Lecture2 RPC and Thread

讲义：http://nil.csail.mit.edu/6.824/2020/notes/l-rpc.txt

视频：https://www.bilibili.com/video/BV1R7411t71W?p=2

Go

why go？

• good support for threads
• convenient RPC
• type safe：memory safe
• garbage-collected (no use after freeing problems)
• threads + GC is particularly attractive!
  • 相比cpp，简单很多，不用判断哪个线程最后使用共享变量并负责释放
• relatively simple
• After the tutorial, use https://golang.org/doc/effective_go.html

Thread

Go calls them goroutines; everyone else calls them threads

Why care？

• 实现并发的重要工具
• 并发是本课程的重点

Why thread？

• They express concurrency, which you need in distributed systems
• I/O concurrency
  • 多个thread，向server并行地发起RPC，等待回应
  • Client sends requests to many servers in parallel and waits for replies.
  • Server processes multiple client requests; each request may block.
  • While waiting for the disk to read data for client X, process a request from client Y.
• Multicore performance
  • Execute code in parallel on several cores.
• Convenience
  • 想在后台做一些事情，例如每过1s检查worker是否存活

替代品：event-driven

Is there an alternative to threads?

Yes: write code that explicitly interleaves activities, in a single thread.

Usually called “event-driven.” aka asynchronous programming

Keep a table of state about each activity, e.g. each client request.

One “event” loop that:

• checks for new input for each activity (e.g. arrival of reply from server),
• does the next step for each activity,
• updates state.

Event-driven gets you I/O concurrency and eliminates thread costs (which can be substantial),but doesn’t get multi-core speedup, and is painful to program.

challenge

• share data 共享内存
  • 不同的写成可以读写同一个东西，可能存在一些临界区
    • 例如，两个线程都在执行 n=n+1，机器码可能如
    • LD x,r1
    • ADD 1,r1
    • STORE r1,x
    • 结果可能会不正确
  • 称为竞争（“Race”）
  • 解决办法：加锁（比如go里的sync.Mutex）、或是直接避免使用共享变量
• coordination
  • 例如，生产消费者问题
  • 特意地互相等待
  • use Go channels or sync.Cond or WaitGroup
• deadlock