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Chapter 12Mass Storage Structure


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Disk-scheduling

DisciplinesOverview


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Disk-scheduling Disciplines(1)

FCFS (First Come, First Served) Simplest form of disk-scheduling.

Fair, but does not provide the fastest service(seek time may be long).

what comes in first is handled first, what

comes in next waits until the first is finished,

etc.


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SSTF (Shortest Seek Time First) Selects the request that is closest to the disk

arm position Good at reducing seeks

May result in starvation


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SCAN Adopts an elevator algorithm. It takes the

closest request in the direction of travel. Worst case: if a disk is heavily loaded withrequests, a new request at a location that hasbeen just recently scanned can wait for almost

two full scans of the disk


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C-SCAN (Circular SCAN) The disk arm always serves requests by

scanning in one direction Once the arm finishes scanning for onedirection, it quickly returns to the 0th track forthe next round of scanning


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LOOK Similar to SCAN

The difference is that LOOK uses informationabout the requests to change the direction ofthe head when it knows that there are norequests beyond the current point

improves both the throughput and theresponse time


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Exercise 12.1

None of the disk-scheduling disciplines,except FCFS, is truly fair (starvation may

occur).a. Explain why this assertion is true.


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Answer 12.1.a

New requests for the track over which thehead is currently resides can arrive as

quickly as these requests are beingserviced.

For example, In SSTF, some process may have to wait a longtime until it's request(s) are satisfied if newrequests with shorter seek time keep arriving.

In SCAN, new arrived requests may be closer tothe current head (in the right direction), hence anold request may be kept waiting.


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Exercise 12.1.b

Describe a way to a modify algorithmssuch as SCAN to ensure fairness.


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Answer 12.1.b

All requests older than somepredetermined age could be forced to

the top of the queue An associated bit for each could be set to

indicate that no new request could bemoved ahead of these requests


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Exercise 12.1.c

Explain why fairness is an important goalin a time sharing system.


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Answer 12.1.c

To prevent unusually long response times.


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Exercise 12.1.d

Give three or more examples ofcircumstances in which it is important that

the operating system be unfair in servingI/O requests.


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Answer 12.1.d

Kernel-initiated I/O take precedence overuser requests.

If the kernel supports real-time processpriorities, the I/O requests for thoseprocesses should be favored.

Paging and swapping should take priorityover user requests.


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Exercise 12.2

Suppose that a disk drive has 5000 cylinders,numbered 0 to 4999. The drive is currently

serving a request at cylinder 143, and theprevious request was cylinder 125. Thequeue pending requests, in FIFO order, is

86, 1470, 913, 1774, 948, 1509, 1022, 1750, 130

Starting from the current head position, whatis the total distance (in cylinders) that thedisk arm moves to satisfy all the pendingrequests


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18000

125

143

86

1470

913

1774

948

1509

1022

1750

130

a. First Come First Served


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FCFS total seek time

From cylinder To cylinder Seek Time

143 86 57

86 1470 13841470 913 557

913 1774 861

1774 948 826

948 1509 561

1509 1022 487

1022 1750 728

1750 130 1620

Total Seek Time 7081


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18000

125

143

86

1470

913

1774

948

1509

1022

1750

130

b. Shortest Seek Time First

Total seek distance is 1745


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18000

125

143

86

1470

913

1774

948

1509

1022

1750

1304999

c. SCAN



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18000

125

143

86

1470

913

1774

948

1509

1022

1750

130

d. LOOK



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18000

125

143

86

1470

913

1774

948

1509

1022

1750

130

4999

e. Circular SCAN


0


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18000

125

143

86

1470

913

1774

948

1509

1022

1750

130

f. Circular LOOK


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