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Synchronous & Asynchronous

I/O models and Sync-Async

  • Four types of I/O models
    • Four types of I/O models

  1. Synchronous blocking I/O

    • Synchronous blocking I/O
    • The application blocks until the system call is complete (data transferred or error).

  2. Synchronous non-blocking I/O

    • Synchronous non-blocking I/O
    • In this model, a device is opened as non-blocking. This means that instead of completing an I/O immediately, a read may return an error code indicating that the command could not be immediately satisfied (EAGAIN or EWOULDBLOCK)
    • Application may call numerous times to await the read completion.
    • This can be extremely inefficient.

  3. Asynchronous blocking I/O

    • Asynchronous blocking I/O
    • In this model, non-blocking I/O is configured, and then the blocking select system call is used to determine when there's any activity for an I/O descriptor.
    • So, IO is non-blocking, notification(kernel to application) is blocking
    • select is not very efficient. Not advisable for high performance I/O.

  4. Asynchronous non-blocking I/O (AIO) Our interest

    • Asynchronous non-blocking I/O (AIO)
    • The read request returns immediately, indicating that the read was successfully initiated.
    • The application can then perform other processing while the background read operation completes.
    • When the read response arrives, a signal or a thread-based callback can be generated to complete the I/O transaction.
    • When many slow I/O call happens, threads can be fully operational regardless of I/O blocking.

ref : Boost application performance using asynchronous I/O ref(translated in Korean) : Asynchronous IO 개념 정리

Asynchronous programming

  • Asynchrony, in computer programming, refers to the occurrence of events independent of the main program flow and ways to deal with such events.
  • 메인 프로그램을 blocking(wait) 하지 않고 수행 -> 병렬 컴퓨팅 가능(parallel computing)
  • 일반적으로 subroutine 을 통해 처리
    • ex) procedure, function(callback), method, routin, subprogram 등으로 불림
  • future or promise

ref : Wikipedia : Asynchrony

node.js

node.js architecture

nodejs architecture

ref : http://biznomy.com/nodejs-development-company

key points

  • single threaded event loop
  • worker thread pool (to enable it, node.js uses libuv)
    • libuv : libuv (Unicorn Velociraptor Library) is a multi-platform C library that provides support for asynchronous I/O based on event loops.
      • It supports epoll(4), kqueue(2), Windows IOCP, and Solaris event ports.
  • V8 javascript engine
    • V8 compiles JavaScript directly to native machine code before executing it