Terminology Recap: Preemptive / Cooperative / Multitasking / Multithreading / Race Condition

Preemptive / Cooperative / Multitasking

根据 MIT 1.124 Lecture 15: Multitasking is the ability of a computer’s operating system to run several programs (or processes) concurrently on a single CPU.

• This is done by switching from one program to another fast enough to create the appearance that all programs are executing simultaneously.

Preemptive 和 Cooperative 指的其实是 scheduling 的形式，可以看做 Multitasking 的两种 type. 根据 Davide Spataro:

• Preemptive
  • It means that threads are not in control on when and/or for how long they are going to use the CPU and run. It is the scheduler (a component of the OS) that decides at any moment which thread can run and which has to sleep. You have no strong guarantees on what will be the next time a thread will run, and for how long. It is completely up to the scheduler.
• Cooperative
  • In cooperative multitasking, what happens is that the scheduler has no say in when a thread can run. Each thread decides for how long it keeps the CPU. If it decided not to share the CPU with any other thread, then no other threads will run causing what is known as starvation.

注意：

• 虽然这里说的是 thread，但应该是指 process
• 另外要注意语法:
  • A process can be preemptable
  • A process is preempted by a scheduler
  • Processes can be coopeartive (to each other)
• 切换 process 的时候，context switch 是额外的 resource overhead. In certain real-time low latency application (like high frequency trading), this can be quite unacceptable.

Multithreading

根据 MIT 1.124 Lecture 15: Multithreading extends the concept of multitasking by allowing individual programs to perform several tasks concurrently. Each task is referred to as a thread and it represents a separate flow of control.

• What then is the difference then between a process and a thread? The answer is that each process has its own set of variables, whereas threads share the same data and system resources.

上面这些都没啥好讲的。

和 Multitasking 一样，Multithreading 也能分成 Preemptive 和 Cooperative:

• 一般的 multi-thread lib 应该有自己的 scheduler 吧？就算没有，也应该能托管给 OS 来做。所以如果我只是创建了一群 threads 让它们自由地跑，应该是 Preemptive 的情况
• 如果我自己要精细控制，我可以搞 synchronized、wait、yield、join 这些操作，那就是稍微添加了点 Cooperative 的意味
  • 要不要做成完全的 Cooperative 形式，要看你自己 application 的需求

Race Condition

根据 MIT 6.031 Reading 19: Concurrency: A race condition means that the correctness of the program (the satisfaction of postconditions and invariants) depends on the relative timing of events in concurrent computations A and B. When this happens, we say “A is in a race with B.”

按这个说法，race condition 不是 multithreading 独有的，广义上的 multitasking 也可以有 race condition (比如两个 git processes 同时 commit 相同的一个 resource)。但一般来说，race condition 还是在 multithreading 的语境下出现得比较多，毕竟我们很少去操控 multitasking.

给 resource 加锁是最常见的避免 race condition 的手段。