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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

手机的CPU和内存装到电脑上还能用

下图里除了处理器和内存，均是外设。

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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: connection point for device
  • controller: usually contains processor, microcode, private memory, bus controller, etc

I/O access can use polling or interrupt.

  • polling: IO主动向CPU发请求，CPU中断

操作系统空闲的时候在等event(interrupt和exception也算event)

timer interrupt：减少时间片，减到0 schedule

  • Some CPU architecture has dedicated I/O instructions.

如 x86 里有 in, out。

  • Devices usually provide registers for data and control I/O of device

  • Registers include data-in/data-out, status, control or command register

  • 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。

spinlock也有busy waiting

解决busy waiting: yield -> moving from running to sleeping

解决busy waiting

Interrupts

Interrupts can avoid busy-wait

cpu中call device的线程(t1 for example)进入device的waiting queue，然后cpu切换到其他线程，等device执行完后给cpu发一个interrupt，同时把t1加入cpu的ready queue

  • 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。

  • 下图虚线中CPU在执行其他线程
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  • 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

SMP IRQ Affinity

  • It allows you to restrict or repartition the work load that you server must do so that it can more efficiently do it's job.
  • "balance" out multiple NiCs in a multi-processor machine. By tying a single NIC to a single CPU, you should be able to scale the amount of traffic your server can handle nicely.

Direct Memory Access

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

GPU/NPU/TPU等XPU 访问内存也算 DMA，只要不经过 CPU 就算。

  • device driver在CPU上跑，所有的controller都是在设备上跑的
  • 第2步使用syscall ioctl
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Application I/O Interface

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

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

抽象成file，因为read和write的第一个参数是fd，这样所有内容都可以用read和write读了

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

img

Devices vary in many dimensions

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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

Synchronous/Asynchronous I/O

  • Synchronous I/O includes blocking and non-blocking I/o

  • blocking I/O: process suspended until I/O completed

  • non-blocking I/O: I/O calls return as much data as available

  • Asynchronous I/O: process runs while I/O executes,

  • I/O subsystem signals process when I/O completed via signal or callback

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.

  • keystrokes can be stolen by a keylogger if keyboard is not protected

    通过键盘输入获取密码

  • to protect I/O devices

  • define all I/O instructions to be privileged

    • I/O must be performed via system calls

  • memory-mapped I/O and I/O ports must be protected too

  • kernel data structures

  • Kernel keeps state info for I/O components
    • e.g., open file tables, network connections, character device state
  • Some OS uses message passing to implement I/O, e.g., Windows
    • message with I/O information passed from user mode into kernel
    • message modified as it flows through to device driver and back to process
img

I/O Requests to Hardware

  • System resource access needs to be mapped to hardware
image-20241204162651181

Performance

I/O is a major factor in system performance

比如磁盘很慢，开机很耗时间

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

Linux IO

  1. Device initialization 需要先注册，注册的时候会把设备对应正确的读写操作的函数也给初始化
  • /dev/tty
    • tty_init
    • create /dev/tty file
image-20241204163150625
  1. Device write: Write (echo) to the file reach vfs_write, which eventually calls tty_write

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