IO and Disks

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I/O and Disks
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Announcements

• Prelim tomorrow Thursday, March 8th, 730-900pm,
  1½ hour
• 203 Phillips
• Closed book, no calculators/PDAs/
• Bring ID
• Make-up exam will be early Thursday morning,
  March 8th 830am-1000am.
• Three people signed up so far.
• Topics Everything up to (and including) Monday,
  March 5th
• Lectures 1-18, chapters 1-9 (7th ed)
• Solutions for Homework 3 available via CMS
• Joy has office hours all day today.

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Goals for Today

• I/O
• How does a computer system interact with its
  environment?
• Disks
• How does a computer system permanently store data?

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The Requirements of I/O

• So far in this course
• We have learned how to manage CPU, memory
• What about I/O?
• Without I/O, computers are useless (disembodied
  brains?)
• But thousands of devices, each slightly
  different
• How can we standardize the interfaces to these
  devices?
• Devices unreliable media failures and
  transmission errors
• How can we make them reliable???
• Devices unpredictable and/or slow
• How can we manage them if we dont know what they
  will do or how they will perform?
• Some operational parameters
• Byte/Block
• Some devices provide single byte at a time (e.g.
  keyboard)
• Others provide whole blocks (e.g. disks,
  networks, etc)
• Sequential/Random
• Some devices must be accessed sequentially (e.g.
  tape)
• Others can be accessed randomly (e.g. disk, cd,
  etc.)
• Polling/Interrupts
• Some devices require continual monitoring

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Modern I/O Systems
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Example Device-Transfer Rates (Sun Enterprise
6000)

• Device Rates vary over many orders of magnitude
• System better be able to handle this wide range
• Better not have high overhead/byte for fast
  devices!
• Better not waste time waiting for slow devices

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The Goal of the I/O Subsystem

• Provide Uniform Interfaces, Despite Wide Range of
  Different Devices
• This code works on many different devices
• int fd open(/dev/something) for (int i
  0 i lt 10 i) fprintf(fd,Count
  d\n,i) close(fd)
• Why? Because code that controls devices (device
  driver) implements standard interface.
• We will try to get a flavor for what is involved
  in actually controlling devices in rest of
  lecture
• Can only scratch surface!

8
Want Standard Interfaces to Devices

• Block Devices e.g. disk drives, tape drives,
  DVD-ROM
• Access blocks of data
• Commands include open(), read(), write(), seek()
• Raw I/O or file-system access
• Memory-mapped file access possible
• Character Devices e.g. keyboards, mice, serial
  ports, some USB devices
• Single characters at a time
• Commands include get(), put()
• Libraries layered on top allow line editing
• Network Devices e.g. Ethernet, Wireless,
  Bluetooth
• Different enough from block/character to have own
  interface
• Unix and Windows include socket interface
• Separates network protocol from network operation
• Includes select() functionality
• Usage pipes, FIFOs, streams, queues, mailboxes

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How Does User Deal with Timing?

• Blocking Interface Wait
• When request data (e.g. read() system call), put
  process to sleep until data is ready
• When write data (e.g. write() system call), put
  process to sleep until device is ready for data
• Non-blocking Interface Dont Wait
• Returns quickly from read or write request with
  count of bytes successfully transferred
• Read may return nothing, write may write nothing
• Asynchronous Interface Tell Me Later
• When request data, take pointer to users buffer,
  return immediately later kernel fills buffer and
  notifies user
• When send data, take pointer to users buffer,
  return immediately later kernel takes data and
  notifies user

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Life Cycle of An I/O Request
User Program
Kernel I/O Subsystem
Device Driver Top Half
Device Driver Bottom Half
Device Hardware
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A Kernel I/O Structure
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Device Drivers

• Device Driver Device-specific code in the kernel
  that interacts directly with the device hardware
• Supports a standard, internal interface
• Same kernel I/O system can interact easily with
  different device drivers
• Special device-specific configuration supported
  with the ioctl() system call
• Device Drivers typically divided into two pieces
• Top half accessed in call path from system calls
• Implements a set of standard, cross-device calls
  like open(), close(), read(), write(), ioctl(),
  strategy()
• This is the kernels interface to the device
  driver
• Top half will start I/O to device, may put thread
  to sleep until finished
• Bottom half run as interrupt routine
• Gets input or transfers next block of output
• May wake sleeping threads if I/O now complete

13
I/O Device Notifying the OS

• The OS needs to know when
• The I/O device has completed an operation
• The I/O operation has encountered an error
• I/O Interrupt
• Device generates an interrupt whenever it needs
  service
• Pro handles unpredictable events well
• Con interrupts relatively high overhead
• Polling
• OS periodically checks a device-specific status
  register
• I/O device puts completion information in status
  register
• Could use timer to invoke lower half of drivers
  occasionally
• Pro low overhead
• Con may waste many cycles on polling if
  infrequent or unpredictable I/O operations
• Some devices combine both polling and interrupts
• For instance High-bandwidth network device
• Interrupt for first incoming packet
• Poll for following packets until hardware empty

14
How does the processor actually talk to the
device?

• CPU interacts with a Controller
• Contains a set of registers that can be read and
  written
• May contain memory for request queues or
  bit-mapped images
• Regardless of the complexity of the connections
  and buses, processor accesses registers in two
  ways
• I/O instructions in/out instructions
• Example from the Intel architecture out 0x21,AL
• Memory mapped I/O load/store instructions
• Registers/memory appear in physical address space
• I/O accomplished with load and store instructions

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Transfering Data To/From Controller

• Programmed I/O
• Each byte transferred via processor in/out or
  load/store
• Pro Simple hardware, easy to program
• Con Consumes processor cycles proportional to
  data size
• Direct Memory Access
• Give controller access to memory bus
• Ask it to transfer data to/from memory directly
• Sample interaction with DMA controller (from
  book)

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Main components of Intel Chipset Pentium 4

• Northbridge
• Handles memory
• Graphics
• Southbridge I/O
• PCI bus
• Disk controllers
• USB controllers
• Audio
• Serial I/O
• Interrupt controller
• Timers

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The Memory Hierarchy

• Each level acts as a cache for the layer below it

CPU
registers, L1 cache
L2 cache
primary memory
disk storage (secondary memory)
random access
tape or optical storage (tertiary memory)
sequential access
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What does the disk look like?
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Some parameters

• 2-30 heads (platters 2)
• diameter 14 to 2.5
• 700-20480 tracks per surface
• 16-1600 sectors per track
• sector size
• 64-8k bytes
• 512 for most PCs
• note inter-sector gaps
• capacity 20M-300G
• main adjectives BIG, slow

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Disk overheads

• To read from disk, we must specify
• cylinder , surface , sector , transfer size,
  memory address
• Transfer time includes
• Seek time to get to the track
• Latency time to get to the sector and
• Transfer time get bits off the disk

Track
Sector
Rotation Delay
Seek Time
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Modern disks
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50 years ago

• On 13th September 1956, IBM 305 RAMAC computer
  system first to use disk storage
• 80000 times more data on the 8GB 1-inch drive in
  his right hand than on the 24-inch RAMAC one in
  his left

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Disks vs. Memory

• Smallest write sector
• Atomic write sector
• Random access 5ms
• not on a good curve
• Sequential access 200MB/s
• Cost .002MB
• Crash doesnt matter (non-volatile)

• (usually) bytes
• byte, word
• 50 ns
• faster all the time
• 200-1000MB/s
• .10MB
• contents gone (volatile)

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Disk Structure

• Disk drives addressed as 1-dim arrays of logical
  blocks
• the logical block is the smallest unit of
  transfer
• This array mapped sequentially onto disk sectors
• Address 0 is 1st sector of 1st track of the
  outermost cylinder
• Addresses incremented within track, then within
  tracks of the cylinder, then across cylinders,
  from innermost to outermost
• Translation is theoretically possible, but
  usually difficult
• Some sectors might be defective
• Number of sectors per track is not a constant

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Non-uniform sectors / track

• Maintain same data rate with Constant Linear
  Velocity
• Approaches
• Reduce bit density per track for outer layers
• Have more sectors per track on the outer layers
  (virtual geometry)

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Disk Scheduling

• The operating system tries to use hardware
  efficiently
• for disk drives ? having fast access time, disk
  bandwidth
• Access time has two major components
• Seek time is time to move the heads to the
  cylinder containing the desired sector
• Rotational latency is additional time waiting to
  rotate the desired sector to the disk head.
• Minimize seek time
• Seek time ? seek distance
• Disk bandwidth is total number of bytes
  transferred, divided by the total time between
  the first request for service and the completion
  of the last transfer.

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Disk Scheduling (Cont.)

• Several scheduling algos exist service disk I/O
  requests.
• We illustrate them with a request queue (0-199).
• 98, 183, 37, 122, 14, 124, 65, 67
• Head pointer 53

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FCFS
Illustration shows total head movement of 640
cylinders.
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SSTF

• Selects request with minimum seek time from
  current head position
• SSTF scheduling is a form of SJF scheduling
• may cause starvation of some requests.
• Illustration shows total head movement of 236
  cylinders.

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SSTF (Cont.)
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SCAN

• The disk arm starts at one end of the disk,
• moves toward the other end, servicing requests
• head movement is reversed when it gets to the
  other end of disk
• servicing continues.
• Sometimes called the elevator algorithm.
• Illustration shows total head movement of 208
  cylinders.

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SCAN (Cont.)
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C-SCAN

• 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 it immediately
  returns to beginning of the disk
• No requests serviced on the return trip.
• Treats the cylinders as a circular list
• that wraps around from the last cylinder to the
  first one.

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C-SCAN (Cont.)
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C-LOOK

• Version of C-SCAN
• Arm only goes as far as last request in each
  direction,
• then reverses direction immediately,
• without first going all the way to the end of the
  disk.

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C-LOOK (Cont.)
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Selecting a Good Algorithm

• SSTF is common and has a natural appeal
• SCAN and C-SCAN perform better under heavy load
• Performance depends on number and types of
  requests
• Requests for disk service can be influenced by
  the file-allocation method.
• Disk-scheduling algo should be a separate OS
  module
• allowing it to be replaced with a different
  algorithm if necessary.
• Either SSTF or LOOK is a reasonable default algo

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Summary

• I/O Devices Types
• Many different speeds (0.1 bytes/sec to
  GBytes/sec)
• Different Access Patterns
• Block Devices, Character Devices, Network Devices
• Different Access Timing
• Blocking, Non-blocking, Asynchronous
• I/O Controllers Hardware that controls actual
  device
• Processor Accesses through I/O instructions,
  load/store to special physical memory
• Report their results through either interrupts or
  a status register that processor looks at
  occasionally (polling)
• Device Driver Device-specific code in kernel
• Disks
• Latency Seek Rotational Transfer
• Also, queuing time
• Rotational latency on average ½ rotation
• Improve performance (decrease queuing time) via
  scheduling