Skip to main content

Scheduling Algorithms

    Scheduling Algorithms

CPU scheduling algorithm deal with the problem of deciding which of the processes in the ready queue is to be allocated the CPU . six commonly used scheduling algorithms are:

1. first-come First-served(FCFS)

2. Shortest job First(SJF)

3. Priority scheduling

4. Round-Robin Scheduling(RR)

5. Multi-Level Queue Scheduling(MLQ)

6. Multi-Level Feedback Queue Scheduling (MFQ)

     

First-Come First-Served Scheduling (FCFS)

·      It is simplest and the most straight forward of all scheduling algorithms.

·      In this scheduling, the process that request the CPU first is allocated CPU first. Thus  the name first come first served.

·      We can say that in FCFS scheduling, a process is allocated CPU time according to the arrival time of a process.

·      The implementation of FCFS policy is easily manged with a FIFO queue

·      When a process enters the ready queue, its PCB is linked onto tail of the queue.

·      When CPU is free, it is allocated to the head of ready queue.the running process is then removed from the queue.

·      FCFS scheduling algorithm is non-preemptive. Once the CPU is allocated to a process, that process keeps the CPU until it releases the CPU, either by terminating or by requesting I/O

 

1.     FCFS tends to favour CPU-bound processes. Consider . in system with one CPU bound process  and a number of I/O bound processes. In such as system, the following scenario may arise:

·      The CPU bound process will get the CPU and holds it.

·      During this all the other processes will finish their I/O and move  into the ready queue, waitin for CPU. When these processes are waiting in ready queue, the I/O devices are idle.

·      After some time, the CPU-bound process finishes its CPU burst (CPU burst time indicates for how much time . the process needs the CPU)and moves to an I/o devices . at this time all the I/O bound processes.

·      The CPU-bound process will then move back to ready queue and be allocated the CPU. Again all the I/O processes end up waiting in the ready queue until the CPU-bound process is done.


  • Advantages –
    1. It is simple and easy to understand.
  • Disadvantages –
    1. The process with less execution time suffer i.e. waiting time is often quite long.
    2. Favors CPU Bound process then I/O bound process.
    3. Here, first process will get the CPU first, other processes can get CPU only after the current process has finished it’s execution. Now, suppose the first process has large burst time, and other processes have less burst time, then the processes will have to wait more unnecessarily, this will result in more average waiting time, i.e., Convey effect.
    4. This effect results in lower CPU and device utilization.
    5. FCFS algorithm is particularly troublesome for time-sharing systems, where it is important that each user get a share of the CPU at regular intervals.

  • Example of FCFS scheduling

    A real-life example of the FCFS method is buying a movie ticket on the ticket counter. In this scheduling algorithm, a person is served according to the queue manner. The person who arrives first in the queue first buys the ticket and then the next one. This will continue until the last person in the queue purchases the ticket. Using this algorithm, the CPU process works in a similar manner.

  • How FCFS Works? Calculating Average Waiting Time

    Here is an example of five processes arriving at different times. Each process has a different burst time.

    ProcessBurst timeArrival time
    P162
    P235
    P381
    P430
    P544

    Using the FCFS scheduling algorithm, these processes are handled as follows.

    Step 0) The process begins with P4 which has arrival time 0

    Step 1) At time=1, P3 arrives. P4 is still executing. Hence, P3 is kept in a queue.

    ProcessBurst timeArrival time
    P162
    P235
    P381
    P430
    P544

    Step 2) At time= 2, P1 arrives which is kept in the queue.

    ProcessBurst timeArrival time
    P162
    P235
    P381
    P430
    P544

    Step 3) At time=3, P4 process completes its execution.

    Step 4) At time=4, P3, which is first in the queue, starts execution.

    ProcessBurst timeArrival time
    P162
    P235
    P381
    P430
    P544

    Step 5) At time =5, P2 arrives, and it is kept in a queue.

    ProcessBurst timeArrival time
    P162
    P235
    P381
    P430
    P544

    Step 6) At time 11, P3 completes its execution.

    Step 7) At time=11, P1 starts execution. It has a burst time of 6. It completes execution at time interval 17

    Step 8) At time=17, P5 starts execution. It has a burst time of 4. It completes execution at time=21

    Step 9) At time=21, P2 starts execution. It has a burst time of 2. It completes execution at time interval 23

    Step 10) Let's calculate the average waiting time for above example.

    Waiting time = Start time - Arrival time

    P4 = 0-0 = 0

    P3 = 3-1 = 2

    PI = 11-2 = 9

    P5= 17-4 = 13

    P2= 21-5= 16

    Average Waiting Time

    = 40/5= 8

 

 

Comments

Post a Comment

Popular posts from this blog

Defination of OS(operating system) and its concepts

    What do you mean by operating system?     Definition :  An operating system is a program that act as an interface between the user of a computer and the                                      Computer hardware. Operating system is a first program that gets loaded into the memory through a process called booting. Concepts of operating system : ·                       The purpose of operating system is to provide an environment in which a user can execute program in a convenient and efficient manner. ·                       Operating system is an integrated set of program that ma...
3 comments
Read more

Multi user Operating System

  Multi user operating system ·           In a multi-user operating system, multiple number of user can access different resources of a computer at a same time. ·           The access is provided using a network that consists of various personal computer attached to a mainframe computer system.                                                              diagram of multi -user operating system       ·           The various personal computer can send and receive information to mainframe computer system. ·           The example    of multi-user OS are UNIX, windows 2000,novell netware.            sing...
Post a Comment
Read more

Batch Processing Operating System

  Batch processing system ·           Batch processing is one of the oldest method    of running the programs ·           The computer in the past were very large in size and their I/O devices were very different from those that are used today. The job processing was not interactive as it is today. ·           The user did not interact directly with computer system.   ·           The process scheduling , memory management, file management and I/Omanagement functions are quite simple in batch processing system   1.         Process scheduling (i.e. allocation strategy for a processor is typically in order of their arrival i.e. first come first served(FCFS)basis.   2.         Memory management  is done by divi...
Post a Comment
Read more

Scheduler

  Scheduler   Scheduling Objectives Here, are important objectives of Process scheduling Maximize the number of interactive users within acceptable response times. Achieve a balance between response and utilization. Avoid indefinite postponement and enforce priorities. It also should give reference to the processes holding the key resources  Process Scheduling handles the selection of a process for the processor on the basis of a scheduling algorithm and also the removal of a process from the processor. It is an important part of multiprogramming in operating system. Process scheduling involves short-term scheduling, medium-term scheduling and long-term scheduling. Details about these are given as follows − Long-Term Scheduling Long-term scheduling involves selecting the processes from the storage pool in the secondary memory and loading them into the ready queue in the main memory for execution. This is handled by the long-term scheduler or job scheduler. The long-term s...
Post a Comment
Read more

Time Sharing System and its Requirements

  Time sharing  system ·           Time sharing refers to the allocation of computer resources in a time dependent fashion to several program simultaneously ·           A time sharing system has many user terminals that are connected to same computer simultaneously. Using these terminal, different users can work on a system at the same time ·           Thus, it uses multi programming with a special CPU scheduling among all the last one, and then again beginning from the first one ·           In time sharing system, the CPU time is divided among all the users on schedule basis. ·           It release the CPU under any of the following three conditions: 1.         When the allotted time slice expires. 2.    ...
Post a Comment
Read more