玩转python（3）全局解释器锁学习心得

这几天我在GitHub上读了GIL的实现和python主循环的源码，总算对python的GIL有了大概的理解，现在来分享一下心得。
GIL源码上有这样一段注释：

The GIL is just a boolean variable (locked) whose access is protected by a mutex (gil_mutex), and whose changes are signalled by a condition variable (gil_cond). gil_mutex is taken for short periods of time, and therefore mostly uncontended.

这段时注释说明了GIL只是一个布尔值。不过为了控制各个线程对它的访问，设置了互斥锁和信号量。这几个变量被保存于结构体_gil_runtime_state之中，下面是结构体的定义：

struct _gil_runtime_state {
    /* microseconds (the Python API uses seconds, though) */
    unsigned long interval;
    /* Last PyThreadState holding / having held the GIL. This helps us
       know whether anyone else was scheduled after we dropped the GIL. */
    _Py_atomic_address last_holder;
    /* Whether the GIL is already taken (-1 if uninitialized). This is
       atomic because it can be read without any lock taken in ceval.c. */
    _Py_atomic_int locked;
    /* Number of GIL switches since the beginning. */
    unsigned long switch_number;
    /* This condition variable allows one or several threads to wait
       until the GIL is released. In addition, the mutex also protects
       the above variables. */
    PyCOND_T cond;
    PyMUTEX_T mutex;
#ifdef FORCE_SWITCHING
    /* This condition variable helps the GIL-releasing thread wait for
       a GIL-awaiting thread to be scheduled and take the GIL. */
    PyCOND_T switch_cond;
    PyMUTEX_T switch_mutex;
#endif
};

这个结构体中还有两个重要变量一个是interval，是一个计时器，默认为5ms，当一个线程执行时间超过5ms，将进行线程切换；另一个是last_holder，用于记录当前运行线程的信息，当前线程准备释放GIL时，last_holder会更新为下一个即将运行的线程的信息，这样能保证当前线程释放GIL之后，其他某个等待的线程可以立刻执行。有个隐含的意思，GIL在任一时刻只允许一个线程执行。GIL源码上对这样做的原因也做出了解释：

This is meant to prohibit the latency-adverse behaviour on multi-core machines where one thread would speculatively release the GIL, but still run and end up being the first to re-acquire it, making the “timeslices” much longer than expected.

这种机制是为了防止出现一个线程刚刚释放GIL又重新获取，导致时间片分配不均，同时简化线程调度。由于一个解释进程在任一时刻只有一个线程在执行，所以在多核心CPU上运行的python多线程程序只会利用一个核心，完全没有做到并行，要实现并行只能使用多进程，然而多进程之间的通信又成了编程中的一大难题。所以python社区也有不少人提出过要废除GIL，然而结果并不好，运行效率不增反降。
接下来可以说说为什么GIL可以保证字节码层面上的线程安全了。首先我们看看python解释器是如何工作的:

for (;;) {
    ...
    if (_Py_atomic_load_relaxed(
                    &_PyRuntime.ceval.gil_drop_request))    //判断是否有GIL释放请求，由interval决定
    {
        /* Give another thread a chance */
        if (PyThreadState_Swap(NULL) != tstate)     //这里进行线程切换，将当前线程置为NULL
            Py_FatalError("ceval: tstate mix-up");
        drop_gil(tstate);   //释放GIL

        /* Other threads may run now */

        take_gil(tstate);
        ...
    } 
    /* execute the instruction ... */ 
}

python解释器中有这样一个循环体，每一次循环都会执行一条解释器指令。进入循环后，先判断是否有释放GIL的请求(gil_drop_request=1)。若有，释放GIL，将CPU使用权交给别的线程；若无，继续执行下一条指令。也就是说，GIL的状态在解释器指令执行期间是不会发生变化的，这样就能保证当前线程不至于被其他线程干扰。

最后提醒一下，GIL只是保证解释器层面的线程安全，要保护源代码层面的对象，还需要python中锁的保护。