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Operating Systems CMPSC 473. I/O Management (1) November 30 2010 - Lecture 23 Instructor: Bhuvan Urgaonkar. Announcements. Project 3 due today Please indicate time to meet TA on sheet Project 4 out and due on Dec 10 You need to study material on content addressable storage

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Operating systems cmpsc 473

Operating SystemsCMPSC 473

I/O Management (1)

November 30 2010 - Lecture 23

Instructor: Bhuvan Urgaonkar


  • Project 3 due today

  • Please indicate time to meet TA on sheet

  • Project 4 out and due on Dec 10

    • You need to study material on content addressable storage

    • Will release slides on this later today

  • Will release practice question set to help you prepare for the final exam (sometime this week)

    • Exam is on Dec 16

I o management topics
I/O Management: Topics

  • General issues in I/O management

  • Secondary storage management

    • Properties of media

      • Magnetic disks and Flash

    • Systems software for using these media

      • File Systems

  • Note: Would like you to read ahead for P4

    • Material on content addressable storage: will post slides

I o hardware
I/O Hardware

  • Incredible variety of I/O devices

  • Two ways of communicating

    • Port

    • Bus

  • Plus … interrupts

    • Unidirectional: From device to CPU

    • Special CPU input lines/pins for these

I o hardware1
I/O Hardware

  • Incredible variety of I/O devices

  • Two ways of communicating

    • Port

    • Bus

  • Controller: Electronics to operate a port/bus or a device

    • More complex controller: host adapter

Some examples
Some examples

PCI slots and card

Some examples1
Some examples

SATA cable

SATA ports on a motherboard

I o hardware2
I/O Hardware

  • I/O instructions control devices

  • Devices have addresses, used by

    • Direct I/O instructions

      • These are part of the ISA

      • E.g., IN and OUT on Intel

    • Memory-mapped I/O

      • Not to be confused with memory mapped file IO!

Direct v memory mapped io
Direct V. Memory Mapped IO

  • Direct I/O

    • Uses special instructions for accessing the I/O devices

      • i.e., different from loads/stores

    • IO devices have separate address space from general memory

      • Either accomplished by an extra IO pin in CPU’s physical interface

      • Or, separate bus for IO devices

      • Also called isolated IO

Direct v memory mapped io1
Direct V. Memory Mapped IO

  • Memory Mapped I/O

    • Uses loads and stores

Communication between host and device
Communication between host and device

  • Employs registers on the port/controller

  • Typically four important registers

    • Data-in: read by the host to get input

    • Data-out: written by the host to send output

    • Status: Bits readable by the host that indicate state of port

      • Has the current command been completed?

      • Has a device error occurred?

      • Is there data available to be read from Data-in register?

    • Control: Writable by the host to start a command, select certain properties of the port (e.g., speed)

  • Interaction between host and controller

    • Polling

    • Interrupts


  • Determines state of device

    • command-ready

    • busy

    • Error

  • Busy-wait cycle to wait for I/O from device


  • CPU Interrupt-request line triggered by I/O device

  • Interrupt handler receives interrupts

  • Maskable to ignore or delay some interrupts

  • Interrupt vector to dispatch interrupt to correct handler

    • Based on priority

    • Some non-maskable

  • Interrupt mechanism also used for exceptions

Direct memory access
Direct Memory Access

  • Used to avoid programmed I/O for large data movement

  • Requires DMA controller

  • Bypasses CPU to transfer data directly between I/O device and memory

Application i o interface
Application I/O Interface

  • I/O system calls encapsulate device behaviors in generic classes

  • Device-driver layer hides differences among I/O controllers from kernel

  • Devices vary in many dimensions

    • Character-stream or block

    • Sequential or random-access

    • Sharable or dedicated

    • Speed of operation

    • read-write, read only, or write only

Block and character devices
Block and Character Devices

  • Block devices include disk drives

    • Commands include read, write, seek

    • Raw I/O or file-system access

    • Memory-mapped file access possible

  • Character devices include keyboards, mice, serial ports

    • Commands include get, put

    • Libraries layered on top allow line editing

Network devices
Network Devices

  • Varying enough from block and character to have own interface

  • Unix and Windows NT/9x/2000 include socket interface

    • Separates network protocol from network operation

    • Includes select functionality

  • Approaches vary widely (pipes, FIFOs, streams, queues, mailboxes)

Clocks and timers
Clocks and Timers

  • Provide current time, elapsed time, timer

  • Programmable interval timer used for timings, periodic interrupts

  • ioctl (on UNIX) covers odd aspects of I/O such as clocks and timers

Synchronous i o
Synchronous I/O

  • Blocking - process suspended until I/O completed

    • Easy to use and understand

    • Likely to be inefficient

      • Quiz: When and why?

Asynchronous i o
Asynchronous I/O

  • Process runs while I/O executes

    • Returns immediately

      • “Event-driven” programming

    • Difficult to use

    • I/O subsystem signals process when I/O completed

Two i o methods
Two I/O Methods



Hybrid io
“Hybrid” IO

  • Nonblocking - I/O call returns as much as available

    • One approach: Implemented via multi-threading

      • Some threads do blocking I/O, while others continue executing

    • Other approach: OS provides non-blocking call

      • Does not halt execution of application for an extended time, returns quickly with count of bytes read or written

      • Good e.g., select () system call for network I/O

Kernel i o subsystem
Kernel I/O Subsystem

  • Scheduling

    • Some I/O request ordering via per-device queue

    • Some OSs try fairness

  • Buffering - store data in memory while transferring between devices

    • To cope with device speed mismatch

    • To cope with device transfer size mismatch

    • To maintain “copy semantics”

Kernel i o subsystem1
Kernel I/O Subsystem

  • Caching - fast memory holding copy of data

    • Always just a copy

    • Key to performance

  • Spooling - hold output for a device

    • If device can serve only one request at a time

    • i.e., Printing

  • Device reservation - provides exclusive access to a device

    • System calls for allocation and deallocation

    • Watch out for deadlock

Error handling
Error Handling

  • OS can recover from disk read, device unavailable, transient write failures

  • Most return an error number or code when I/O request fails

  • System error logs hold problem reports

I o protection
I/O Protection

  • User process may accidentally or purposefully attempt to disrupt normal operation via illegal I/O instructions

    • All I/O instructions defined to be privileged

    • I/O must be performed via system calls

      • Memory-mapped and I/O port memory locations must be protected too

Use of a system call to perform i o
Use of a System Call to Perform I/O

Monitor = privileged/kernel mode

Life cycle of an i o request
Life Cycle of An I/O Request


  • I/O a major factor in system performance:

    • Demands CPU to execute device driver, kernel I/O code

    • Context switches due to interrupts

    • Data copying

Improving performance
Improving Performance

  • Reduce number of context switches

  • Reduce data copying

  • Reduce interrupts by using large transfers, smart controllers, polling

  • Use DMA

  • Balance CPU, memory, bus, and I/O performance for highest throughput

Unified buffer cache
Unified Buffer Cache

  • A unified buffer cache uses the same page cache to cache both memory-mapped pages and ordinary file system I/O


  • Consistency checking – compares data in directory structure with data blocks on disk, and tries to fix inconsistencies

  • Use system programs to back up data from disk to another storage device (floppy disk, magnetic tape, other magnetic disk, optical)

  • Recover lost file or disk by restoring data from backup