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I/O Systems

Overview

I/O management is a major component of OS design and operation.

  • important aspect of computer operation

  • I/O devices vary greatly

    IO 设备差异很大，如网卡、鼠标键盘、显示器。

  • new types of devices frequently emerge

下图里除了处理器和内存，均是外设。

I/O Hardware

Common concepts: signals from I/O devices interface with computer

  • bus

    用来做设备和 CPU 的互连。

  • port

  • controller

I/O access can use polling or interrupt.

  • Some CPU architecture has dedicated I/O instructions.

    如 x86 里有 in, out。

  • Devices are assigned addresses for registers or on-device memory

    • direct I/O instructions

      扩展性差，现在使用的少。

    • memory-mapped I/O

      把外设映射到内存地址空间，这样就可以用内存访问指令来访问外设。

Polling

CPU 主动询问设备，是否需要服务。

For each I/O operation:

  • busy-wait if device is busy (status register)

    • Cannot accept any command if busy

  • send the command to the device controller (command register)

  • read status register until it indicates command has been executed
  • read execution status, and possibly reset device status

Polling requires busy wait.
busywait 需要锁，会 sleep。所以如果设备很快那么轮询是合理的；如果设备很慢那么会很低效。

Interrupts

Interrupts can avoid busy-wait

  • device driver (part of OS) send a command to the controller (on device), and return
  • OS can schedule other activities
  • device will interrupt the processor when command has been executed
  • OS retrieves the result by handling the interrupt

Interrupt-based I/O requires context switch at start and end.
如果中断发生的频率很高，那么上下文切换会浪费很多 CPU 时间。

  • Interrupt is also used for exceptions
    • protection error for access violation
    • page fault for memory access error
    • software interrupt for system calls
  • Multi-CPU systems can process interrupts concurrently
    • sometimes a CPU may be dedicated to handle interrupts
    • interrupts can also have CPU affinity

Direct Memory Access

DMA transfer data directly between I/O device and memory.

GPU 访问内存也算 DMA，只要不经过 CPU 就算。

Application I/O Interface

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

  • in Linux, devices can be accessed as files; low-level access with ioctl.

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

Devices vary in many dimensions

如果想对设备发起控制、命令，会使用 ioctl 这个系统调用。

设备可以被大致分为：

  • Block and Character Devices

    以块为单位访问数据。支持 read, write, seek 操作。可以通过内存映射访问，也有 DMA。

    其中 character I/O 指逐个字节传输（Stream）。

  • Network Devices

    socket

  • Clocks and Timers

    provide current time, elapsed time, timer.

  • memory-mapped file access

Kernel I/O Subsystem

  • I/O scheduling
  • Buffering - store data in memory while transferring between devices.
  • Caching: hold a copy of data for fast access.
  • Spooling: A spool is a buffer that holds the output (device’s input) if device can serve only one request at a time.
  • Device reservation: provides exclusive access to a device.
  • OS needs to protect I/O devices.

Transforming I/O Requests to Hardware Operations

Performance

I/O is a major factor in system performance.

Improve Performance

  • Reduce number of context switches
  • Reduce data copying
  • Reduce interrupts by using large transfers, smart controllers, polling
  • Use DMA
  • Use smarter hardware devices
  • Balance CPU, memory, bus, and I/O performance for highest throughput
  • Move user-mode processes / daemons to kernel threads
最后更新: 2024年1月7日 14:20:35
创建日期: 2023年12月16日 01:08:34

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