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[UCSD CSE120]输入/输出系统-I/O system

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本文是我在上UCSD的 CSE 120: Principles of Operating Systems (Winter 2020) 整理的笔记，这一课主要介绍了操作系统里面输入系统系统的概念，包括硬件上的组成以及软件的结构。

I/O basic

  1. Intro

    • I/O = Input/Output

      • Input from attached device to CPU/memory
      • Output from CPU/memory to device

    • Synchronization and transferring data

  2. Issues

    • Problems:

      • So many different types of I/O devices
      • Wide range: speed, operation, data transfer units

    • Questions:

      • How does a process initiate I/O?
      • How is synchronization achieved?
      • How is data transferred?

  3. Background: I/O Hardware

    • CPU and device (controller) communicate via

      • I/O instructions
      • Memory instructuions (memory-mapped)

    • Data transfer: progammed I/O vs. DMA (direct memory access)

    • Synchronization: polling vs. interrupts

  4. Buffered/Unbuffered I/O

    • Pros:

      • What if pages containing buffer are paged out?
      • What if entire process is swapped out?
      • Can pin pages, but if too many processes do this?

    • Cons:

      • Memory copying is expensive
      • Consider effect on caches

  5. Dealing with Complexity of Devices

    • Many different types of devices

      • Classify by shared characteristics
      • Imposes structure: shared code, lower complexity

    • Dimensions

      • Varaible vs. fixed size units
      • Sequential vs. random-access
      • Synchronous vs. asynchronous
      • Speed of operation

I/O system

  1. I/O system Intro

    • Software that deals with I/O

      • Mostly in the kernel
      • Also in processes (in form of library, e.g., stdio)

    • Separated into two portions

      • Device-dependent
      • Device- independent

    • Structure: Layered

  2. Device Dependent: Device Drivers

    • Encapsulates device-dependent code

      • Contains device-specific register reads/write

    • Implements a standard interface

      • open(), close(), read(), write()

    • Interrupt handlers

      • Executes when I/O completes
      • Updates data structure
      • Wakes up waiting process

  3. Device-Independent I/O

    • Uniform interfacing for device drivers
    • Naming, protection
    • Uniform block size
    • Buffering, caching
    • Storage allocation
    • Locking
    • Error handling

  4. User-space I/O

    • Convenient interface

      • printf() vs. write()

    • User-level buffering

      • Unix: stdio library

    • Spooling daemons

      • Printer

  5. Overall Operation

Example: UNIX

  1. I/O Model

    • Uses file system interface
    • stdio.h: C standard I/O library

    • Block devices (disks, USB cameras, …)

      • Fixed-size blocks
      • Randomly addressable
      • Uses buffer cache

    • Character devices (serial ports, parallel ports, sound cards,…)

      • Variable sequence of bytes
      • For non-block devices

  2. I/O System Call Interface

    • fd = open(“/dev/devname”, …)
    • close(fd)
    • nr = read(fd, buf, n)
    • nw = write(fd, buf, n)
    • ioctl(fd, cmd, buf) //(input/output control)

  3. Standard I/O Library

    • fopen, fread, fwrite, fprintf, fscanf, fclose, …
    • Private buffer kept in user space
    • Minimizes the number of I/O system calls

  4. Software Block Cache Design

    • Has copies of blocks that are also on disk

    • Upon read or write

      • Check if a buffer contains the block
      • If not, get from disk
      • To make room, remove LRU block