Chapter 11 I/O Management and Disk SchedulingNo Buffering
Without a buffer, OS directly accesses the device as and when it needs
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busy waiting (like programmed I/O)
process suspension on an interrupt (like interrupt-driven I/O or DMA)
Problems
the program is hung up waiting for the relatively slow I/O to complete
interferes with swapping decisions by OS
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No Buffering
It is impossible to swap the process out completely because the data area must be locked in main memory before I/O
Otherwise, data may be lost or single-process deadlock may happen
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I/O Buffering
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disks and
USB drives
Transfers are made a block at a time
Can reference data by block number
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terminals
printers
most other devices that are not secondary storage
Transfer information as a stream of bytes
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Single Buffer
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Input transfers are made to system buffer
Block moved to user space when needed
The next block is immediately requested, expecting that the block will eventually be needed
Read ahead or Anticipated Input
A reasonable assumption as data is usually accessed sequentially
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Provide a speedup
User process can be processing one block of data while the next block is being read in
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Complicate the logic in OS
OS must keep track of the assignment of system buffers to user processes
Affect the swapping logic
Consider both the I/O operation and swapping involve the same disk
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Line-at-time or Byte-at-a-time
Terminals often deal with one line at a time with carriage return signaling the end of the line
Also line printer
Byte-at-a-time suits devices where a single keystroke may be significant
Also sensors and controllers
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Use two system buffers instead of one
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Each individual buffer is one unit in a circular buffer
Used when I/O operation must keep up with process
Follows the bounded-buffer producer/consumer model
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Buffering smoothes out peaks in I/O demand
But with enough demand eventually all buffers become full and their advantage is lost
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Disk Performance Parameters
Currently, disks are at least four orders of magnitude slower than main memory
performance of disk storage subsystem is of vital concern
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Positioning the
Read/Write Heads
When the disk drive is operating, the disk is rotating at constant speed.
To read or write, the head must be positioned at the desired track and at the beginning of the desired sector on that track.
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Access Time is the sum of:
Seek time: The time it takes to position the head at the desired track
Rotational delay or rotational latency: The time it takes for the beginning of the sector to reach the head
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Ta = Ts + 1 / (2r) + b / (rN)
where Ts = average seek time
b = no. of bytes to be transferred
N = no. of bytes on a track
r = rotation speed, in revolutions / sec.
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Policies
To compare various schemes, consider a disk head is initially located at track 100.
assume a disk with 200 tracks and that the disk request queue has random requests in it.
The requested tracks, in the order received by the disk scheduler, are
55, 58, 39, 18, 90, 160, 150, 38, 184.
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Fair to all processes
May have good performance if most requests are to clustered file sectors
Approaches random scheduling in performance if there are many processes
disk arm movement
Priority
Control of the scheduling is outside the control of disk management software
Goal is not to optimize disk use but to meet other objectives
Often, short batch jobs & interactive jobs are given higher priority than longer jobs
Provide high throughput & good interactive response time
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Good for transaction processing systems
The device is given to the most recent user so there should be little arm movement for moving through a sequential file
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Shortest Service
Time First
Select the disk I/O request that requires the least movement of the disk arm from its current position
Always choose the minimum seek time
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SCAN
Arm moves in one direction only, satisfying all outstanding requests until it reaches the last track in that direction then the direction is reversed
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does not exploit locality
SCAN favors
jobs whose requests are for tracks nearest to both innermost and outermost tracks and
the latest-arriving jobs
Restricts scanning to one direction only
When the last track has been visited in one direction, the arm is returned to the opposite end of the disk and the scan begins again
Reduces the maximum delay experienced by new requests
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N-step-SCAN
With SSTF, SCAN, C-SCAN, the arm may not move if processes monopolize the device by repeated requests to one track: arm stickiness
Segments the disk request queue into subqueues of length N
Subqueues are processed one at a time, using SCAN
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Two subqueues
When a scan begins, all of the requests are in one of the queues, with the other empty
During the scan, all new requests are put into the other queue
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