PROCESSES
Process
Process is a program in execution. It is
the unit of work in most system.
It should reside in the main memory
It is occupied the CPU to execute the
instructions.
A process is basically a program in execution. The
execution of a process must progress in a sequential fashion.
A
process is defined as an entity which represents the basic unit of work to be
implemented in the system.
The
execution of a process must programs in a sequential manner. At any time at
most one instruction is executed. The process includes the current activity as
represented by the value of the program counter and the content of the
processors registers. Also it includes the process stack which contain
temporary data (such as method parameters return address and local variables)
& a data section which contain global variables.
The Process Concept: -
A
process is an instance of a program in execution.
Batch systems work in terms of
"jobs". Many modern process concepts are still expressed in terms of
jobs, ( e.g. job scheduling ), and the two terms are often used interchangeably
A process is an instance of a program in
execution. Basically, a process is not the same ass “program”.12398
1) A program is a passive text of executable
codes resides in disk.
2) Process is a basically a programming
execution.
3) The execution of a process must
process in sequential fashion.
4) A process is an active entity ready
for execution which includes; a program counter, stack and data section.
5) When a task is loaded into the memory, it becomes a process.
Figure 3.1 - A
process in memory
Stack
:- The process stack contains the temporary data such as method ,function
,parameters return address and local variable
HEAP
:- This is dynamically
allocated memory to a process during its run time
TEXT
:- This include the
current activity represented by the value of program counter and the contents
of the processing register
DATA:-
This section contains the global and static variable
System Program View and Process
A program is pieces of code which may be
a single line or millions of lines . A computer program is usually written by a
computer programmer in a programming language.
For Ex-
#include
#include
void main()
{
Printf(“ Technical Point “);
Getch();
}
A program is collection of instruction
that performing a special task when executed by a computer , we can conclude
instance of a computer program
Process control block: (PCB):-
A
process in an OS is represented by a data structure known as process Control
Block (PCB) or process desperiptor .
A PCB keeps all the information needed to keep track of a process . The
architecture of PCB is completely depended on operating system and may control
different information in different OS
Each process is represented in the OS by a process control block. It is
also by a process control block. It is also known as task control block.
A process control block contains many pieces of information associated
with a specific process.
PCB contains the following information
Process state: The state may be new, ready, running, waiting or terminated state.
• Program counter:- it indicates the address of the next
instruction to be executed for this purpose.
• CPU registers:- The registers vary in number &
type depending on the computer architecture. It includes accumulators, index
registers, stack pointer & general purpose registers, plus any condition-
code information must be saved when an interrupt occurs to allow the process to
be continued correctly after- ward.
• CPU scheduling information:- This information includes process priority pointers to scheduling
queues & any other scheduling parameters.
• Memory management
information: This information may
include such information as the value of the bar & limit registers, the
page tables or the segment tables, depending upon the memory system used by the
operating system.
• Accounting
information: This information
includes the amount of CPU and real time used, time limits, account number, job
or process numbers and so on.
• I/O Status Information: This information includes the list of I/O devices allocated to this
process, a list of open files and so on. The PCB simply serves as the
repository for any information that may vary from process to process.
Process State
|
Process Number
|
Program counter
|
Register
|
Memory limits
|
List of open files
|
.......
|
Fig: Process Control
Block
v Attributes /Context of Process:
1. Process ID
2. Process State
3. Program Counter
4. Priority
5. General purpose registers
6. List of open files
7. List of open device
8. Protection information etc
1 . Process ID :- Process Id is the unique identification number given
by operating system to process of time of process creation.
Unique identification for each of
the process in the operating system.
2 .
Process State :- Process state is the current activity of
that process. In which particular state process is currently residing
Eg-
New , Ready , Run
3 .
Program Counter :- PC
contains the address next instruction to be executed
4 .
Priority :-
Priority is parameter which is assigned by OS to the process
5 .
General Purpose Register :-what
kind of register the process is using that register information is maintained
in general purpose register
Note :- Context
of process will be stored in process control block (PCB) each process has its
own PCB will be implemented using double linked list
Note :- PCB
of process will be started in main memory
A process may be in one of the following states :-
§ NEW :- Initial the process is in the new
state , it means the process is under creation or the process is being created
§ READY STATE :- Once
the process is created it moves to the ready state in the ready state there
will be multiple number of process and one of the process will be selected and
that will be dispatched on to the remaining state
§ RUN TIME
:-
In the running state there will be only one process at any point of time . In
the running state the process will perform CPU time and it is executing
instructions on the CPU.
§ WAIT OR BLOCK
:- In the wait state there will be multiple number of process .It means
multiple process will perform input operation parallel
§ Terminated:- The process has finished execution.
§ SUSPENDED READY :- When the ready queue becomes full , Some
processes are moved to suspended ready state
§ SUSPENDED BLOCK :-When waiting queue becomes full.
Multiprogramming OS :-
None
pre-emptive :- Running process will not get disturb
forcefully
Pre-Emptive /Multitasking
/time sharing :-
Forcefully
interrupting or disturbing the process
Note:- When the Process is in the ready state ,
running state and wait state block it is in the main memory
Note :- If the resources are not sufficient to
manage the process in the ready state then process in the ready state then sum
of the process will be suspended and they will be moved on to suspend ready
state .
Note
:- When the process in suspend ready
state .it is in the secondary memory when resources available process from
suspend ready state move back to ready state .
DEGREE
OF MULTIPROGRAMMING:-
Number
of process present in main memory at any point of time is called degree of
multiprogramming
DISPATCHER:- Responsibility of dispatcher is
loaded a process from ready state to run state . It is also responsible for
performing context switching .
DISPATCHER
LATENCY: - Time taken to
start one process and stop another process.
CPU BOUND PROCESS :- The process which required more amount
of CPU are called CPU bound . These processes will spend more time in running
state
I/O BOUND PROCESS:- The process which required more
amount of i/o time are called i/o bound processes . This processes will spend more
time in wait state .
CONTEXT SWITCHING:
Each and every time when the process is moving from one state to another
state the context ( attributes ) of the process will change .
DEFINITION
OF CONTEXT SWITCHING :-
Saving the
context of process and loading the context of other process is called Context
Switching
The Context
switching time is considered as the overload for the system.
Similarly, a context switch occurs when the time
slice for one process has expired and a new process is to be loaded from the
ready queue. This will be instigated by a timer interrupt, which will then
cause the current process's state to be saved and the new process's state to be
restored.
Context switching happens VERY VERY frequently, and the overhead
of doing the switching is just lost CPU time, so context switches ( state saves
& restores ) need to be as fast as possible. Some hardware has special
provisions for speeding this up, such as a single machine instruction for
saving or restoring all registers at once.
Note :- In some special cases if there is only one
process then still it is considered as context switching .
Eg-
Round Robin scheduler algorithm
Schedulers :-
Schedulers are special system software which handle process
scheduling in various way . Their main task is to select the job to be
submitted into system and to decide which program run .
Scheduler are three types:-
1 .
Long
Term scheduler
2 .
Short
Term Scheduler
3 .
Medium
Term Scheduler
v LTS (Long Term Scheduler ) :
LTS is responsible for creating new process and beginning
them into system. LTS select process from job pool (where process are kept run
later execution ) and load them into memory for execution . LTS control degree
of Multiprogramming.
Job scheduler/ Long term scheduler is responsible for
creating and bringing the new process into the system.
It is also called job scheduler. It selects processes from the pool and
loads them into memory for execution
Long term scheduler selects process from the disk &
loads them into
memory for execution. It controls the degree of
multi-programming i.e. no. of processes in memory. It executes less frequently
than other schedulers. If the degree of multiprogramming is stable than the
average rate of process creation is equal to the average departure rate of
processes leaving the system. So, the long term scheduler is needed to be
invoked only when a process leaves the system. Due to longer intervals between
executions
it can afford to take more time to decide which process
should be selected for execution. Most processes in the CPU are either I/O
bound or CPU bound. An I/O bound process (an interactive ‘C’ program is one
that spends most of its time in I/O operation than it spends in doing I/O
operation. A CPU bound process is one that spends more of its time in
doing computations than I/O operations (complex sorting
program). It is important that the long term scheduler should select a good mix
of I/O bound & CPU bound processes.
v STS
/ CPU SCHEDULER :-
STS is responsible for selecting one of the process in the
ready state for scheduling on to running state or CPU. It is also called CPU
scheduler
The short term scheduler selects among the process that are ready
to execute & allocates the CPU to one of them. The
primary distinction between these two schedulers is the frequency of their
execution. The short-term scheduler must select a new process for the CPU quite
frequently. It must execute at least one in 100ms. Due to the
short duration of time between executions, it must be very
fast.
The short-term scheduler, or CPU Scheduler, runs very frequently, on the
order of 100 milliseconds, and must very quickly swap one process out of the
CPU and swap in another one.
v MTS/SWAPPER :-
MTS is responsible for suspending and resuming the process
the job done by MTS is called as Swapping.
some operating systems introduce an additional intermediate
level of scheduling known as medium - term scheduler. The
main idea behind this scheduler is that sometimes it is advantageous to remove
processes from memory & thus reduce the degree of multiprogramming. At some
later time, the process can be reintroduced into memory & its execution can
be continued from where it had left off. This is called as
swapping. The process is swapped out & swapped in later
by medium term scheduler. Swapping is necessary to improve the process miss or
due to some change in memory requirements, the available memory limit is
exceeded which requires some memory to be freed up.
Some systems also employ
a medium-term scheduler. When system loads get
high, this scheduler will swap one or more processes out of the ready queue
system for a few seconds, in order to allow smaller faster jobs to finish up
quickly and clear the system
Ø
Dispatcher :-
·
The
dispatcher is responsible of performing the context switching .the Dispatcher
is also responsible of loading the selected job on to the CPU
·
Selecting
of job is done by STS & loading by dispatcher
·
The
LTS should select the good combination of both CPU bound and i/o bound process
in order to get good throughput for the system
·
If
LTS selects only CPU bound time then CPU utilization increase but throughput
decrease if LTS selects only i/o bound time then CPU idleness increase.
Note :- LTS should select the good combination
of both CPU bound and I/O bound process in order to get good throughput for the
system.
Note:- LTS controls degree of
multiprogramming
Q.
consider a system which has n CPU
processors then what is the min & the max no. of processes that may present
in the ready, running & block
min
|
max
|
|
Ready
|
0
|
∞
|
Running
|
0
|
n
|
Wait/block
|
0
|
∞
|
- SCHEDULING
ALGORITHM :-
Scheduling algorithm are
used distributing resources among processes which simultaneously and asynchronous
request them
A process scheduler different process to be a
sign to the CPU based on particular scheduling algorithm . It is the primitive
and a other process executed for a given time period.
Schedu
ling deals with the
problem of deciding which of the requests is to be allocated resources.
CPU Scheduling deals with the problem
of deciding which of the processes in the ready queue is to be allocated first
to the CPU. There are four types of CPU scheduling that exist
There are many different scheduling algorithms they are as
follows:
I.
FCFS (First Come
First Serve)
II.
SJN or SJF (Shortest Job Nest / First)
III.
Round Robin
Scheduling
IV.
Shortest Test
Remaining Time
V.
Priority Scheduling
VI.
Multi Level Queues
Scheduling
v AT (Arrival Time )
:- The time when the process is arrived into the ready state
v BT (Burst Time ):-
The time required by process to complete its execution
v CT (Completion Time ) :- The time when the process is completed its total
execution is called CT
v TAT (Turn Around Time) :- The time difference between the CT and AT is called TAT (TAT=CT-AT)
v WT (Waiting Time ) :- The difference
between TAT and BT is time called as WT of the process (WT=TAT-BT)
v RT (Response Time ) :- The time difference between first response and the AT
Note :- STS is responsible for schedule scheduling .
Goals of CPU Scheduling :-
1.
To maximize CPU
utilization & throughput of the system
2.
To minimize the
average TAT, Avg. WT & Avg. RT of the processes
FIRST COME FIRST SERVE (FCFS)
First Come, First Served Scheduling
(FCFS) Algorithm:- This is the
simplest CPU scheduling algorithm. In this scheme, the process which requests
the CPU first, that is allocated to the CPU first. The implementation of the
FCFS algorithm is easily managed with a FIFO queue. When a process enters the
ready queue its PCB is linked onto the rear of the queue
First Come First Serve is the scheduling of processes
where the process leaves the queue in the order in which they arrive.
It is based on Queuing.
The FCFS scheduling algorithm in
non-preemptive.
Criteria :- Arrival Time (AT)
Mode :-
None pre-emptive algorithm
Q. Find out the avg.TAT & Avg. WT.
P.NO
|
AT
|
BT
|
CT
|
TAT
|
WT
|
P1
|
0
|
3
|
3
|
3
|
0
|
P2
|
1
|
1
|
4
|
3
|
2
|
P3
|
2
|
4
|
8
|
6
|
2
|
P4
|
3
|
5
|
13
|
10
|
5
|
P5
|
4
|
2
|
15
|
11
|
9
|
Total number of
process = 5
|
33
|
18
|
|||
Gantt chart
P1
|
P2
|
P3
|
P4
|
P5
|
0 3 4 8 13 15
Sum of TAT
= 33
Sum of WT
=18
AVG. TAT
=33/5=6.6
AVG. WT
=18/5=3.6
NOTE: -
if the Arrival time (AT) of the process are matching then schedule the process
which has lowest process ID.
Q. Find out the avg.TAT & Avg. WT.
|
P.NO
|
AT
|
BT
|
CT
|
TAT
|
WT
|
|
p1
|
6
|
3
|
29
|
23
|
20
|
|
P2
|
3
|
8
|
18
|
15
|
7
|
|
p3
|
4
|
4
|
22
|
18
|
14
|
|
p4
|
2
|
6
|
10
|
8
|
2
|
|
p5
|
1
|
3
|
4
|
3
|
0
|
|
p6
|
5
|
4
|
26
|
21
|
17
|
|
Total number of
process = 6
|
+88
|
+60
|
|||
Gantt chart
|
CPU idle
|
P5
|
P4
|
P2
|
P3
|
P6
|
P1
|
0 1 4 10 18 11
26
29
Sum of TAT
= +88
Sum of WT
= +60
AVG. TAT
= 88/6=14.6
AVG. WT
= 60/6=10
Q. Find out the avg.TAT & Avg. WT.
|
P.NO
|
AT
|
BT
|
CT
|
TAT
|
WT
|
|
P1
|
12
|
8
|
21
|
9
|
1
|
|
P2
|
9
|
4
|
13
|
4
|
0
|
|
P3
|
3
|
3
|
6
|
3
|
0
|
|
P4
|
2
|
1
|
3
|
1
|
0
|
|
P5
|
3
|
2
|
8
|
5
|
3
|
|
P6
|
15
|
9
|
30
|
15
|
6
|
|
Total number of
process = 6
|
+37
|
+10
|
|||
Gantt chart
|
CPU idle
|
P4
|
P3
|
P5
|
CPU idle
|
P2
|
P1
|
P5
|
0 2 3 6 3 9 13 21 30
Sum of TAT
= +37
Sum of WT
= +10
AVG. TAT
= 37/6 = 6.16
AVG. WT
= 10/6= 1.66
Convey Effect –
In the FCFS If
the process is having large BT (CPU bound) then it will have the major effect
on avg. WT of the process. This effect is called as Convey effect.
Ex--
Q. Find out the avg.TAT & Avg. WT.
|
P.NO
|
AT
|
BT
|
CT
|
TAT
|
WT
|
|
P1
|
0
|
20
|
20
|
20
|
0
|
|
P2
|
1
|
2
|
22
|
21
|
19
|
|
P3
|
2
|
2
|
24
|
22
|
20
|
|
Total number of
process = 3
|
+63
|
+39
|
|||
Gantt chart
|
P1
|
P2
|
P3
|
0 20 22 24
Sum of TAT
= 63
Sum of WT
= 39
AVG. TAT
= 63/3=21 AVG.
WT = 39/3=13
Q. Find out the avg.TAT & Avg. WT.
|
P.NO
|
AT
|
BT
|
CT
|
TAT
|
WT
|
|
P1
|
0
|
2
|
2
|
2
|
0
|
|
P2
|
1
|
2
|
4
|
3
|
2
|
|
P3
|
2
|
20
|
24
|
22
|
1
|
|
Total number of
process = 3
|
+28
|
+3
|
|||
Gantt chart
|
P1
|
P2
|
P3
|
0 2 4 24
Sum of TAT
= 28
Sum of WT
= 3
AVG. TAT
= 28/3= 9.33 AVG.
WT = 3/3=1SHORTEST JOB FIRST (SJF)
Shortest Job First Scheduling (SJF) Algorithm:- This algorithm associates with each
process if the CPU is available. This scheduling is also known as shortest next
CPU burst, because the scheduling is done by examining the length of the next
CPU burst of the process rather than its total length
Shortest Job First, which connects with each process the
length of second next CPU bursts
Whichever process requires less time to complete the
execution is processed first
If there is a case,
where two processes in ready queue has same CPU burst, FCFS is used to decide
among them.
This algorithm takes minimum average
waiting time.
To schemes to find the shortest job first : -
a. Non-pre-emptive:
- once CPU given to the process it cannot be pre-empted until completes its CPU bursts.
b. Pre-emptive: - if a new process arrives with CPU
burst length less than remaining time of current executing process, pre-empt.
This scheme is known as the Shortest-Remaining Time-First (SRTF).
Criteria :-
Burst Time (BT)
Mode:- 1. Non- pre-emptive 2. Pre-emptive
1.
Non- pre-emptive:-
Q. Find out the
avg.TAT & Avg. WT
|
P.NO
|
AT
|
BT
|
CT
|
TAT
|
WT
|
|
P1
|
1
|
7
|
8
|
7
|
0
|
|
P2
|
2
|
5
|
16
|
14
|
9
|
|
P3
|
3
|
1
|
9
|
6
|
5
|
|
P4
|
4
|
2
|
11
|
7
|
5
|
|
P5
|
5
|
8
|
24
|
19
|
11
|
|
Total number of
process = 5
|
+53
|
+30
|
|||
Gantt chart
|
CPU idle
|
P1
|
P3
|
P4
|
P2
|
P5
|
0 1 8 9 11 16
24
Sum of TAT
= 53
Sum of WT
= 30
AVG. TAT
= 53/5=10.6
AVG. WT
= 30/5=6
·
Note :- if the BT
of the processes are matching then schedule the process which has lowest AT.
Q. Find out the avg.TAT & Avg. WT
|
P.NO
|
AT
|
BT
|
CT
|
TAT
|
WT
|
|
P1
|
6
|
1
|
8
|
2
|
1
|
|
P2
|
3
|
3
|
13
|
10
|
7
|
|
P3
|
4
|
6
|
19
|
15
|
9
|
|
P4
|
1
|
5
|
6
|
5
|
0
|
|
P5
|
2
|
2
|
10
|
8
|
6
|
|
P6
|
5
|
1
|
7
|
2
|
1
|
|
Total number of
process = 6
|
+42
|
+24
|
|||
Gantt chart
|
CPU
idle
|
P4
|
P6
|
P1
|
P5
|
P2
|
P3
|
Sum of TAT
= 42
Sum of WT
= 24
AVG. TAT
= 42/6=7
AVG. WT
= 24/6=4
Q. Find out the avg.TAT & Avg. WT
|
P.NO
|
AT
|
BT
|
CT
|
TAT
|
WT
|
|
P1
|
0
|
2
|
2
|
2
|
0
|
|
P2
|
1
|
3
|
5
|
11
|
1
|
|
P3
|
2
|
5
|
14
|
12
|
07
|
|
P4
|
3
|
4
|
09
|
06
|
02
|
|
P5
|
4
|
6
|
20
|
16
|
10
|
|
Total number of
process = 5
|
+40
|
+20
|
|||
Gantt chart
|
P1
|
P2
|
P4
|
P3
|
P5
|
0 2 5 9 14 20
Sum of TAT
= 40
Sum of WT
= 20
AVG. TAT
= 40/5=8
AVG. WT
= 20/5=4
SHORTEST JOB FIRST / SHORTEST REMAINING TIME FIRST (SRTF) PRE -EMTIVE .
Criteria: - Burst
Time (BT)
Mode:- Pre-emptive.
Q. Find out the avg.TAT & Avg. WT
|
P.NO
|
AT
|
BT
|
CT
|
TAT
|
WT
|
|
P1
|
0
|
|
14
|
14
|
7
|
|
P2
|
1
|
6
|
20
|
19
|
13
|
|
P3
|
2
|
|
6
|
4
|
1
|
|
P4
|
3
|
|
4
|
1
|
0
|
|
P5
|
4
|
|
9
|
5
|
3
|
|
P6
|
5
|
|
7
|
2
|
1
|
|
Total number of
process = 6
|
+45
|
+25
|
|||
Gantt chart
|
P1
|
P1
|
P3
|
P4
|
P3
|
P3
|
P6
|
P5
|
P1
|
P2
|
0 1 2 3 4 5 6 7 9 14 20
Sum of TAT
= 45
Sum of WT
= 25
AVG. TAT
= 45/6 = 7.5
AVG. WT
= 25/6=4.16
Q. Find out the avg.TAT & Avg. WT
|
P.NO
|
AT
|
BT
|
CT
|
TAT
|
WT
|
|
P1
|
3
|
4
|
13
|
10
|
6
|
|
P2
|
4
|
|
9
|
5
|
3
|
|
P3
|
5
|
|
7
|
2
|
1
|
|
P4
|
2
|
6
|
19
|
17
|
11
|
|
P5
|
1
|
|
26
|
25
|
17
|
|
P6
|
2
|
|
6
|
4
|
0
|
|
Total number of
process = 6
|
+63
|
+38
|
|||
Gantt chart
|
CPU idle
|
P5
|
P6
|
P6
|
P6
|
P6
|
P3
|
P2
|
P2
|
P1
|
P4
|
P5
|
Sum of TAT
= 63
Sum of WT
= 38
AVG. TAT
= 63/6=10.5
AVG. WT
= 38/6=6.3
Priority
Scheduling
The priority will be assigned t each
of the process . The higher the priority the sooner will the process get the
CPU .If the priority of the two process is some then they will be schedule
according to their arrival time.
In this scheduling a priority is associated
with each process and the CPU is allocated to the process with the highest
priority. Equal priority processes are scheduled in FCFS manner
Priority is associated with each
process in the queue and the process with the highest priority is processed. SJF is an example of priority algorithm.
Criteria :-
Priority
Mode :- 1. Non-pre-emptive
2. pre-emptive
Note :- If the priority of the process are matching then schedule the
process which has lower AT
1. Non-pre-emptive:-
Q. Find out the avg.TAT & Avg. WT
|
Priority
|
P.NO
|
AT
|
BT
|
CT
|
TAT
|
WT
|
|
4
|
P1
|
0
|
4
|
4
|
4
|
0
|
|
5
|
P2
|
1
|
5
|
16
|
15
|
10
|
|
Highest 7
|
P3
|
2
|
1
|
5
|
3
|
2
|
|
2
|
P4
|
3
|
2
|
18
|
15
|
13
|
|
Lowest-
1
|
P5
|
4
|
3
|
21
|
17
|
14
|
|
6
|
P6
|
5
|
6
|
11
|
6
|
0
|
|
Total number of
process = 6
|
+60
|
+39
|
||||
Gantt chart
|
P1
|
P3
|
P6
|
P2
|
P4
|
P5
|
0 4 5 11 16 18 21
Sum of TAT
= 60
Sum of WT
= 39
AVG. TAT
= 60/6=10
AVG. WT
= 39/6= 6.5
Q. Find out the avg.TAT & Avg. WT
|
Priority
|
P.NO
|
AT
|
BT
|
CT
|
TAT
|
WT
|
|
4
|
P1
|
4
|
6
|
17
|
13
|
7
|
|
5
|
P2
|
6
|
3
|
11
|
5
|
2
|
|
6
|
P3
|
3
|
4
|
8
|
5
|
1
|
|
6
|
P4
|
2
|
2
|
4
|
2
|
0
|
|
High 7
|
P5
|
1
|
1
|
2
|
1
|
0
|
|
Low 3
|
P6
|
2
|
2
|
19
|
17
|
2
|
|
Total number of
process = 6
|
+43
|
+25
|
||||
Gantt chart
|
CPU idle
|
P5
|
P4
|
P3
|
P2
|
P1
|
P6
|
0 1 2 4 8 11 17 19
Sum of TAT
= 43
Sum of WT
= 25
AVG. TAT
= 43/6=7.1
AVG. WT
= 25/6=4.1
PRE-EMPTIVE
Q. Find out the avg.TAT & Avg. WT
|
Priority
|
P.NO
|
AT
|
BT
|
CT
|
TAT
|
WT
|
|
Low 4
|
P1
|
1
|
|
18
|
17
|
17
|
|
5
|
P2
|
2
|
2
|
14
|
12
|
10
|
|
7
|
P3
|
2
|
|
10
|
8
|
5
|
|
High
8
|
P4
|
3
|
|
8
|
5
|
0
|
|
5
|
P5
|
3
|
1
|
15
|
12
|
11
|
|
6
|
P6
|
4
|
2
|
12
|
8
|
6
|
|
Total number of process = 6
|
+62
|
+45
|
||||
Gantt
chart
|
CPU idle
|
P1
|
P3
|
P4
|
P3
|
P6
|
P2
|
P5
|
P1
|
0 1 2 3 8 10 12 14 15
18
Sum of TAT
= 62
Sum of WT
= 45
AVG. TAT
= 62/6=10.33
AVG. WT
= 45/6=7.5
Q. Find out the avg.TAT & Avg. WT
|
Priority
|
P.NO
|
AT
|
BT
|
CT
|
TAT
|
WT
|
|
5
|
P1
|
1
|
|
16
|
15
|
11
|
|
2
|
P2
|
2
|
5
|
21
|
19
|
14
|
|
6
|
P3
|
3
|
|
14
|
11
|
5
|
|
4
|
P4
|
0
|
|
1
|
1
|
0
|
|
7
|
P5
|
4
|
|
9
|
5
|
3
|
|
8
|
P6
|
5
|
3
|
8
|
3
|
0
|
|
Total number of
process = 6
|
54
|
33
|
||||
Gantt chart
|
P4
|
P1
|
P1
|
P3
|
P5
|
P6
|
P5
|
P3
|
P1
|
P2
|
0 1 2 3 4 5 8
9
14 16 21
Sum of TAT
= 54
Sum of WT
= 33
AVG. TAT
= 54/6=9
AVG. WT =
33/6=5.5Round Robin Scheduling
Round
Robin Scheduling Algorithm: This type of algorithm is designed only for the
time sharing system. It is similar to FCFS scheduling with preemption condition
to switch between processes. A small unit of time called quantum time or time
slice is used to switch between the processes. The average waiting time under
the round robin policy is quiet long.
It is especially used with time sharing systems. It is called as
process sharing approach.
The queue is seen as a circular
queue.
The queue is kept as FIFO queue.
As the time quantum is completed the queue is dispatched
Criteria :- AT & TQ (TIME QUANTUM) (or) Time
Slice
Mode :- pre-emptive
TIME QUANTUM (TQ) :- Whenever the
process is scheduled for execution the Maximum Time the process will
execute at that time
Note :- Round Robin Scheduling algorithm
is implemented using circular queue
Note:- If Arrival time of the process are
not given in the question then by default consider it as Zero.
Note:- In the Round Robin if TQ is very-
very large then the Round Robin
Algorithm behaves like FCFS OR FIFO Scheduling .
Note
:- In Round Robin if TQ is less then number of context switches will in
increase and response time will be less
Note:- If TQ is large then numbers of
context switches will be less and response time will be more
Q. Find out the avg.TAT & Avg. WT
& Avg. RT
TQ=2
|
P.NO
|
AT
|
BT
|
CT
|
TAT
|
WT
|
RT
|
|
P1
|
0
|
|
8
|
8
|
4
|
0
|
|
P2
|
1
|
|
18
|
17
|
12
|
1
|
|
P3
|
2
|
|
6
|
4
|
2
|
2
|
|
P4
|
3
|
1
|
9
|
6
|
5
|
5
|
|
P5
|
4
|
|
21
|
17
|
11
|
5
|
|
P6
|
6
|
|
19
|
13
|
10
|
7
|
|
Total number of
process = 6
|
65
|
44
|
20
|
|||
RQ (Ready Queue
) (in which order the process will come)
RQ= P1
P2 P3 P1 P4 P5 P2 P6 P5 P2 P6 P5
Gantt chart
|
P1
|
P2
|
P3
|
P1
|
P4
|
P5
|
P2
|
P6
|
P5
|
P2
|
P6
|
P5
|
0 2 4 6 8 9 11 13 15 17 18 19 21
Avg. TAT = 65/6=10.8
Avg. WT = 44/6 Avg.
RT= 20/6= 3.33 (RT= first response-AT)
Q. Find out the avg.TAT & Avg. WT
& Avg. RT
TQ= 3
|
P.NO
|
AT
|
BT
|
CT
|
TAT
|
WT
|
RT
|
|
P1
|
5
|
5
|
32
|
27
|
22
|
10
|
|
P2
|
4
|
6
|
27
|
23
|
17
|
5
|
|
P3
|
3
|
7
|
33
|
30
|
23
|
3
|
|
P4
|
1
|
9
|
30
|
29
|
20
|
0
|
|
P5
|
2
|
2
|
6
|
4
|
2
|
2
|
|
P6
|
6
|
3
|
21
|
15
|
12
|
12
|
|
Total number of
process = 6
|
128
|
96
|
32
|
|||
READY QUEUE
:- P4 P5 P3 P2 P4 P1 P6 P3 P2 P4 P1 P3
Gantt
chart
|
CPU idle
|
P4
|
P5
|
P3
|
P2
|
P4
|
P1
|
P6
|
P3
|
P2
|
P4
|
P1
|
P3
|
0 1
4 6 9 12 15 18 21 24 27 30 32 33
Avg. RT = 32/6=5.3
Avg. WT= 96/6=12
Avg.
TAT = 128//6= 21.3
Operating System Services for Process
Management: -
1. Process
management is integral part of any model operating system.
An operating system
provides an environment for the execution of the program. Following are the
various services provided by an operating system: -
• Program Execution: The operating system must
provide the ability to load a program into memory at run time.
• I/O Operation: Program in running process must
require I/O. It may involve in a file or specific I/O device.
• File System Manipulation: The operating system must
provide the capability to read, write, create and delete a file for user program.
Therefore each and every files must maintained correctly by the operating
system.
• Communication: The process of sharing message via shared memory or
by the technique of message passing in which packets of information are moved
between the processes by the operating system this process is called
communication.
• Error detection: Operations like arithmetic
overflow, access to the illegal memory location and too large user CPU time,
the operating system should take the appropriate actions for these occurrences.
• Research Allocation: It is the ability to allocate
multiple users, which are logged on to the system the resources must be
allocated to each of them. Among the various processes the operating system
uses the CPU scheduling algorithms which determine which process will be
allocated with the resource.
• Accounting: Operating system keep track of the users which use
how many and which kind of computer resources.
• Protection: The
protection of hardware as well as software is the responsibility of operating
system.
SYSTEM CALL :- System
call is the request made by user program to OS i order to get any kind of
service
Eg- printf-write() monitor
Printf internally calls write system call in
order to get interact with keyboard
The Fork():
1.
System Call fork() is used to create process
. it take no arguments and returns a process ID
2.
The purpose of fork() is to create a new
process, which becomes child process of the caller .
3.
After a new child process is created, both
processes will executed the nest instruction following the fork() system call,
4.
Therefore we have to distinguish the parent from the child .
This
can be done by testing the return value of fork()
·
If Fork() return a negative value , the
creation of child process was unsuccessful .
·
If fork() return a Zero to the newly created child process
·
If fork() return a positive value , the
process ID of the child process to the parent . Normally the process ID is an
integer moreover, a process can use
function getpid() to retrieve the process ID assigned to this process
Conclusion :-
·
If the program contains n fork() calls it
will create 2n-1 child process
·
When the child process is created by using
fork system call . both parent and child will have
·
Relative address = same for both parent and
child process
·
Absolute Address = different for both parent
and child process
Fork()
system call implemented –
Main()
{
Int
pid;
Pid=fork();
If(pid<0 o:p="">
{
Printf(“child
process creation failed “);
Else
if(pid==0)
{
Printf(“child
process”);
}
Else
{
Printf(“parent
process” )
}
}
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