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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Operating System</title>
<style>
* {
margin: 0;
padding: 0;
box-sizing: border-box;
color-scheme: dark;
font-family: sans-serif;
color: rgb(202, 202, 202);
}
table tr,
table tr td,
table tr th {
text-align: center;
padding: 10px;
border: 1px solid white;
}
</style>
</head>
<body>
<center><h1>Operating System</h1></center>
<strong>CPU bound process: </strong>These are those processes that require
most of the time on CPU.<br />
<strong>I/O bound process: </strong>These are those processes that require
most of the time on I/O devices.<br />
<strong>N.B: </strong>A good CPU scheduling idea should choose the mixture
of both so that both I/O devices and CPU can be utilized correctly.<br />
<strong>CPU scheduling are of two types: </strong>i) Non-Pre-emptive, ii)
Pre-emptive<br />
<strong>i) Non-Pre-emptive: </strong> We can't forcefully terminate this
type of process. This process complete it's execution and leave CPU to
perform other operation.<br />
<strong>ii) Pre-emptive: </strong>If a process enters in the ready state
from new or waiting state and high priority process.<br />
<center><strong>CPU scheduling terminiology</strong></center>
<strong>i) Burst Time / Execution Time / Running Time: </strong>It's the
time process require for running on CPU. <br />
<strong>ii) Waiting Time: </strong>Time spend by a process in ready state
waiting for CPU. <br />
<strong>iii) Arrival Time: </strong>When a process enters ready state.
<br />
<strong>iv) Exit Time: </strong>When a process completes execution and exit
from system. <br />
<strong>v) Turn Around Time: </strong>It's the time process require for
running on CPU. T.A.T == E.T - A.T == B.T + W.T<br />
<strong>vi) Response Time: </strong>It's the time between a process enters
ready queue and get scheduled on the CPU for the first time. <br />
<center><strong>CPU scheduling algorithm criteria</strong></center>
<strong>i) Average waiting time: </strong> Average waiting time should be as
low as possible <br />
<strong>ii) Average response time: </strong> Average response time should be
as low as possible<br />
<strong>iii) CPU utilization: </strong>CPU utilization should be a high all
time. <br />
<strong>iv) Throughput: </strong> Number of process execution per unit time.
<br />
<center><strong>FCFS (First Come First Serve)</strong></center>
i) Simplest scheduling algorithm, it assigns CPU to the process which
arrives first. <br />
ii) Easy to understand and can easily be implemented using Queue data
structure. <br />
iii) Always non-pre-emptive in nature. <br />
<h3>Chart</h3>
<table>
<tr>
<th>Pid</th>
<th>A.T</th>
<th>B.T</th>
<th>T.A.T = E.T - A.T</th>
<th>W.T = T.A.T - B.T</th>
</tr>
<tr>
<td>A</td>
<td>3</td>
<td>4</td>
<td>7 - 3 = 4</td>
<td>4 - 4 = 0</td>
</tr>
<tr>
<td>B</td>
<td>5</td>
<td>3</td>
<td>13 - 5 = 8</td>
<td>8 - 3 = 5</td>
</tr>
<tr>
<td>C</td>
<td>0</td>
<td>2</td>
<td>2 - 0 = 2</td>
<td>2 - 2 = 0</td>
</tr>
<tr>
<td>D</td>
<td>5</td>
<td>1</td>
<td>14 - 5 = 9</td>
<td>9 - 1 = 8</td>
</tr>
<tr>
<td>E</td>
<td>4</td>
<td>3</td>
<td>10 - 4 = 6</td>
<td>6 - 3 = 3</td>
</tr>
</table>
<h3>Gantt Chart</h3>
<table>
<tr>
<td>C <br />0 - 2</td>
<td>
idle <br />
2 - 3
</td>
<td>
A <br />
3 - 7
</td>
<td>
E <br />
7 - 10
</td>
<td>
B <br />
10 - 13
</td>
<td>
D <br />
13 - 14
</td>
</tr>
</table>
<center><strong>Convoy Effect</strong></center>
Smaller process have to wait for long time for bigger process to release
CPU. <br />
<strong>Advantage: </strong> Simple, easy to use, easy to understand, easy
to implement, must be used for background process where execution is not
urgent. <br />
<strong>Disadvantage: </strong> Suffer from convoy effect, normally higher
average waiting time, no consideration on priority or burst time, should not
be used for interactive system. <br />
<center>
<strong
>Shortest Job First (Non-Pre-emptive) / Shortest Remaining Time First
(Pre-emptive)</strong
>
</center>
i) Process, who have small burst time will assigned first. <br />
ii) In case of tie, FCFS is used. <br />
iii) It uses both pre-emptive and non-pre-emptive approach. <br />
<h3>Chart (Non-Pre-emptive)</h3>
<table>
<tr>
<th>Pid</th>
<th>A.T</th>
<th>B.T</th>
<th>T.A.T = E.T - A.T</th>
<th>W.T = T.A.T - B.T</th>
</tr>
<tr>
<td>A</td>
<td>3</td>
<td>1</td>
<td>7 - 3 = 4</td>
<td>4 - 1 = 3</td>
</tr>
<tr>
<td>B</td>
<td>1</td>
<td>4</td>
<td>16 - 1 = 15</td>
<td>15 - 4 = 11</td>
</tr>
<tr>
<td>C</td>
<td>4</td>
<td>2</td>
<td>9 - 4 = 5</td>
<td>5 - 2 = 3</td>
</tr>
<tr>
<td>D</td>
<td>0</td>
<td>6</td>
<td>6 - 0 = 6</td>
<td>6 - 6 = 0</td>
</tr>
<tr>
<td>E</td>
<td>2</td>
<td>3</td>
<td>12 - 2 = 10</td>
<td>10 - 3 = 7</td>
</tr>
</table>
<h3>Gantt Chart</h3>
<table>
<tr>
<td>D<br />0 - 6</td>
<td>
A <br />
6 - 7
</td>
<td>
C <br />
7 - 9
</td>
<td>
E <br />
9 - 12
</td>
<td>
B <br />
12 - 16
</td>
</tr>
</table>
<h3>Chart SRTF (Pre-emptive) Optimal (uses purely greedy)</h3>
<table>
<tr>
<th>Pid</th>
<th>A.T</th>
<th>B.T</th>
<th>T.A.T = E.T - A.T</th>
<th>W.T = T.A.T - B.T</th>
</tr>
<tr>
<td>A</td>
<td>3</td>
<td>1</td>
<td>4 - 3 = 1</td>
<td>1 - 1 = 0</td>
</tr>
<tr>
<td>B</td>
<td>1</td>
<td>4</td>
<td>6 - 1 = 5</td>
<td>5 - 4 = 1</td>
</tr>
<tr>
<td>C</td>
<td>4</td>
<td>2</td>
<td>8 - 4 = 4</td>
<td>4 - 2 = 2</td>
</tr>
<tr>
<td>D</td>
<td>0</td>
<td>6</td>
<td>16 - 0 = 16</td>
<td>16 - 6 = 10</td>
</tr>
<tr>
<td>E</td>
<td>2</td>
<td>3</td>
<td>11 - 2 = 9</td>
<td>9 - 3 = 6</td>
</tr>
</table>
<h3>Gantt Chart</h3>
<table>
<tr>
<td>D<br />0 - 1</td>
<td>
B<br />
1 - 3
</td>
<td>
A<br />
3 - 4
</td>
<td>
B<br />
4 - 6
</td>
<td>
C<br />
6 - 8
</td>
<td>
E<br />
8 - 11
</td>
<td>
D<br />
11 - 16
</td>
</tr>
</table>
<strong>Advantages: </strong> <br />
i) SRTF generates minimal average waiting time. <br />
ii) Provide a standard for other algo in terms of average waiting time.
<br />
iii) Better average response time compare to FCFS. <br />
<strong>Disadvantages: </strong> <br />
i) Algo can't be implemented as there is no way to know the burst time of a
process. <br />
ii) Process with longer CPU burst time requirement will go into starvation.
<br />
iii) No idea of priority, process with large burst time have poor response
time. <br />
<center><strong>Priority Algorithm (Non-Pre-emptive)</strong></center>
i) Here a priority is associated with each process. <br />
ii) At any instance of time out of all available process, CPU s allocated to
the process which has the highest priority (number maybe higher or lower)
<br />
iii) Tie is broken using FCFS order. <br />
iv) No importace to A.T or B.T. <br />
v) Supports both non-pre-emptive and pre-emptive version <br />
<h3>Chart (Non- pre-emptive)</h3>
<table>
<tr>
<th>Pid</th>
<th>A.T</th>
<th>B.T</th>
<th>Priority</th>
<th>T.A.T = E.T - A.T</th>
<th>W.T = T.A.T - B.T</th>
</tr>
<tr>
<td>A</td>
<td>0</td>
<td>4</td>
<td>2</td>
<td>4 - 0 = 4</td>
<td>4 - 4 = 0</td>
</tr>
<tr>
<td>B</td>
<td>1</td>
<td>3</td>
<td>3</td>
<td>15 - 1 = 14</td>
<td>14 - 3 = 11</td>
</tr>
<tr>
<td>C</td>
<td>2</td>
<td>1</td>
<td>4</td>
<td>12 - 2 = 10</td>
<td>10 - 1 = 9</td>
</tr>
<tr>
<td>D</td>
<td>3</td>
<td>5</td>
<td>5</td>
<td>9 - 3 = 6</td>
<td>6 - 5 = 1</td>
</tr>
<tr>
<td>E</td>
<td>4</td>
<td>2</td>
<td>5</td>
<td>11 - 4 = 7</td>
<td>7 - 2 = 5</td>
</tr>
</table>
<h3>Gantt Chart</h3>
<table>
<tr>
<td>
A<br />
0 - 4
</td>
<td>
D<br />
4 - 9
</td>
<td>
E<br />
9 - 11
</td>
<td>
C<br />
11 - 12
</td>
<td>
B<br />
12 - 15
</td>
</tr>
</table>
<h3>Chart (Pre-emptive)</h3>
<table>
<tr>
<th>Pid</th>
<th>A.T</th>
<th>B.T</th>
<th>Priority</th>
<th>T.A.T = E.T - A.T</th>
<th>W.T = T.A.T - B.T</th>
</tr>
<tr>
<td>A</td>
<td>0</td>
<td>4</td>
<td>2</td>
<td>15 - 0 = 15</td>
<td>15 - 4 = 11</td>
</tr>
<tr>
<td>B</td>
<td>1</td>
<td>3</td>
<td>3</td>
<td>12 - 1 = 11</td>
<td>11 - 3 = 8</td>
</tr>
<tr>
<td>C</td>
<td>2</td>
<td>1</td>
<td>4</td>
<td>3 - 2 = 1</td>
<td>1 - 1 = 0</td>
</tr>
<tr>
<td>D</td>
<td>3</td>
<td>5</td>
<td>5</td>
<td>8 - 3 = 5</td>
<td>5 - 5 = 0</td>
</tr>
<tr>
<td>E</td>
<td>4</td>
<td>2</td>
<td>5</td>
<td>10 - 4 = 6</td>
<td>6 - 2 = 4</td>
</tr>
</table>
<h3>Gantt Chart</h3>
<table>
<tr>
<td>
A<br />
0 - 1
</td>
<td>
B<br />
1 - 2
</td>
<td>
C<br />
2 - 3
</td>
<td>
D<br />
3 - 8
</td>
<td>
E<br />
8 - 10
</td>
<td>
B<br />
10 - 12
</td>
<td>
A<br />
12 - 15
</td>
</tr>
</table>
<strong>Advantages: </strong> <br />
i) provides a facility of priority speacially for system process. <br />
ii) Allows to run important process first even if it is a userprocess.
<br />
<strong>Disadvantages: </strong> <br />
i) Here process with the smaller priority may starve for the CPU. <br />
ii) No idea of response time or waiting time. <br />
<strong>N.B: -</strong> Ageing is a technique of gradually increasing the
parity of process that wait in the system for long time. <br />
<center><strong>Round-Robin</strong></center>
<br />
i) Designed for time sharing system. <br />
ii) It divides time of the CPUamong the processes in ready state, and
circularly repeat the same process. <br />
iii) Here we fix a time quantum for a process to execute a process. After
time quantum limit exceed the next process will come to queuefor executing.
<br />
iv) RR is pre-emptive in nature. <br />
<h3>Chart (Pre-emptive) TQ = 2</h3>
<table>
<tr>
<th>Pid</th>
<th>A.T</th>
<th>B.T</th>
<th>T.A.T = E.T - A.T</th>
<th>W.T = T.A.T - B.T</th>
</tr>
<tr>
<td>A</td>
<td>0</td>
<td>5</td>
<td>13 - 0 = 13</td>
<td>13 - 5 = 8</td>
</tr>
<tr>
<td>B</td>
<td>1</td>
<td>3</td>
<td>12 - 1 = 11</td>
<td>11 - 3 = 8</td>
</tr>
<tr>
<td>C</td>
<td>2</td>
<td>1</td>
<td>5 - 2 = 3</td>
<td>3 - 1 = 2</td>
</tr>
<tr>
<td>D</td>
<td>3</td>
<td>2</td>
<td>9 - 3 = 6</td>
<td>6 - 2 = 4</td>
</tr>
<tr>
<td>E</td>
<td>4</td>
<td>3</td>
<td>11 - 4 = 7</td>
<td>7 - 3 = 4</td>
</tr>
</table>
<h3>Gantt Chart</h3>
<table>
<tr>
<td>
A<br />
0 - 2
</td>
<td>
B<br />
2 - 4
</td>
<td>
C<br />
4 - 5
</td>
<td>
A<br />
5 - 7
</td>
<td>
D<br />
7 - 9
</td>
<td>
E<br />
9 - 11
</td>
<td>
B<br />
11 - 12
</td>
<td>
A<br />
12 - 13
</td>
<td>
E<br />
13 - 14
</td>
</tr>
</table>
<strong>Advantages: </strong> <br />
i) Perform best in terms of response time. <br />
ii) Works well in case of time sharing, client server architecture and
intermediate system. <br />
iii) Kind of SJF implementation. <br />
<strong>Disadvantages: </strong> <br />
i) Bigger process may starve. <br />
ii) performance depends ehavily on time quantum. <br />
iii) No idea of priority. <br />
<center><strong>Process Synchronization</strong></center>
<strong>Race Condition: - </strong>multiple processes are involved in the
system and these process access some shared resources now if they access in
a mix fashion then system gives different result sometime. <br />
<strong>Critical Section Problem</strong> <br />
i) Mutual Exclusion - Critical Sectin will only be accessed by single
process at a time.<br />
ii) Progress - Those process should enter C.S who wants to.<br />
iii) Bounded Wait (optional) - After a specific space of time the process
should exit CS to overcome starving. <br />
<strong>Semaphore</strong> <br />
A semaphore is an integer variable int s that apart from initialization is
accessed through two atomic standards.
<pre>
i) wait(s) => wait(s){
while(s <= 0);
s--;
}
ii) signal(s) => signal(s){
s++;
}
</pre>
<pre>
<strong>Code</strong>
do{
wait(s);
//critical Section
signal(s)
//remainder section
}
while(true);
</pre>
<strong>Uses of Semaphore: </strong> <br />
i) Critical Section Problem solving. <br />
ii) To decide order of execution. <br />
iii) For managing resources. <br />
<strong>What is Deadlock?</strong>
If all the resources are circularly depending on each other then the state
is called deadlock.
<strong>Four condition of deadlock: </strong> <br>
i) Mutual Exclusion <br> ii) Hold and Wait <br> iii) non-pre-emtion process <br> iv)
Circular wait <br>
<strong>Deadlock handling method: </strong> <br>
i) Prevention <br> ii) Avoidance (banker's algo) <br> iii) detection and recovery <br> iv) Ignorance /
ostrich method <br>
</body>
</html>