I/O Models

Unix I/O 模型說明

I/O Models

• Synchronous IO
  • Blocking I/O Model (Blocking / Synchronous) : BIO, 阻塞 IO
  • Non-Blocking I/O (Non-Blocking / Synchronous) : NIO, 非阻塞 IO
  • I/O Multiplexing (Blocking / Asynchronous): I/O 多工, 多路複用 IO
  • Signal-driven I/O (SIGIO): 訊號驅動，屬於 Synchronous I/O, 訊號驅動 IO
• Asynchronous I/O (Non-Blocking / Asynchronous): AIO, 異步 IO

BIO

  sequenceDiagram
    participant App as Application
    participant K as Kernel

	Note over App: process blocks in a call to recvfrom
	App ->> K: recvfrom - system call
	K ->> K: no datagram ready
	Note over K: wait for data
	K ->> K: no datagram ready
	Note over K: wait for data
	K ->> K: datagram ready
	K ->> K: copy datagram
	K ->> K: copy complete
	Note over K: copy data from kernel to user
	K ->> App: return OK
	App ->> App: process datagram

NIO

  sequenceDiagram
    participant App as Application
    participant K as Kernel

	Note over App: process repeatedly calls recvfrom, waiting for an OK return (polling)
	App ->> K: recvfrom - system call
	K ->> K: no datagram ready
	K ->> App: EWOULDBLOCK
	App ->> K: recvfrom - system call
	K ->> K: no datagram ready
	K ->> App: EWOULDBLOCK
	App ->> K: recvfrom - system call
	K ->> K: no datagram ready
	K ->> App: EWOULDBLOCK
	Note right of K: wait for data
	App ->> K: recvfrom - system call
	K ->> K: datagram ready
	Note right of K: copy data from kernel to user
	K ->> K: copy datagram
	K ->> K: copy complete
	K ->> App: return OK
	App ->> App: process datagram

IO Multiplexing

  sequenceDiagram
    participant App as Application
    participant K as Kernel

	Note over App: process repeatedly calls select, waiting for one possibly many sockets to become readable
	App ->> K: select - system call
	K ->> K: no datagram ready
	Note over K: wait for data
	K ->> K: datagram ready
	K ->> App: return readable
	Note over App: process repeatedly calls select, waiting for one possibly many sockets to become readable
	App ->> K: recvfrom - system call
	K ->> K: copy datagram
	Note right of K: copy data from kernel to user
	K ->> K: copy complete
	K ->> App: return OK
	App ->> App: process datagram

Signal-Driven IO

  sequenceDiagram
    participant App as Application
    participant K as Kernel

	Note over K: wait for data
	App ->> App: establish SIGIO signal handler
	App ->> K: sigaction - system call
	K ->> App: return
	Note over App: process continues executing
	K ->> K: datagram ready
	Note over App: process blocks while data copied into application buffer
	Note left of App: signal handler
	K ->> App: deliver SIGIO
	App ->> K: recvfrom - system call
	K ->> K: copy datagram
	Note right of K: copy data from kernel to user
	K ->> K: copy complete
	K ->> App: return OK
	App ->> App: process datagram

AIO

  sequenceDiagram
    participant App as Application
    participant K as Kernel

	Note over K: process continues executing
	App ->> K: aio_read - system call
	Note over App: wait for data
	K ->> K: no datagram ready
	K ->> App: return
	K ->> K: datagram ready
	Note right of K: copy data from kernel to user
	K ->> K: copy complete
	K ->> App: deliver signal specified in aio_read
	Note over App: signal handler process datagram

Network IO

輸入輸出 (IO) 操作的核心概念在於 User Space Buffer 與 Kernel Space Buffer 之間的互相複製數據。

它通常包含以下兩個步驟：

1. 等待網路數據到達網卡 (NIC) (read-ready) / 等待網卡 (NIC) Writable (write-ready) → read / write 到 Kernel Space Buffer
2. 從 Kernel Space Buffer 複製數據到 User Space Buffer (read) 從 User Space Buffer 複製數據到 Kernel Space Buffer (write)