Go 读后感

The Go Memory Model

Posted by w@hidva.com on September 7, 2016

Introduction

略

Advice

当数据被多个 goroutines 同时 accesse(存取) 时, 这些 access must serialize.

To serialize access, protect the data with synchronization primitives(同步原语), such as channel operations, those in the sync and sync/atomic packages.

Happens Before

  1. 不同 goroutine 观测到的 execution order 可能不同, 其实, 一直不是很理解这里的 “观测” 究竟是个啥意思, 然后发现原文举了个栗子, 还挺形象的.

介绍了若干概念. 与 C++11 的 spec 相比, 这里介绍的概念真是太少了. 如下

  1. happens before, happen after, happen concurrently; 关于这仨概念, 具体参考原文.

    Within a single goroutine, the happens-before order is the order expressed by the program. 也就是讲, single goroutine 中所有 memory operations 之间的 happens-before order 就是代码里面 指定的顺序; 即若在代码里面, memory operation A 在 memory operation B 之前, 则 A happens before B.

    The initialization of variable v with the zero value for v’s type behaves as a write in the memory model.

    Reads and writes of values larger than a single machine word behave as multiple machine-word-sized operations in an unspecified order. 这里 multiple machine-word-sized operations 理解为: 多个 machine-word-sized operation, 每一个 operation 作用在一个机器字上.

  2. allowed to observe, 对于 variable v 上一个 read operation r 来说, 其 allowed to observe 的 write operation 其实是一个集合, 并且对于集合中每一个 write operation w 来说, 都满足指定的 条件, 这里具体的条件参考原文.

  3. To guarantee that a read op r of a variable v observes a particular(指定的) write op w to v, 需要达成的条件参见原文. 此时也可以从原文的条件中推断出: w is the only write r is allowed to observe.

  4. 结论: Within a single goroutine, there is no concurrency, so the two definitions are equivalent: a read r observes the value written by the most recent write w to v. When multiple goroutines access a shared variable v, they must use synchronization events to establish happens-before conditions that ensure reads observe the desired writes.

Synchronization

接下来指定了若干条条例, 这些条例满足 happens before 的情况.

Initialization

  1. 参见 Package initialization与原文, 这里用 happens before 这个概念重新定义了一下.

Goroutine creation

  1. The go statement that starts a new goroutine happens before the goroutine’s execution begins.

    这个条例也就解释了当使用 pthread_create() 创建线程时, 当前线程对某个变量的修改对新线程是否可见 这个问题. 实际上, 据我了解的并没有 C 语言标准中并未指定修改对新线程是否可见.

Goroutine destruction

  1. The exit of a goroutine is not guaranteed to happen before any event in the program.

Channel communication

  1. 参见原文, 了解 4 个条例就行.

Locks

  1. 参见原文, 了解 2 个条例就行.

    总结来说的话, 就是若某次 unlock()(可能是 RUnlock() 或者 Unkock()) 操作导致了 lock()(可能是 Lock(), 或者 RLock()) 操作返回, 则这个 unlock() 操作 happens before 这次 lock() 操作.

Once

  1. A single call of f() from once.Do(f) happens (returns) before any call of once.Do(f) returns.

Incorrect synchronization

略

参考

  1. The Go Memory Model 1.7
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