
 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 */
 Operations on Processes

 System must provide mechanisms for:

• Process creation
• Process termination
 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
 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
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
 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
Process Scheduling
 Process Scheduling

 Program scheduler
• Initialize each program
• Only concerned with selecting programs from a queue of incoming
programs
• Places them in a process queue (READY queue)
 Process scheduler
• Determines which processes get the CPU, when, and for how long.
• Also decides when processing should be interrupted decides when
each step will be executed recognizes when a process has
concluded and should be terminated
 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
Representation of Process Scheduling
CPU Switch From Process to Process
A context switch occurs when the CPU switches from
one process to another.
 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
C Program Forking Separate Process
Creating a Separate Process via Windows API