CS 2200 Lecture 18 IO 1

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CS 2200 Lecture 18I/O (1)

• (Lectures based on the work of Jay Brockman,
  Sharon Hu, Randy Katz, Peter Kogge, Bill Leahy,
  Ken MacKenzie, Richard Murphy, and Michael
  Niemier)

2
What is it exactly?

• To anyone in computer science or computer
  engineering I/O probably has many different
  meanings
• My research in computer architecture focuses on
  processor design
• So I/O generally just involves a processor/memory
  interface
• For a DRAM chip designer, I/O might involve
• A processor/memory interface
• A memory/disk interface
• For an OS designer, I/O might be
• An interrupt from a device, input from the user,
  etc. etc.
• Basically it can mean lots of different things
• In computer architecture levels of memory
  hierarchy beyond main memory are often ignored

3
Why study I/O?

• Weve talked a lot about the CPU time metric
• (In fact Ive probably stressed it quite a bit!)
• CPU time is important
• for measuring how fast an instruction or program
  is actually executed
• But whats perhaps more important is response
  time
• The time between when the user types a command
  and when the results appear
• This might be a better measure of performance
• A brief study of I/O will help complete the
  picture of a general computer architecture or
  organization

4
A quick example

• The difference between CPU time and response time
  is 10
• (So, I/O overhead basically adds 10 to our
  execution time before user sees results)
• Can speed up CPU by a factor of 10, but I/O
  overhead/time will stay the same
• With no changes in the I/O performance, Amdahls
  law states that
• Well only get a speedup of 5.5! ½ of CPU
  improvement is wasted
• What if we make the CPU 100 times faster?
• Well only get a speed up of 10! 90 of speedup
  wasted!
• With CPU performance skyrocketing, if we dont
  improve I/O, tasks will become I/O bound

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Our Road Map
Processor
Memory Hierarchy
I/O Subsystem
Parallel Systems
Networking
6
Five Classic Components of a Computer Systemall
computers since 1946
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... and the software abstractions atop them!
operating systems, networking
Computation Processes Threads
Communication I/O devices, the internet
Storage Virtual Memory, Files
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I/0 Plan

• I/O devices in general
• magnetic disks in particular
• networks in particular
• Hardware interface issues
• tradeoff of performance and convenience
• dealing with external events
• Software abstractions
• example filesystems
• disk head scheduling
• POSIX models all I/O as files device drivers

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I/O Types and Rates

• Device Behavior Partner DataRate
  kb/s
• Keyboard I Human 0.01
• Mouse I Human 0.02
• Voice Input I Human 0.02
• Scanner I Human 400
• Voice Output O Human 0.6
• Line Printer O Human 1
• Laser Printer O Human 200
• Graphics Display O Human 60,000
• Modem IO Machine 8
• Network IO Machine 6,000
• Floppy Disk S Machine 100
• Optical Disk S Machine 1,000
• Magnetic Tape S Machine 2,000
• Magnetic Disk S Machine 10,000

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Mouse
I got the idea for the mouse while attending a
talk at a computer conference. The speaker was so
boring that I started daydreaming and hit upon
the idea. Doug Englebart

• Uses mechanical counters or optical devices to
  generate pulses which increment or decrement
  counters
• Counter values determined by polling.

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Magnetic Disks

• Drums
• Disks
• Removable disk packs
• Floppy disk
• Invented for IBM Field Engineers
• Contact
• Slow speed

12
Magnetic Disks

• Most common form of long term, rewriteable
  storage devices
• Usually considered the lowest level of memory
  hierarchy
• How does a magnetic disk work?
• Collection of platters rotates on a spindle at
  some RPM
• Platters are metal disks covered with magnetic
  recording material on both sides
• Disk diameters can vary
• Usually the wider faster, narrower cheaper
• Disk surface divided into tracks which are
  divided into sectors
• Sectors are the smallest unit that can be written

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A disk, pictorially

• When accessing data we read or write to a sector
• All sectors the same size, outer tracks just less
  dense
• To read or write, moveable arm with read/write
  head moves over each surface
• Cylinder all tracks under the arms at a given
  point on all surfaces
• To read or write
• Disk controller moves arm over proper track a
  seek
• The time to move is called the seek time
• When sector found, data is transferred

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Disk Terminology
Cylinder Track 'x' on all platters/surfaces
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The speed of light? No.

• Time required for a requested track sector to
  rotate under the read/write head is called the
  rotation latency or rotational delay
• Involves mechanical components on the order of
  milliseconds
• i.e. were no longer moving at the speed of light
  like in our CPU!
• Time required to actually write or read data is
  called the transfer time
• (a function of block size, rotation speed,
  recording density on a track, and speed of the
  electronics connecting the disk to the computer)

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Disk odds n ends

• Often transfer time is a very small portion of a
  full access
• Its possible to use techniques (discussed in
  caches) to help reduce disk overhead. Any
  thoughts?
• To help reduce complexity theres usually
  additional HW called a disk controller
• Disk controller helps manage disk accesses
• but also adds more overhead controller time
• (Can also have a queuing delay)
• (Time spent waiting for a disk to become free if
  its already in use for another access)

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Example average disk access time

• What is the average time to read or write a
  512-byte sector for a typical disk?
• The average seek time is given to be 9 ms
• The transfer rate is 4 MB per second
• The disk rotates at 7200 RPM
• The controller overhead is 1 ms
• The disk is currently idle before any requests
  are made (so there is no queuing delay)
• Average disk access time average seek time
  average rotational delay transfer time
  controller overhead

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Capacity trends and disks

• Capacity of disks usually referred to as areal
  density

Cost for 1GB of magnetic disk space has
decreased/ will decrease almost exponentially
over time!
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Magnetic Disks short overview

• Hard disk
• Higher speed (3600 - 7200)
• Larger
• Higher Density
• Multiple platters
• Performance
• Seek time (8-20 ms or faster)
• Rotational latency (4-8 ms)
• Transfer rate 2-40 MB/sec

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Disk Latency
Disk Latency Queuing Time Controller time
Seek Time Rotation Time Transfer Time
Order of magnitude times for 4K byte transfers
Seek 8 ms or less Rotate 4.2 ms _at_ 7200
rpm Transfer 1 ms _at_ 7200 rpm
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Technology Trends
Disk Capacity now doubles every 18
months before 1990 every 36 months
Today Processing Power Doubles Every 18
months  Today Memory Size Doubles Every 18
months(4X/3yr)  Today Disk Capacity Doubles
Every 18 months  Disk Positioning Rate (Seek
Rotate) Doubles Every Ten Years!
The I/O GAP
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Historical Perspective

• 1956 IBM Ramac early 1970s Winchester
• Developed for mainframes
• Had proprietary interfaces
• Steady shrink in form factor 27 in. to 14 in.
• 1970s developments
• 5.25 inch floppy disk formfactor (microcode into
  mainframe)
• early emergence of industry standard disk
  interfaces
• ST506, SASI, SMD, ESDI

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Historical Perspective

• Early 1980s
• PCs and first generation workstations
• Mid 1980s
• Client/server computing
• Centralized storage on file server
• accelerates disk downsizing 8 inch to 5.25 inch
• Mass market disk drives become a reality
• industry standards SCSI, IPI, IDE
• 5.25 inch drives for standalone PCs, End of
  proprietary interfaces

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Disk History
Data density Mbit/sq. in.
Capacity of Unit Shown Megabytes
1973 1. 7 Mbit/sq. in 140 MBytes
1979 7. 7 Mbit/sq. in 2,300 MBytes
source New York Times, 2/23/98, page C3,
Makers of disk drives crowd even more data into
even smaller spaces
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Historical Perspective

• Late 1980s/Early 1990s
• Laptops, notebooks, (palmtops)
• 3.5 inch, 2.5 inch, (1.8 inch formfactors)
• Formfactor plus capacity drives market, not so
  much performance
• Recently Bandwidth improving at 40/ year
• Challenged by DRAM, flash RAM in PCMCIA cards
• still expensive
• unattractive MBytes per cubic inch
• Optical disk fails on performace (e.g., NEXT) but
  finds niche (CD ROM)

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Disk History
1989 63 Mbit/sq. in 60,000 MBytes
1997 1450 Mbit/sq. in 2300 MBytes
1997 3090 Mbit/sq. in 8100 MBytes
source New York Times, 2/23/98, page C3,
Makers of disk drives crowd even more data into
even smaller spaces
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Something cool

• This iPod mini
• A 4 GB disk in a 2 x 3.6 x 0.5 space

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Magnetic Disks
illustration source unknown
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Second Major Example Networks

• Examples
• System Area Networks (SP2) 100s nodes 25
  meters per link
• Local Area Networks (Ethernet) 100s nodes
  1000 meters
• Wide Area Network (ATM) 1000s nodes 5,000,000
  meters

a.k.a. end systems, hosts
a.k.a. network, communication subnet
Interconnection Network
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ABCs of Networks

• Starting Point Send bits between 2 computers
• Queue (FIFO) on each end
• Information sent called a message
• Can send both ways (Full Duplex)
• Rules for communication? protocol
• Inside a computer
• Loads/Stores Request (Address) Response (Data)
• Need Request Response signaling

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Trivial Example

• What is the format of mesage?
• Fixed? Number bytes?

Request/ Response
Address/Data
1 bit
32 bits
0 Please send data from Address 1 Packet
contains data corresponding to request

• Header/Trailer information to deliver a message
• Payload data in message (1 word above)

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Extensions

• What if more than 2 computers want to
  communicate?
• Need computer address field (destination) in
  packet
• What if packet is garbled in transit?
• Add error detection field in packet (e.g., CRC)
• What if packet is lost?
• More elaborate protocols to detect loss
• What if multiple processes/machine?
• Queue per process to provide protection
• Simple questions such as these lead to elaborate
  protocols and packet formats complexity
• note complexity often slow

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A Simple Example Revisted

• What is the format of packet?
• Fixed? Number bytes?

Address/Data
CRC
Code
2 bits
32 bits
4 bits
00 RequestPlease send data from Address 01
ReplyPacket contains data corresponding to
request 10 Acknowledge request 11 Acknowledge
reply
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Network Media

• There are different ways to connect computers
  together
• Can kind of think of it like a memory hierarchy
• Different kinds of media vary in cost,
  performance, and reliability
• There are several different kinds well consider
• Twisted Pair
• Coaxial Cable
• Fiber Optics
• Air
• (first, see board for summary discussion)

35
Twisted pair media

• Just a twisted pair of copper wires
• Insulated, about 1mm thick
• Twisted together to reduce electrical
  interference
• Makes sure we dont turn it into an antenna!
• Data transfer speeds of
• A few Mbs over a few kilometers 10s of Mbs over
  shorter distances
• Uses
• Used lots in the telephone industry
• OK for LANs because of reasonable data transfer
  rates

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Coaxial (coax) cable

• A picture of it is included below
• Consists of copper center surrounded by
  insulator, a mesh, and a plastic coating
• Originally developed for cable companies to
  transmit at a higher rate over a few kms
• Good bandwidth 50 ohm coax cable can deliver 10
  Mbs over a kilometer
• Good for LAN

37
Coax cable junctions

• Its harder to connect things to this media
  however
• One method is the T-junction
• The typical way this is handled
• Cable cut in 2 and a connector is inserted that
  reconnects the cable and adds a 3rd wire to the
  computer
• But, if you add a new connector, you have to
  split the network and therefore bring it down for
  a short period of time
• Additional maintenance is a headache b/c any user
  can disconnect the network
• Better the vampire tap
• Drill a hole to terminate in the copper core
• Screw in connector no cable cut, no network
  down time

38
Fiber optics

• Replaces copper with plastic and electrons with
  photons
• Information is now transmitted via pulses of
  light
• Usually, 3 basic components
• Transmission medium fiber optic cable
• Light source LED or laser diode
• Light detector photodiode
• A simplex media data can only go in 1 direction
• How it works

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Fiber optics how it really works

• Because light is bent/refracted at interfaces, it
  can slowly spread out as it travels down the
  diameter of a cable
• Unless that is we transfer a single wavelength of
  light
• Then itll travel in a straight line
• With this in mind, let consider the 2 kinds of
  fiber optic cable
• Multimode Fiber
• Allows light to be dispersed
• Uses inexpensive LEDs
• Useful for transmissions of about 2 kms 600 Mbs
  in 1995
• Single-mode Fiber
• A single-wavelength fiber
• Uses more expensive laser diodes as light sources
• Transmits Gbs over 100s of kms great for phone
  companies!

40
Fiber optics practical issues

• Single mode fiber is a better transmitter but
  more difficult to attach connectors
• Also, less reliable, more expensive, cant bend
  as much
• Usually in LAN, multimode is the weapon of
  choice
• So, how do you connect fiber optics to a
  computer?
• Passive Mode
• Taps are fused into the fiber and a photodiode
  looks at passing light
• Electrical output passes to the computer
  interface
• A failure cuts off just 1 computer
• Active Mode
• Really a break in the cable
• Light converted to electrical signals, sent to
  computer, converted back to light, sent back down
  cable
• Problem tap failure causes net failure
• Advantage light source refreshed, can go longer
  distances

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Some comparisons
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The bottom line

• Bandwidth problems can be fixed with more money
  for more wires
• Improving your latency is somewhat more difficult
  to do
• After all, 299792.5 km/s is kinda fixed

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I/O Device Summary

• Disks/Networks very different but consider these
  similarities
• Data handled in batches (sectors, messages)
• Lots of waiting around for external events
• Compatibility is important (more than
  performance)
• Reliability is important (and requires work to
  achieve)
• Slow devices are simple (and boring)
• Fast devices may be substantially autonomous
• graphics

44
I/O Hardware Interface Issues
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I/O Hardware

• Basic memory-map w/polling and/or interrupts
• Project 2!
• Advanced bus issues
• Performance vs. compatibility - multiple busses
• Namespaces
• Smart device controllers
• Direct Memory Access (DMA)
• Arbitration
• Caching issues
• I/O processors
• the wheel of reincarnation
• (see board for preliminary examples)

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Basic I/O devices as memorya la project 2
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Performance vs. Compatibility

• Problem
• Processor - memory is a performance-crucial path
  ... improve as often as possible!
• I/O controllers made by many vendors ... change
  is expensive!

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(No Transcript)
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Multiple Busses
Cache Bus e.g. 256b, 533MHz
Memory Bus e.g. 64b, 533MHz
Processor
interrupts
Cache
I/O Bus e.g. 64b, 66MHz
Memory Bus
bridge
Main Memory
I/O Bus (e.g. PCI)
I/O Controller
I/O Controller
I/O Controller
Disk Drive Bus e.g. SCSI 16b, 20MHz
Graphics
Disk
Disk
Network
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Smart Device Controllers
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Polling

• Computer
• Busy bit set? Yes. Busy bit set? Yes.
• Busy bit set? Yes.
• Busy bit set? Yes.
• Busy bit set? Yes.
• Busy bit set? Yes.
• Busy bit set? Yes.
• Busy bit set? Yes.
• Busy bit set? Yes.
• Busy bit set? Yes.

• Controller

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Polling

• Computer
• Busy bit set? No.
• Set write bit in command register
• Write a byte (or word) of data to Data-out
• Set command ready bit in control register
• Busy bit set? Yes.
• Busy bit set? Yes.

• Controller
• Controller clears busy bit
• Sees command ready
• Set busy bit

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Polling

• Computer
• Busy bit set? Yes.
• Busy bit set? Yes.
• Busy bit set? Yes.
• Busy bit set? Yes.
• Busy bit set? Yes.
• Busy bit set? Yes.
• Busy bit set? Yes.
• Busy bit set? Yes.
• Busy bit set? Yes.
• Busy bit set? No.

• Controller
• Checks write bit
• Reads data-out
• Does I/O with device
• Clears command ready bit
• Clears error bit
• Clears busy bit

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Polling

• Appropriate when controller and device very fast
• Very inefficient when most of the time the
  controller is busy
• Better solution...Interrupts

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(Recall) Interrupt Mechanism Hardware
Address Bus
Processor
Data Bus
Int
Inta
Device 1
Device 2
If the processor decides to handle the interrupt
it asserts the inta (interrupt acknowledege) line
56
(Recall) Interrupts/Exceptions/Traps protection
I/O (kernel) space
a loop
user space
PC (mem. addr.)
a system call
kernel space
an interrupt
time
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(Recall) Process States
New
Terminated
Ready
Running
A longer example using these states later on in
lecture
Waiting
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(recall) Convoy Effect
CPU
Ready Queue
IOB
IOB
IOB
CPUB
I/O
I/O Request
I/O Queue
CPU is running Compute bound jobs while I/O Bound
jobs wait.
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Interrupts

• Used by I/O controllers to communicate to
  Processor
• Also used by applications to communicate with
  operating system
• Software Interrupt or Trap
• OS can now use same device registers as before

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Interrupts

• Processor
• Initiate I/O
• Context switch to something else
• Receive interrupt transfer to handler
• Interrupt handler processes data, returns from
  interrupt
• Resume processing of interrupted task

• I/O Controller
• Initiate I/O with physical device
• Completion (good or bad)
• Generate interrupt

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DMA

• Preceding scheme effective but wasteful for large
  blocks of data.
• Using sophisticated general-purpose processor for
  very specialized function
• Solution Add enough processing power to device
  controller (and possibly bus controller) to allow
  direct transfer between device and memory.

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DMA
N
Processor tells controller to make DMA
transfer. Assume disk to memory. (Includes N
number of bytes)
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DMA
N
Controller gets sector of data from disk.
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DMA
N-1
Controller transfers one word to memory and
updates count. Checks for termination. If not...
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DMA
N-2
Controller transfers one word to memory and
updates count. Checks for termination. If not...
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DMA
N-3
Controller transfers one word to memory and
updates count. Checks for termination. If not...
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DMA
N-4
Controller transfers one word to memory and
updates count. Checks for termination. If not...
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DMA
N-5
Controller transfers one word to memory and
updates count. Checks for termination. If not...
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DMA
0
Controller transfers one word to memory and
updates count. Checks for termination. If done...
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DMA
Controller interrupts processor
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DMA
Processor acknowledges interrupt
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DMA
Controller sends interrupt vector
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DMA
Processor can now have scheduler take
appropriate action (i.e. move process waiting
for I/O into ready queue, etc.)
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Arbitration

• DMA implies multiple owners of the bus
• must decide who owns the bus from cycle to cycle
• Arbitration
• Daisy chain
• Centralized parallel arbitration
• Distributed arbitration by self selection
• Distributed arbitration by collision detection
• (see board for detailed examples and pictures)

75
Daisy Chain
Simple but not fair and slow.
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Centralized Parallel Arbitration

• Requires central arbiter
• Each device has separate line
• Central arbiter may become bottleneck
• Used in PCI bus

77
Distributed Arbitration by Self Selection

• Each device sees all requestors
• Priority scheme allows each to know if they get
  bus
• Requires lots of request lines
• Used by Apple NuBus (backplane)

78
Distributed Arbitration by Collision Detection

• Devices independently request bus
• Devices have ability to detect simultaneous
  requests or Collisions.
• Upon collision a variety of schemes are used to
  select among requestors
• Used by Ethernet

79
Caching Issues

• What happens if the processor has a cached copy
  of data when a device does DMA?
• short answer is that theres a cache
  coherance problem the DMA may change memory and
  the processor doesnt see the change. Two
  solutions
• Device driver (software) flushes cache before
  using DMA
• Elaborate bus hardware maintains consistency by
  checking the cache on every external bus
  transaction

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wheel of reincarnation

• Start with simple devices
• Add cute functionality
• Add lots of functionality
• Declare it to be a processor in its own right
• Repeat...
• Graphics community has been around this wheel a
  couple of times now.

81
Summary

• Example Devices
• often work in blocks
• spend lots of time waiting
• Bus Issues
• memory map w/polling and/or interrupts (project
  2)
• Performance vs. compatibility - multiple busses
• Namespaces
• Smart device controllers
• Direct Memory Access (DMA)
• Arbitration
• Caching issues
• I/O processors
• the wheel of reincarnation