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Disk scheduling.49

Disk scheduling.49






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    Disk scheduling.49 Disk scheduling.49 Presentation Transcript

    • Disk Scheduling ,FCFS diskscheduling algorithms http://raj-os.blogspot.in/ 1
    • RecapIn the last class, you have learnt: Various allocation methods - Their advantages and disadvantages http://raj-os.blogspot.in/ 3
    • ObjectivesOn Completion of this period, you would be able to know• The Need for Disk Scheduling• FCFS disk scheduling algorithm http://raj-os.blogspot.in/ 4
    • Need for Disk Scheduling• Most jobs depend heavily on disk• It is important that disk service be as fast as possible for – Loading – Input – Output of files http://raj-os.blogspot.in/ 5
    • Need for Disk Scheduling• The operating system can improve average disk service time – By scheduling the requests for disk access http://raj-os.blogspot.in/ 6
    • Need for Disk Scheduling• Disk speed is composed of three parts• First – To access a block the system must move the head to the appropriate track or cylinder http://raj-os.blogspot.in/ 7
    • Need for Disk Scheduling• This head movement is called a SEEK• The time to complete it is SEEK TIME http://raj-os.blogspot.in/ 8
    • Need for Disk Scheduling• Once the head is at the right track – It must wait until the desired block rotates under read write head• This operation is called LATENCY(ROTATIOAL)• The delay is called LATENCY TIME http://raj-os.blogspot.in/ 9
    • Need for Disk Scheduling• The Transfer Time – The time required to transfer of data between the disk and main memory• The total time to service a disk request is the sum of – Seek time – Latency (rotational) time & – Transfer time http://raj-os.blogspot.in/ 10
    • Need for Disk Scheduling• The disk bandwidth is the ratio of The total number of bytes transferred The time between the first request of service and completion of transfer http://raj-os.blogspot.in/ 11
    • Disk Scheduling• Whenever a process needs a I/O from the disk – It issues a system call to the OS• The request has several pieces of information http://raj-os.blogspot.in/ 12
    • Disk Scheduling• Whether this operation is input or output• What the disk address for the transfer is• What is the memory address for the transfer• What the number of sectors to be transferred http://raj-os.blogspot.in/ 13
    • Disk Scheduling• If the desired disk drive and controller are available – The service can be serviced immediately• If the drive controller is busy – Any new requests will be placed in the queue http://raj-os.blogspot.in/ 14
    • Disk Scheduling• Multiprogramming system with many process• The disk queue may often have several pending requests• Thus when one request is completed• The operating system chooses which pending request is to service next http://raj-os.blogspot.in/ 15
    • Disk Scheduling• How does operating system makes this choice – Any of the disk scheduling algorithms can be used• We shall discuss various disk scheduling algorithms http://raj-os.blogspot.in/ 16
    • Various Disk Scheduling• FCFS Scheduling• SSTF Scheduling• SCAN Scheduling• C-SCAN Scheduling• LOOK Scheduling• C-LOOK Scheduling http://raj-os.blogspot.in/ 17
    • Disk Scheduling• All these algorithms are based on criteria i.e. – Minimize seek time• Seek time ≈ Seek distance http://raj-os.blogspot.in/ 18
    • Quiz1. The operating system can improve average disk service time by scheduling the requests for disk access [T/F] TRUE2. The time required to transfer of data between the disk and main memory is called TRANSFER TIME http://raj-os.blogspot.in/ 19
    • Quiz3. If the desired disk drive and controller are available the service can be serviced immediately [T/F] TRUE4. The disk scheduling algorithms are based on criteria Minimize seek time http://raj-os.blogspot.in/ 20
    • Quiz1. The FCFS algorithm does not provide best (average) access [T/F] TRUE2. The head movement in the FCFS algorithm is minimal in all cases [T/F] FALSE http://raj-os.blogspot.in/ 21
    • Other subject materials• Web designing• Micro processors• C++ tutorials• javahome http://raj-os.blogspot.in/ 22