Chapter 2:

Process Concept

COURSE INSTRUCTOR:
ENGR. FARHEEN QAZI, ENGR. SYED HARIS MEHBOOB, MS. FALAK SALEEM

PROGRAM : SOFTWARE ENGINEERING

COURSE : OPERATING SYSTEMS (SE-204T)
BATCH : 2021F SPRING : 2023

DEPARTMENT OF SOFTWARE ENGINEERING

SIR SYED UNIVERSITY OF ENGINEERING & TECHNOLOGY
Today’s Agenda

 Process Concept
 Process Scheduling
 Operations on Processes
 Interprocess Communication
 Examples of IPC 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 inter-process communication using shared

memory and message passing
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
o Function parameters, return addresses, local variables

 Data section containing global variables

 Heap containing memory dynamically allocated during run time
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
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
Five State Process Model
 Process State Transition

 New → Ready: OS ready to schedule the new process.

 Ready → Running: OS Scheduler selects one of the processes in
the ready queue to run.
 Running → Exit: Process notifies OS to exit or abort.
 Running → Ready: Processes has reached its quantum and OS
uses scheduling algorithm to find the next process in Ready
state. Process can also release the processor.
 Running → Blocked: Process issues a request and must wait for
the event.
 Blocked → Ready: Event for which the process is waiting has
occurred.
 Ready → Exit: Parent process terminates a child process. Parent
process terminates and all child processes also terminates.
Seven State Model
Suspended state and swapping

 Suspended : Another process has explicitly told this process to

sleep. It will be awakened when a process explicitly awakens
it.
 So far, all the processes had to be (at least partly) in main
memory
 The OS may need to suspend some processes, i.e: to swap
them out to disk. We add 2 new states:

 Blocked Suspend: blocked processes which have been

swapped out to disk

 Ready Suspend: ready processes which have been swapped

out to disk
Suspended Processes

 Processor is faster than I/O so all processes could be waiting

for I/O
 Swap these processes to disk to free up more memory and
use processor on more processes

 Blocked state becomes suspend state when swapped to disk

 Two new states

 Blocked/Suspend
 Ready/Suspend
New state transitions (mid-term
scheduling)

 Blocked --> Blocked Suspend

 When all processes are blocked, the OS will make room to
bring a ready process in memory
 Blocked Suspend --> Ready Suspend
 When the event for which it has been waiting occurs (state
info is available to OS)
 Ready Suspend --> Ready
 when no more ready process in main memory or process has
higher priority than other Ready processes.
 Ready--> Ready Suspend (unlikely)
 OS needs to free up Main Memory for current process or next
scheduled process.
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
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
CPU Switch From Process to Process
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
Ready Queue And Various I/O Device
Queues
Representation of Process Scheduling

 Queueing diagram represents queues, resources, flows

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
Life cycle of a typical process
Dispatcher (short-term scheduler)

 Swaps processes out to secondary storage.

 It prevents a single process from monopolizing processor time.

 It decides who goes next according to a scheduling

algorithm. (chapter 4)

 The CPU will always execute instructions from the dispatcher

while switching from process A to process B.
Dispatcher at Work
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
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
A Tree of Processes in Linux

init
pid = 1

login kthreadd sshd

pid = 8415 pid = 2 pid = 3028

bash khelper pdflush sshd

pid = 8416 pid = 6 pid = 200 pid = 3610

emacs tcsch
ps
pid = 9204 pid = 4005
pid = 9298
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
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
Interprocess Communication

 Independent process cannot affect or be affected by the execution

of another process
 Cooperating process can affect or be affected by other processes,
including sharing data
 Reasons for cooperating processes:
 Information sharing (ex.: shared file)
 Computation speedup (break up process into sub tasks to run
faster and can be achieved only if the computer has multiple
processing elements – CPUs or I/O channels)
 Modularity (dividing system functions into separate processes or
threads)
 Convenience (individual user may work on many tasks at the
same time could be editing, printing, and compiling in parallel)
Interprocess Communication

 Mechanism for processes to communicate and to synchronize

their actions
 Two models of IPC:
1) shared memory
cooperating processes exchange information by reading and
writing data to a shared region of memory.
* allows maximum speed and convenience of communication.
* faster than message passing (system calls only to establish the
region. All accesses are routine memory accesses, no
assistance from the kernel).
 Contd…

2) message passing
messages are exchanged between the cooperating processes
useful for exchanging smaller amounts of data.
easier to implement than is shared memory for
intercomputer communications.
implemented using system calls (more time, kernel
intervention).
Communications Models
End of Chapter 2
Thank you