异步I/O机制分类

发布于 2017-09-01 15:06:16

Some forms

  • Process
    • Expensive to create and maintain
    • Usally the number of processes is small and pooled
  • Polling
    • Available in traditional Unix and Windows
    • Waster CPU time polling repeatedly when there is nothing else for the issuing process to do
    • Essentially single-threaded, so may be unable to exploit I/O parallelism the hardware is capable of
  • Select(/poll) loops
    • Available in BSD Unix, and almost TCP/IP protocol stack
    • Is a variation of polling
    • A select loop uses the select system call to sleep until a condition occurs on a file descriptor
    • Reliable and relatively efficient, depends haveily on the Unix paradigm tha “everything is a file”
    • select system call allow tuning of the acceptable event set
  • Signals
    • Essentially interrupts
    • Available in BSD and POSIX Unix
    • The main flow of the process could have been interruptted at nearly any point, resulting in inconsistent data structures as seen by the signal handler
      • The signal handler is usually not able to issue further asynchronous I/O by itself
      • Every blocking system call is potentially interruptible
      • The programmer must usually incoroprate retry code at each call
  • Callback functions
    • Available in classic Mac OS, MVS and Windows
    • Bear many of the characteristics of the signal method
    • Each I/O request usually can have its own completion function, whereas the signal system has a single callback
    • Stack depth can grow unmanageably
  • Light-weight processes(LWPs) or threads
    • Available in more modern Unixes
    • Like the process, but without the data isolation
      • Introduce its own problems, requrie kernel-provided synchronizaion mechanisms and thread-safe libraries
      • Each LWP use traditional blocking synchronous I/O
  • Completion queues/ports
    • Available in Microsoft Windows, Solaris, AmigaOS, and DNIX
    • I/O requests are issued asynchronously, but notifications of completion are provided via a synchronizing queue mechanism in the order they are completed
  • Event flags
    • It is essentially a completion queue of depth one
    • To simulate the effect of queue ‘depth’, an additional event flag is required for each potential unprocessed (but completed) event, or event information can be lost.
  • Channel I/O
    • Available in mainframes（大型机） by IBM, Groupe Bull, and Unisys
    • Channel I/O is designed to maximize CPU utilization and throughput by offloading most I/O onto a coprocessor（协处理器）
    • The coprocessor has onboard DMA, handles device interrupts, is controlled by main CPU, and only interrupts main CPU when it’s truly necessary.
  • Registered I/O
    • Available in Windows Server 2012 and Windows 8

The vast majority of general-purpose computing hardware relies entirely upon two methods of implementing asynchronous I/O: polling and interrupts.

Usually both methods are used together, the balance depends heavily upon the design of the hardware and its required performance characteristics.

  • Pure polling systems are entirely possible, small microcontrollers are often built this way.
  • Most general-purpose computing systems rely heavily upon interrupts.
  • Hybrid approaches are also possible, wherein an interrupt can trigger the beginning of some burst of asynchronous I/O, and polling is used within the burst itself.

C10K problem 翻译文章句子节选

一个线程服务多个客户端，并使用非阻塞I/O和电平触发的就绪通知

在Linux上/dev/poll有很多种实现，但是没有一种性能与epoll一样好，这些实现从来没有真正完整过。在linux上/dev/poll是不建议的。

一个线程服务多个客户端，并使用非阻塞 I/O 和变更就绪通知

“边缘触发”是“水平触发”就绪通知的对立面。这对编程错误有点不宽容，因为如果你错过了唯一的事件，连接事件就将永远的卡住。

一个服务器线程处理多个客户端请求，并使用异步I/O

一个服务器线程服务一个客户端，使用阻塞I/O

这样不好的地方在于需要为每个客户端使用一个完整的栈，从而比较浪费内存。

你可以减小每个线程所拥有的栈内存大小，但是由于大部分线程库在一旦线程创建后就不能增大线程栈大小，所以这样做就意味着你必须使你的程序最小程度地使用内存。

1:1 线程 vs M:N 线程

从某个角度来说, M:N被认为拥有更好的性能，但是由于很难被正确的编写，所以大部分人都远离了该方法。

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