Dear All,

1
Dear All, 1 Remember to bring in the notes that
we have not completed on week 9. And we will
continue on 26th (Monday) from where we
stopped. 2 This notes for week 10 is the Part
2 continuation. 3 The tutorial document
consists of 2 weeks of tutorial material for week
10 and week 11 contact hour, respectively. 4
Next in line is the Week 11(Storage Files).
Week 12(Multiprocessing) and Week
13(Revision).

2
Week 09 19/March/2007
Week 10 26/March/2007

• I/O and low level support (part 2 of 2)

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Input Class
Random Access
Mouse, cursor key, numeric key pad
Block Device
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Harddisk as a Block device
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Block versus Stream I/O

• Block-oriented
• Information is stored in fixed sized blocks
• Transfers are made a block at a time
• Used for disks and tapes
• Stream-oriented
• Transfer information as a stream of bytes
• Used for terminals, printers, communication
  ports, mouse and other pointing devices, and most
  other devices that are not secondary storage

6
I/O Buffering

• Reasons for buffering
• Processes must wait for I/O to complete before
  proceeding
• Certain pages must remain in main memory during
  I/O

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Single Buffer

• Operating system assigns a buffer in main memory
  for an I/O request
• Block-oriented
• Input transfers made to buffer
• Block moved to user space when needed
• Another block is moved into the buffer
• Read ahead

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Single Buffer

• Stream-oriented
• Used a line at time
• User input from a terminal is one line at a time
  with carriage return signaling the end of the
  line
• Output to the terminal is one line at a time

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I/O Buffering
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Double Buffer

• Use two system buffers instead of one
• A process can transfer data to or from one buffer
  while the operating system 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

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Harddisk as a Block device
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Disk Structure

• Disk drives are addressed as large 1-dimensional
  arrays of logical blocks
• The logical block is the smallest unit of
  transfer.
• The 1-dimensional array of logical blocks is
  mapped into the sectors of the disk sequentially
• Sector 0 is the first sector of the first track
  on the outermost cylinder.
• Mapping proceeds in order through that track,
  then the rest of the tracks in that cylinder, and
  then through the rest of the cylinders from
  outermost to innermost.

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I/O Architecture

• Host accessed storage through I/O ports talking
  to I/O buses
• SCSI is a I/O bus with up to 16 devices per cable
• SCSI initiator requests operation and SCSI
  targets perform tasks
• Each target can have up to 8 logical units
  (disks attached to device controller)

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Disk Types

• Disks can be removable
• Drive attached to computer via I/O bus
• Variations include EIDE, ATA, SATA, USB, SCSI
• Host controller in computer uses bus to talk to
  disk controller built into drive or storage array

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Disk Performance Parameters

• To read or write, the disk head must be
  positioned at the desired track and at the
  beginning of the desired sector
• Seek time
• Time it takes to position the head at the desired
  track
• Rotational delay or rotational latency
• Time its takes for the beginning of the sector
  to reach the head

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Timing of a Disk I/O Transfer
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Disk Performance Parameters

• Access time
• Sum of seek time and rotational delay
• The time it takes to get in position to read or
  write
• Data transfer occurs as the sector moves under
  the head

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Transfer Time for one sector

• The transfer time from disk
• T (b/rN)
• where
• b number of bytes to be transferred
• N number of bytes on a track
• r rotation speed (rev./sec)
• The average access time
• Ta T_seek (1/2r) (b/rN)

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Disk Scheduling Policies

• Seek time is the reason for differences in
  performance
• For a single disk there will be a number of I/O
  requests
• If requests are selected randomly, we will poor
  performance

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Disk Scheduling Policies

• Priority
• Goal is not to optimize disk use but to meet
  other objectives
• Short batch jobs may have higher priority
• Provide good interactive response time

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Disk Scheduling Policies

• 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
• Possibility of starvation since a job may never
  regain the head of the line

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Disk Scheduling Policies

• For a disk, a queue of I/O requests (Read/ Write)
  for processes
• Random scheduling
• Selected requests in random, tracks visited will
  occur randomly giving poor performance
• Use as benchmark for evaluating other scheduling
  techniques
• To improve performance, one needs to reduce
  average time spent on seeks

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Disk Scheduling Policies

• First-in, first-out (FIFO)
• Process request sequentially
• Fair to all processes
• Approaches random scheduling in performance if
  there are many processes
• Requests sequence 55,58,39,18,90,160, 150,38,184

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Disk Scheduling Policies

• 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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Disk Scheduling Policies

• SCAN
• Arm moves in one direction only, satisfying all
  outstanding requests until it reaches the last
  track in that direction
• Direction is reversed

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Disk Scheduling Policies

• C-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

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Disk sectors are typically cached for subsequent
update before finally committing and write back
to sector
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Least Frequently Used Caching

• The block that has experienced the fewest
  references is replaced
• A counter is associated with each block
• Counter is incremented each time block accessed
• Block with smallest count is selected for
  replacement
• Some blocks may be referenced many times in a
  short period of time and the reference count is
  misleading

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Linux I/O

• Elevator scheduler
• Maintains a single queue for disk read/write
  requests
• Keeps list of requests sorted by block number
• Drive moves in a single direction to satisfy each
  request

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Linux I/O

• Deadline scheduler
• Uses three queues
• Incoming requests
• Read requests go to the tail of a FIFO queue
• Write requests go to the tail of a FIFO queue
• Each request has an expiration time

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Linux I/O
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Windows I/O

• Basic I/O modules
• Cache manager
• File system drivers
• Network drivers
• Hardware device drivers

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Low Level I/O support consideration
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From Lecture on Threads
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Microkernel Design from threads lecture

• Low-level memory management
• Mapping each virtual page to a physical page
  frame
• 2pages example
• Interprocess communication
• 2 pages example
• I/O and interrupt management
• 3 pages example

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NS Cache Management embedded in Processor
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MC68000 on chip Error Handling
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Lightweight Interprocess Structure
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NS on chip support for Interprocess
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MC68000 on chip DMA interfacing
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MC68000 Comprehensive Interrupt Structure
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Small System Implementation with auto-vector
interrupt via VPA low
End of week 10 Lecture