Chapter 11
I/O Management and Disk Scheduling
– Operating System Design Issues
– I/O Buffering
– Disk Scheduling
– Disk Cache
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Goal: Generality
• For simplicity and freedom from error, it’s better to handle
all I/O devices in a uniform manner
• Due to the diversity of device characteristics, it is difficult in
practice to achieve true generality
• Solution: use a hierarchical modular design of I/O functions
– Hide details of device I/O in lower-level routines
– User processes and upper levels of OS see devices in terms of
general functions, such as read, write, open, close, lock, unlock
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A Model of I/O Organization
• Logical I/O:
– Deals with the device as a logical resource
and is not concerned with the details of
actually controlling the device
– Allows user processes to deal with the device
in terms of a device identifier and simple
commands such as open, close, read, write
• Device I/O:
– Converts requested operations into sequence
of I/O instructions
– Uses buffering techniques to improve
utilization
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A Model of I/O Organization
• Scheduling and Control:
– Performs actual queuing / scheduling and
control operations
– Handles interrupts and collects and
reports I/O status
– Interacts with the I/O module and hence
the device hardware
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Goal: Efficiency
• Most I/O devices are extremely slow
compared to main memory
 I/O operations often form a bottleneck
in a computing system
• Multiprogramming allows some processes
to be waiting on I/O while another process
is executing
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Goal: Efficiency
• Swapping brings in ready processes but
this is an I/O operation itself
• A major effort in I/O design has been
schemes for improving the efficiency of I/O
– I/O buffering
– Disk scheduling
– Disk cache
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Roadmap
– Operating System Design Issues
– I/O Buffering
– Disk Scheduling
– Disk Cache
7
No Buffering
• Without a buffer, OS directly accesses the
device as and when it needs
• A data area within the address space of
the user process is used for I/O
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No Buffering
• Process must wait for I/O to complete
before proceeding
– 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
• the suspended process is blocked waiting on the
I/O event, and the I/O operation is blocked waiting
for the process to be swapped in
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I/O Buffering
• It may be more efficient to perform input
transfers in advance of requests being
made and to perform output transfers
some time after the request is made.
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Block-oriented Buffering
• For block-oriented I/O devices such
as
– disks and
– USB drives
• Information is stored in fixed sized
blocks
• Transfers are made a block at a time
• Can reference data by block number
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Stream-Oriented
Buffering
• For stream-oriented I/O devices such as
– terminals
– printers
– communication ports
– mouse and other pointing devices, and
– most other devices that are not secondary
storage
• Transfer information as a stream of bytes
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Single Buffer
• OS assigns a buffer in the system portion
of main memory for an I/O request
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Block Oriented
Single Buffer
• 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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Block Oriented
Single Buffer
•  Provide a speedup
– User process can be processing one block of
data while the next block is being read in
•  OS is able to swap the process out
because the I/O operation is taking place
in system memory
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Block Oriented
Single Buffer
•  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
• Does it make sense to swap the process out after
the I/O operation finishes?
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Stream-oriented
Single Buffer
• 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
– Interaction between OS and user process follows the
producer/consumer model
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Double Buffer
• Use two system buffers instead of one
• A process can transfer data to or from one
buffer while OS empties or fills the other
buffer
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Circular Buffer
• More than two buffers are used
• 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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Buffer Limitations
• Buffering smoothes out peaks in I/O
demand
– But with enough demand eventually all buffers
become full and their advantage is lost
• In multiprogramming environment,
buffering can increase the efficiency of OS
and the performance of individual
processes
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Roadmap
– Operating System Design Issues
– I/O Buffering
– Disk Scheduling
– Disk Cache
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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
• A general timing diagram of disk I/O transfer is
shown here.
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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.
• Track selection involves moving the head
in a movable-head system.
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Disk Performance
Parameters
• 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
• Transfer Time is the time taken to transfer
the data (as the sector moves under the
head)
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Disk Performance
Parameters
• Total average access time Ta
where
Ta = Ts + 1 / (2r) + b / (rN)
Ts = average seek time
b = no. of bytes to be transferred
N = no. of bytes on a track
r = rotation speed, in revolutions / sec.
• Due to the seek time, the order in which
sectors are read from disk has a
tremendous effect on I/O performance
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Disk Scheduling
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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First-in, first-out (FIFO)
• Process requests sequentially
• 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
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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
– Longer jobs may have to wait an excessively
long time
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Last-in, first-out
• 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
• Possibility of starvation since a job may
never regain the head of the line
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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
• LOOK policy: reverse direction when there
are no more requests in a direction
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SCAN
• SCAN is biased against the area most
recently traversed
 does not exploit locality
• SCAN favors
– jobs whose requests are for tracks
nearest to both innermost and
outermost tracks and
– the latest-arriving jobs
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C-SCAN (Circular SCAN)
• 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
• New requests are added to other queue when a
queue is being processed
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FSCAN
• 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
• Service of new requests is deferred until all of
the old requests have been processed
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Performance Compared
Comparison of Disk Scheduling Algorithms
37
Roadmap
– Operating System Design Issues
– I/O Buffering
– Disk Scheduling
– Disk Cache
38
Disk Cache
• Buffer in main memory for disk sectors
• Contains a copy of some of the sectors
• When an I/O request is made for a
particular sector, a check is made to
determine if the sector is in the disk cache
– If so, the request is satisfied via the cache
– If not, the requested sector is read into the
disk cache from the disk
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Disk Cache
• Locality of reference
– When a block of data is fetched into the cache
to satisfy a single I/O request, it is likely that
there will be future references to that same
block
• One design issue: replacement strategy
– When a new sector is brought into the disk
cache, one of the existing blocks must be
replaced
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Least Recently Used (LRU)
• The block that has been in the cache the
longest with no reference to it is replaced
• A stack of pointers reference the cache
– Most recently referenced block is on the top of
the stack
– When a block is referenced or brought into
the cache, it is placed on the top of the stack
– The block on the bottom of the stack is to be
replaced
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LRU Disk Cache
Performance
• The miss ratio is, principally, a function of
the size of the disk cache
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Least Frequently Used (LFU)
• The block that has experienced the fewest
references is replaced
• A counter is associated with each block
– Incremented each time the block is accessed
• When replacement is required, the block
with the smallest count is selected.
Consider certain blocks are referenced
repeatedly in short intervals due to locality, but
are referenced relatively infrequently overall
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Frequency-Based
Replacement
• Blocks are logically organized in a stack,
similar to LRU
On a cache hit
On a miss, the block with the
smallest count that is not in the new
section is chosen for replacement
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Refined Frequency-Based
Replacement
• Only blocks in the old section are eligible for
replacement
• Allows relatively frequently referenced blocks a
chance to build up their reference counts before
becoming eligible for replacement
• Simulation studies indicate that this refined
policy outperforms LRU and LFU
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