CPU scheduling algorithms in OS

Aug 24, 2013Download as PPTX, PDF20 likes36,079 views

This document discusses different CPU scheduling algorithms. It describes the First Come First Serve (FCFS), Shortest Job First (SJF), Priority Scheduling (PS), and Round Robin (RR) algorithms. Each algorithm is evaluated based on criteria like average turnaround time, waiting time, and CPU utilization. FCFS is found to have the highest CPU utilization but higher average turnaround times. SJF provides the lowest average turnaround times but can cause starvation of longer jobs. RR provides fairness but the time quantum setting impacts efficiency. The best algorithm depends on the specific performance measures and system requirements.

INTRODUCTION
o The CPU scheduling is used to improve its
efficiency.
o It is used to allocate resources among competing
processes.
o Maximum CPU utilization is obtained with
multiprogramming.
• The processes which are to be executed are in ready queue.
• Every time the running process is blocked , the process in the
ready queue takes over the CPU.
o A process may be
• I/O BOUND: Involves more I/O operation. Ex : reading from
disk.
• CPU BOUND: Involves more computation. Ex : calculating the
sum of 2 numbers from memory.

PROCESS STATES
o The 3 states of a process are ready , running and
wait states.
o CPU scheduling is affected by the following set
of circumstances:
• A process switches from running to waiting state.
• A process switches from running to ready state .
• A process switches from waiting to ready state .
• A processes switches from running to terminated state.

CPU SCHEDULING CRITERIA
o CPU Utilization: Percent of time that the CPU is
busy executing a process.
o Throughput: Number of processes executed per
unit time.
o Turnaround Time: The interval of time between
submission of a process and its completion.
o Waiting Time: The amount of time the process
spends in the ready queue waiting for the CPU.
o Response Time: The time between submission
of requests and first response to the request.

First Come First Served(FCFS)
o The process which enters the queue first is
executed first.
o It is non-preemptive.
Process Arrival Service TT NTT
P1 0 6 6 1
P2 2 4 8 2
P3 4 1 7 7
P1 P2 P3
0 6 10 11

FCFS
o Advantages:
It is simple and easy to understand.
o Disadvantages:
The process with less execution time suffer.
Favours CPU bound process then I/O bound
process.

Shortest Job First(SJF)
o The process with the lowest execution time is
executed first.
o It can be either preemptive or non preemptive.
Process Arrival Service TT NTT
P1 0 6 6 1
P2 2 4 9 2.2
P3 4 1 3 3
P1 P3 P2
0 6 7 11

SJF
o Advantages:
Shortest jobs are favoured.
o Disadvantages:
When there are more number of shorter jobs,
longer jobs starve.

Priority Scheduling(PS)
o Each process is associated with priority and the
CPU is allocated to each of them based on their
priorities.
o Multiple process with same priority are dealt
using FCFS.
Process Arrival Service TT NTT
P1 0 6 6 1
P2 2 4 8 2
P3 4 1 7 7
P1 P2 P3
0 6 10 11

PS
o Advantages:
Priority scheduling provides a good mechanism
where the relative importance of each process may be
precisely defined.
o Disadvantages:
• If high priority processes use up a lot of CPU
time, lower priority processes may starve and be
postponed indefinitely.
• The situation where a process never gets
scheduled to run is called starvation.
• Another problem with priority scheduling is
deciding which process gets which priority level
assigned to it.

Round Robin(RR)
o Each process is allotted a time slot(q). After this
time has elapsed, the process is pre-empted and
added to the end of the ready queue.
o Performance of the round robin algorithm
◦ q large  FCFS
◦ q small  q must be greater than the context
switch time; otherwise, the overhead is too high

RR
Process Arrival Service TT NTT
P1 0 6 12 2
P2 2 4 8 2
P3 4 2 2 1
q=2
o Advantages:
Round robin scheduling is fair in that every process
gets an equal share of the CPU.
P1
0 62
P2
4
P3 P1
8
P2
10
P1
12

RR
o Disadvantages:
Setting the quantum too short, increases the overhead
and lowers the CPU efficiency, but setting it too long
may cause poor response to short processes.

ANALYSIS AND RESULTS
o Comparing the average turnaround time and
waiting time, the algorithms are ranked as
SJF,FCFS and PS.
o Based on average CPU utilization the algorithms
are ranked as FC,PS and SJF.
o In terms of execution speed, FCFS was found to
be consistently faster than PS and SJF.
Thus according to the performance
measure the corresponding scheduling algorithm
is recommended to the CPU.

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Introduction All the materials around us are made up of elements. These elements can be broadly divided into two major groups: Metals Non-Metals Each group has its own unique physical and chemical properties. Let's understand them one by one. Physical Properties 1. Appearance Metals: Shiny (lustrous). Example: gold, silver, copper. Non-metals: Dull appearance (except iodine, which is shiny). 2. Hardness Metals: Generally hard. Example: iron. Non-metals: Usually soft (except diamond, a form of carbon, which is very hard). 3. State Metals: Mostly solids at room temperature (except mercury, which is a liquid). Non-metals: Can be solids, liquids, or gases. Example: oxygen (gas), bromine (liquid), sulphur (solid). 4. Malleability Metals: Can be hammered into thin sheets (malleable). Non-metals: Not malleable. They break when hammered (brittle). 5. Ductility Metals: Can be drawn into wires (ductile). Non-metals: Not ductile. 6. Conductivity Metals: Good conductors of heat and electricity. Non-metals: Poor conductors (except graphite, which is a good conductor). 7. Sonorous Nature Metals: Produce a ringing sound when struck. Non-metals: Do not produce sound. Chemical Properties 1. Reaction with Oxygen Metals react with oxygen to form metal oxides. These metal oxides are usually basic. Non-metals react with oxygen to form non-metallic oxides. These oxides are usually acidic. 2. Reaction with Water Metals: Some react vigorously (e.g., sodium). Some react slowly (e.g., iron). Some do not react at all (e.g., gold, silver). Non-metals: Generally do not react with water. 3. Reaction with Acids Metals react with acids to produce salt and hydrogen gas. Non-metals: Do not react with acids. 4. Reaction with Bases Some non-metals react with bases to form salts, but this is rare. Metals generally do not react with bases directly (except amphoteric metals like aluminum and zinc). Displacement Reaction More reactive metals can displace less reactive metals from their salt solutions. Uses of Metals Iron: Making machines, tools, and buildings. Aluminum: Used in aircraft, utensils. Copper: Electrical wires. Gold and Silver: Jewelry. Zinc: Coating iron to prevent rusting (galvanization). Uses of Non-Metals Oxygen: Breathing. Nitrogen: Fertilizers. Chlorine: Water purification. Carbon: Fuel (coal), steel-making (coke). Iodine: Medicines. Alloys An alloy is a mixture of metals or a metal with a non-metal. Alloys have improved properties like strength, resistance to rusting.

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CPU scheduling algorithms in OS

2. INTRODUCTION o The CPU scheduling is used to improve its efficiency. o It is used to allocate resources among competing processes. o Maximum CPU utilization is obtained with multiprogramming. • The processes which are to be executed are in ready queue. • Every time the running process is blocked , the process in the ready queue takes over the CPU. o A process may be • I/O BOUND: Involves more I/O operation. Ex : reading from disk. • CPU BOUND: Involves more computation. Ex : calculating the sum of 2 numbers from memory.

3. PROCESS STATES o The 3 states of a process are ready , running and wait states. o CPU scheduling is affected by the following set of circumstances: • A process switches from running to waiting state. • A process switches from running to ready state . • A process switches from waiting to ready state . • A processes switches from running to terminated state.

4. CPU SCHEDULING CRITERIA o CPU Utilization: Percent of time that the CPU is busy executing a process. o Throughput: Number of processes executed per unit time. o Turnaround Time: The interval of time between submission of a process and its completion. o Waiting Time: The amount of time the process spends in the ready queue waiting for the CPU. o Response Time: The time between submission of requests and first response to the request.

5. First Come First Served(FCFS) o The process which enters the queue first is executed first. o It is non-preemptive. Process Arrival Service TT NTT P1 0 6 6 1 P2 2 4 8 2 P3 4 1 7 7 P1 P2 P3 0 6 10 11

6. FCFS o Advantages: It is simple and easy to understand. o Disadvantages: The process with less execution time suffer. Favours CPU bound process then I/O bound process.

7. Shortest Job First(SJF) o The process with the lowest execution time is executed first. o It can be either preemptive or non preemptive. Process Arrival Service TT NTT P1 0 6 6 1 P2 2 4 9 2.2 P3 4 1 3 3 P1 P3 P2 0 6 7 11

8. SJF o Advantages: Shortest jobs are favoured. o Disadvantages: When there are more number of shorter jobs, longer jobs starve.

9. Priority Scheduling(PS) o Each process is associated with priority and the CPU is allocated to each of them based on their priorities. o Multiple process with same priority are dealt using FCFS. Process Arrival Service TT NTT P1 0 6 6 1 P2 2 4 8 2 P3 4 1 7 7 P1 P2 P3 0 6 10 11

10. PS o Advantages: Priority scheduling provides a good mechanism where the relative importance of each process may be precisely defined. o Disadvantages: • If high priority processes use up a lot of CPU time, lower priority processes may starve and be postponed indefinitely. • The situation where a process never gets scheduled to run is called starvation. • Another problem with priority scheduling is deciding which process gets which priority level assigned to it.

11. Round Robin(RR) o Each process is allotted a time slot(q). After this time has elapsed, the process is pre-empted and added to the end of the ready queue. o Performance of the round robin algorithm ◦ q large  FCFS ◦ q small  q must be greater than the context switch time; otherwise, the overhead is too high

12. RR Process Arrival Service TT NTT P1 0 6 12 2 P2 2 4 8 2 P3 4 2 2 1 q=2 o Advantages: Round robin scheduling is fair in that every process gets an equal share of the CPU. P1 0 62 P2 4 P3 P1 8 P2 10 P1 12

13. RR o Disadvantages: Setting the quantum too short, increases the overhead and lowers the CPU efficiency, but setting it too long may cause poor response to short processes.

14. ANALYSIS AND RESULTS o Comparing the average turnaround time and waiting time, the algorithms are ranked as SJF,FCFS and PS. o Based on average CPU utilization the algorithms are ranked as FC,PS and SJF. o In terms of execution speed, FCFS was found to be consistently faster than PS and SJF. Thus according to the performance measure the corresponding scheduling algorithm is recommended to the CPU.

CPU scheduling algorithms in OS

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