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.

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