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Inter-Process Communication

Reasons for cooperating processes

  • Information sharing
  • Computation speedup
  • Modularity
  • Convenience

IPC Communication Models

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  • Most OSes implement both 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
  • One process creates a shared memory segment

  • Processes can then “attach” it to their address spaces

  • Processes communicate by reading/writing to the shared memory region

  • They are responsible for not stepping on each other’s toes
  • The OS is not involved at all

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ipcs -a 可以查看当前 IPC 的状态。

  • Question: How do processes find out the ID of the shared memory segment?
  • Better solution: one could use message-passing to communicate the id!

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。

Message Passing

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

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”

  1. 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。每对之间仅存在一个link，link通常是bi-directional的

    • Indirect

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

      • 多个进程接收时，有以下solution
      • 一个link最多只允许与两个process建立联系
      • 同时间只允许有一个process执行receive操作
      • Allow the system to select arbitrarily the receiver

    • Processes can communicate only if they share a mailbox

    • Each pair of processes may share several communication links 允许系统任意选择接收机

  • 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

    • Non-blocking send -- the sender sends the message and continue

      Non-blocking receive -- the receiver receives a valid message, or null message

    • 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

  1. Ordinary Pipes：没有名字，只能通过 fork() 来传播。 Ordinary pipes are unidirectional - Producer writes to one end (the write-end of the pipe) - Consumer reads from the other end (the read-end of the pipe)
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注意 fd[0] 是 read-end，fd[1] 是 write-end（对于双方都是）

Windows calls these anonymous pipes

  1. Named Pipes - communication是双向的 - 不需要parent-child relationship - 多个进程间可以使用named pipes 进行通信

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

  • The command “ls | grep foo” creates two processes that communicate via a pipe
  • The ls process writes on the write-end
  • The grep process reads on the read-end

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(Remote Procedure Calls)

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

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