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关于IO多路复用为什么搭配非阻塞IO？ #482

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jinshuai86 opened this issue Dec 5, 2018 · 6 comments
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关于IO多路复用为什么搭配非阻塞IO？ #482

jinshuai86 opened this issue Dec 5, 2018 · 6 comments
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@jinshuai86
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@jinshuai86 jinshuai86 commented Dec 5, 2018

  • 关于Java IO介绍NIO的一段：“ 通过配置监听的通道 Channel 为非阻塞，那么当 Channel 上的 IO 事件还未到达时，就不会进入阻塞状态一直等待，而是继续轮询其它 Channel，找到 IO 事件已经到达的 Channel 执行。”
  • 我觉得配置SocketChannel为非阻塞是为了在IO复用第二阶段(copy data from kernel to user) 使得对socket的操作是非阻塞。
  • Linux对于select()为什么搭配非阻塞IO使用: Under Linux, select() may report a socket file descriptor as “ready for reading”, while nevertheless a subsequent read blocks. This could for example happen when data has arrived but upon examination has wrong checksum and is discarded. There may be other circumstances in which a file descriptor is spuriously reported as ready. Thus it may be safer to use O_NONBLOCK on sockets that should not block. 我理解的是如果socket配置为阻塞，当socket可用后，内核会校验socket的数据，如果校验错误会丢弃这些数据，而此时调用read()由于没有数据了会阻塞当前线程。相反的是如果配置为非阻塞就不会阻塞当前线程。
  • “那么当 Channel 上的 IO 事件还未到达时，就不会进入阻塞状态一直等待” 我觉得Channel对应的IO事件没有发生时，调用select()会一直阻塞线程，即使配置Channel为非阻塞。
  • 参考了知乎的一个回答：链接
  • 望大佬解惑😭
@CyC2018
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@CyC2018 CyC2018 commented Dec 6, 2018

@jinshuai86

Java NIO 被称为非阻塞 IO，这是与传统的 BIO（阻塞 IO）相对的。BIO 执行 accept 等操作时会一直阻塞，而 NIO 不会阻塞。只要理解了 Reactor 模型就能理解这里的非阻塞意义。

上面的 select 文档提到的非阻塞用途和这里的不太一样。
@jinshuai86
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@jinshuai86 jinshuai86 commented Dec 6, 2018

  • 谢谢大佬回复。不过还是不太明白这句话 “通过配置监听的通道 Channel 为非阻塞，那么当 Channel 上的 IO 事件还未到达时，就不会进入阻塞状态一直等待，而是继续轮询其它 Channel，找到 IO 事件已经到达的 Channel 执行。” 主要不太明白socketChannel.configureBlocking(false)的目的。大佬的意思是为了在select()轮询Channel时，如果Channel不可用时不会阻塞当前线程吗？ 还是不会阻塞XXX？这里不太清楚😭
  • 我理解的socketChannel.configureBlocking(false)目的是为了搭配非阻塞IO使用。当调用Selecor的select()，尽管设置了非阻塞(socketChannel.configureBlocking(false))，但是当注册的事件没发生时还是会一直阻塞当前线程。
  • 关于这个函数的介绍文档：链接
    • block - If true then this channel will be placed in blocking mode; if false then it will be placed non-blocking mode
  • 关于函数介绍中提到的blocking mode文档介绍：链接，里面提到了通过设置非阻塞可以在I/O Operation时不阻塞当前线程，我理解的I/O Operation是把数据从内核态拷贝到用户态的过程，此时如果内核态丢弃数据导致没有了数据而不会阻塞线程就是select手册里提到的更安全一些？
    • Blocking mode
      • A selectable channel is either in blocking mode or in non-blocking mode. In blocking mode, every I/O operation invoked upon the channel will block until it completes. In non-blocking mode an I/O operation will never block and may transfer fewer bytes than were requested or possibly no bytes at all. The blocking mode of a selectable channel may be determined by invoking its isBlocking method.
      • Newly-created selectable channels are always in blocking mode. Non-blocking mode is most useful in conjunction with selector-based multiplexing. A channel must be placed into non-blocking mode before being registered with a selector, and may not be returned to blocking mode until it has been deregistered.
  • 另外看了个关于select的博客，我理解的是当前线程调用select以后，好像就不是在当前线程对应的内核态去执行(轮询Channel)，而是通过内核态其它线程执行？链接。有点转不过弯来😭
@CyC2018
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@CyC2018 CyC2018 commented Dec 6, 2018

@jinshuai86

Non-blocking mode is most useful in conjunction with selector-based multiplexing.

这里的非阻塞特性允许一个线程处理多个 Channel，而传统的 BIO 一个线程只能处理一个 Channel。

我理解的 I/O Operation 是把数据从内核态拷贝到用户态的过程

你理解感觉不对，这里的非阻塞描述的是第一个阶段，而 select 文档那里写的非阻塞描述的是第二个阶段。
@jinshuai86
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@jinshuai86 jinshuai86 commented Dec 6, 2018

谢谢大佬，之前理解的IO复用第一阶段(调用select()直到有可用socket)一直会阻塞当前线程，然后在第二阶段(copy data from kernel to user)如果socket设置了非阻塞，在内核没有数据时(就像select手册里说的由于检验错误丢弃了已经准备好的数据)进行I/O operation 不会阻塞当前线程。我理解应该有问题，我再去查查资料😄😄
@CyC2018
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@CyC2018 CyC2018 commented Dec 7, 2018

@jinshuai86 IO 多路复用在第一阶段既可以说成是阻塞，也可以说成是非阻塞，就看从哪方面去考虑。
@makeghyours
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@makeghyours makeghyours commented Dec 8, 2018

هل تدعم العربية
@CyC2018 CyC2018 closed this Dec 8, 2018
@CyC2018 CyC2018 added the question label Feb 24, 2019
@CyC2018 CyC2018 reopened this Feb 25, 2019
@CyC2018 CyC2018 closed this Jun 27, 2019
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