I/O Systems¶

Overview¶

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

下图里除了处理器和内存，均是外设。

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

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。

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 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

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。

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

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.

I/O is a major factor in system performance.

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