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Chap03 | Inter-Process Communication#

Processes within a host may be independent or cooperating.
Reasons for cooperating processes:

  • Information sharing
  • Computation speedup
  • Modularity
  • Convenience

进程保护的太好了，需要有互相通信的手段。

Models of IPC

  • Shared memory
  • Message passing
  • Signal
  • Pipe
  • Socket

IPC Communication Models#

  • Message-passing
    需要内核空间支持
    • useful for exchanging small amounts of data
    • simple to implement in the OS
    • sometimes cumbersome for the user as code is sprinkled with send/recv operations
    • high-overhead: one syscall per communication operation
  • Shared memory
    非内核空间
    • low-overhead: a few syscalls initially, and then none
    • more convenient for the user since we’re used to simply reading/writing from/to RAM
    • more difficult to implement in the OS

Shared Memory#

Processes need to establish a shared memory region.

producer/consumer example

Example

ipcs -a 可以查看当前 IPC 的状态。

存在问题：不安全。任何人拿到 share_id 都可以把共享内存 attach 到自己进程上，可以观察到其他进程的数据、甚至做 DOS 攻击。

而且很 cubersome，会发生各种 error 需要处理。（现在使用不多）

Message Passing#

Two fundamental operations:

  • send: to send a message (i.e., some bytes)
  • recv: to receive a message

If processes P and Q wish to communicate they

  • establish a communication “link” between them

    This “link” is an abstraction that can be implemented in many ways (even with shared memory!!)

  • place calls to send() and recv()

  • optionally shutdown the communication “link”

Implementing Message-Passing#

Implementation of communication link

  • Physical:
    • Shared memory
    • Hardware bus
    • Network

  • Logical:

    • Direct or indirect

      • Direct

        有一个 P 和 Q，直接发信息。如果有 n 个进程，需要建立 \(C_2^n\) 个连接。

      • Indirect

        有一个 mailbox，发信息相当于发给一个 mailbox。如果有多个进程，我们需要确定是由哪个进程接收信息。

    • Synchronous or asynchronous

      • Synchronous: Blocking is considered synchronous

        即我们发信息，如果接收者没收到信息，发送者就堵塞着不走；我们收信息，如果发送者没有发送信息，接送者就堵塞着不走。

      • Asynchronous: Non-blocking is considered asynchronous

      • 异步效率更高，同步时效性更高。
        • Automatic or explicit buffering
      • Zero capacity - no messages are queued on a link. Sender must wait for receiver.
      • Bounded capacity - finite length of n messages. Sender must wait if link full.X
      • Unbounded capacity - infinite length. Sender never waits.

Pipes#

  • Ordinary pipes

    没有名字，只能通过 fork() 来传播。

    • Producer writes to one end (the write-end of the pipe)
    • Consumer reads from the other end (the read-end of the pipe)

    注意 fd[0] 是 read-end，fd[1] 是 write-end（对于双方都是）

  • Named pipes

    可以把名字通过网络/文件传播，这样就能交互。（可以使用 mkfifo 创建 named pipes）

In UNIX, a pipe is mono-directional.
要实现两个方向一定需要两个 pipe。

Client-Server Communication#

广义上的 IPC，因为是跑在两个物理机器上的交互。

  • Sockets

  • RPCs

    所有的交互都是和 stub 通信，stub 会和远端的 server 通信。 存在网络问题，如丢包。

  • Java RMI

    RPC in Java

Takeaway#

Takeaway

  • Communicating processes are the basis for many programs/services
  • OSes provide two main ways for processes to communicate
    • shared memory
    • message-passing
  • Each way comes with many variants and in many flavors
  • Signals, Pipes, Sockets, RPCs, RMIs, etc.
Last update: 2024年11月9日 17:03:32
Created: 2024年9月17日 14:52:21