Disk Scheduling
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Secondary-Storage DISK-SCHEDULING ALGOS Lecture Series By : Website ::
Er. Kanwalvir Singh Dhindsa www.dhindsa.info http://groups.google.com/group/os-2007
E-Mail ::
sys.operating @ gmail.com O.S. by Er. K.S.Dhindsa © 2007
Selecting a Disk-Scheduling Algorithm • SSTF is common and has a natural appeal • SCAN and C-SCAN perform better for systems that place a heavy load on the disk • Performance depends on the number and types of request • Requests for disk service can be influenced by the fileallocation method
O.S. by Er. K.S.Dhindsa © 2007
Secondary-Storage Structures • Magnetic disks provide bulk of secondary storage of modern computers – Transfer rate is rate at which data flow between drive and computer – Positioning time (random-access time) is time to move disk arm to desired cylinder (seek time) and time for desired sector to rotate under the disk head (rotational latency) – Head crash results from disk head making contact with the disk surface
O.S. by Er. K.S.Dhindsa © 2007
Moving-head Disk Mechanism
O.S. by Er. K.S.Dhindsa © 2007
Secondary-Storage Structures • Magnetic tape – – – –
Was early secondary-storage medium Access time slow Random access ~ many times slower than disk Mainly used for backup, storage of infrequently-used data, transfer medium between systems – Kept in spool and wound or rewound past read-write head – Once data under head, transfer rates comparable to disk – From MB’s to lots of GB’s of storage O.S. by Er. K.S.Dhindsa © 2007
Disk Scheduling OS is responsible for using hardware efficiently — for the disk drives, this means having a fast access time and disk bandwidth Access time has two major components – Seek time is the time for the disk to move the heads to the cylinder containing the desired sector – Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head Disk bandwidth is the total number of bytes transferred, divided by the total time between the first request for service and the completion of the last transfer O.S. by Er. K.S.Dhindsa © 2007
Disk Scheduling • Several algorithms exist to schedule the servicing of disk I/O requests • Illustrate these with a request queue (0-199) 98, 183, 37, 122, 14, 124, 65, 67 Head pointer 53
O.S. by Er. K.S.Dhindsa © 2007
FCFS SCHEDULING
O.S. by Er. K.S.Dhindsa © 2007
SSTF SCHEDULING Selects the request with the minimum seek time from the current head position SSTF scheduling is a form of SJF scheduling; may cause starvation of some requests Not an optimal scheduling algorithm O.S. by Er. K.S.Dhindsa © 2007
SSTF (Cont.)
O.S. by Er. K.S.Dhindsa © 2007
SCAN SCHEDULING Disk arm starts at one end of the disk, and moves toward the other end, servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues Also called as Elevator algorithm O.S. by Er. K.S.Dhindsa © 2007
SCAN (Cont.)
O.S. by Er. K.S.Dhindsa © 2007
C-SCAN SCHEDULING Provides a more uniform wait time than SCAN The head moves from one end of the disk to the other, servicing requests as it goes. When it reaches the other end, however, it immediately returns to the beginning of the disk, without servicing any requests on the return trip Treats the cylinders as a circular list that wraps around from the last cylinder to the first one O.S. by Er. K.S.Dhindsa © 2007
C-SCAN (Cont.)
O.S. by Er. K.S.Dhindsa © 2007
C-LOOK SCHEDULING Version of C-SCAN Arm only goes as far as the last request in each direction, then reverses direction immediately, without first going all the way to the end of the disk
O.S. by Er. K.S.Dhindsa © 2007
C-LOOK (Cont.)
O.S. by Er. K.S.Dhindsa © 2007
Characteristics of I/O Devices
O.S. by Er. K.S.Dhindsa © 2007
Secondary-Storage DISK-SCHEDULING ALGOS
Lecture Series By : Website ::
Er. Kanwalvir Singh Dhindsa www.dhindsa.info http://groups.google.com/group/os-2007
E-Mail ::
sys.operating @ gmail.com
O.S. by Er. K.S.Dhindsa © 2007
Er. Kanwalvir Singh Dhindsa www.dhindsa.info http://groups.google.com/group/os-2007
E-Mail ::
sys.operating @ gmail.com O.S. by Er. K.S.Dhindsa © 2007
Selecting a Disk-Scheduling Algorithm • SSTF is common and has a natural appeal • SCAN and C-SCAN perform better for systems that place a heavy load on the disk • Performance depends on the number and types of request • Requests for disk service can be influenced by the fileallocation method
O.S. by Er. K.S.Dhindsa © 2007
Secondary-Storage Structures • Magnetic disks provide bulk of secondary storage of modern computers – Transfer rate is rate at which data flow between drive and computer – Positioning time (random-access time) is time to move disk arm to desired cylinder (seek time) and time for desired sector to rotate under the disk head (rotational latency) – Head crash results from disk head making contact with the disk surface
O.S. by Er. K.S.Dhindsa © 2007
Moving-head Disk Mechanism
O.S. by Er. K.S.Dhindsa © 2007
Secondary-Storage Structures • Magnetic tape – – – –
Was early secondary-storage medium Access time slow Random access ~ many times slower than disk Mainly used for backup, storage of infrequently-used data, transfer medium between systems – Kept in spool and wound or rewound past read-write head – Once data under head, transfer rates comparable to disk – From MB’s to lots of GB’s of storage O.S. by Er. K.S.Dhindsa © 2007
Disk Scheduling OS is responsible for using hardware efficiently — for the disk drives, this means having a fast access time and disk bandwidth Access time has two major components – Seek time is the time for the disk to move the heads to the cylinder containing the desired sector – Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head Disk bandwidth is the total number of bytes transferred, divided by the total time between the first request for service and the completion of the last transfer O.S. by Er. K.S.Dhindsa © 2007
Disk Scheduling • Several algorithms exist to schedule the servicing of disk I/O requests • Illustrate these with a request queue (0-199) 98, 183, 37, 122, 14, 124, 65, 67 Head pointer 53
O.S. by Er. K.S.Dhindsa © 2007
FCFS SCHEDULING
O.S. by Er. K.S.Dhindsa © 2007
SSTF SCHEDULING Selects the request with the minimum seek time from the current head position SSTF scheduling is a form of SJF scheduling; may cause starvation of some requests Not an optimal scheduling algorithm O.S. by Er. K.S.Dhindsa © 2007
SSTF (Cont.)
O.S. by Er. K.S.Dhindsa © 2007
SCAN SCHEDULING Disk arm starts at one end of the disk, and moves toward the other end, servicing requests until it gets to the other end of the disk, where the head movement is reversed and servicing continues Also called as Elevator algorithm O.S. by Er. K.S.Dhindsa © 2007
SCAN (Cont.)
O.S. by Er. K.S.Dhindsa © 2007
C-SCAN SCHEDULING Provides a more uniform wait time than SCAN The head moves from one end of the disk to the other, servicing requests as it goes. When it reaches the other end, however, it immediately returns to the beginning of the disk, without servicing any requests on the return trip Treats the cylinders as a circular list that wraps around from the last cylinder to the first one O.S. by Er. K.S.Dhindsa © 2007
C-SCAN (Cont.)
O.S. by Er. K.S.Dhindsa © 2007
C-LOOK SCHEDULING Version of C-SCAN Arm only goes as far as the last request in each direction, then reverses direction immediately, without first going all the way to the end of the disk
O.S. by Er. K.S.Dhindsa © 2007
C-LOOK (Cont.)
O.S. by Er. K.S.Dhindsa © 2007
Characteristics of I/O Devices
O.S. by Er. K.S.Dhindsa © 2007
Secondary-Storage DISK-SCHEDULING ALGOS
Lecture Series By : Website ::
Er. Kanwalvir Singh Dhindsa www.dhindsa.info http://groups.google.com/group/os-2007
E-Mail ::
sys.operating @ gmail.com
O.S. by Er. K.S.Dhindsa © 2007
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