boost.python c ++多線程

Question

我正在編寫一個包含c ++模塊的python程序（ .so ，使用boost.python ）。
我正在啟動幾個運行c ++函數的python線程。

這就是C ++代碼的樣子：

#include <boost/python.hpp>
using namespace boost;
void f(){
    // long calculation

    // call python function

    // long calculation
}

BOOST_PYTHON_MODULE(test)
{
    python::def("f", &f);
}

和python代碼：

from test import f
t1 = threading.Thread(target=f)
t1.setDaemon(True)
t1.start()
print "Still running!"

我遇到了一個問題：“還在運行！” 消息未顯示，我發現c ++線程持有GIL。

在我從python代碼運行c ++代碼的情況下，處理GIL的最佳方法是什么？

謝謝！ 加爾

Answer 1

我經常發現使用RAII風格的類來管理全局解釋器鎖 （GIL）提供了一個優雅的異常安全解決方案。

例如，使用以下with_gil類，當創建with_gil對象時，調用線程將獲取GIL。 當破壞with_gil對象時，它將恢復GIL狀態。

/// @brief Guard that will acquire the GIL upon construction, and
///        restore its state upon destruction.
class with_gil
{
public:
  with_gil()  { state_ = PyGILState_Ensure(); }
  ~with_gil() { PyGILState_Release(state_);   }

  with_gil(const with_gil&)            = delete;
  with_gil& operator=(const with_gil&) = delete;
private:
  PyGILState_STATE state_;
};

而補充的without_gil類則相反：

/// @brief Guard that will unlock the GIL upon construction, and
///        restore its staet upon destruction.
class without_gil
{
public:
  without_gil()  { state_ = PyEval_SaveThread(); }
  ~without_gil() { PyEval_RestoreThread(state_); }

  without_gil(const without_gil&)            = delete;
  without_gil& operator=(const without_gil&) = delete;
private:
  PyThreadState* state_;
};

它們在函數中的用法如下：

void f()
{
  without_gil no_gil;       // release gil
  // long calculation
  ...

  {
    with_gil gil;           // acquire gil
    // call python function
    ...
  }                         // restore gil (release)

  // long calculation
  ...
}                           // restore gil (acquire)

也可以使用更高級別的方便類來提供類似std::lock_guard體驗。 GIL的獲取和釋放，保存和恢復語義與普通的互斥鎖略有不同。 因此， gil_guard接口是不同的：

• gil_guard.acquire()將獲得GIL
• gil_guard.release()將發布GIL
• gil_guard_restore()將恢復以前的狀態

/// @brief Guard that provides higher-level GIL controls.
class gil_guard
{
public:
  struct no_acquire_t {} // tag type used for gil acquire strategy
  static no_acquire;

  gil_guard()             { acquire(); }
  gil_guard(no_acquire_t) { release(); }
  ~gil_guard()            { while (!stack_.empty()) { restore(); } }

  void acquire()          { stack_.emplace(new with_gil); }
  void release()          { stack_.emplace(new without_gil); }
  void restore()          { stack_.pop(); }

  static bool owns_gil()
  {
    // For Python 3.4+, one can use `PyGILState_Check()`.
    return _PyThreadState_Current == PyGILState_GetThisThreadState();
  }

  gil_guard(const gil_guard&)            = delete;
  gil_guard& operator=(const gil_guard&) = delete;

private:
  // Use std::shared_ptr<void> for type erasure.
  std::stack<std::shared_ptr<void>> stack_;
};

它的用法是：

void f()
{
  gil_guard gil(gil_guard::no_acquire); // release gil
  // long calculation
  ...

  gil.acquire();                        // acquire gil
  // call python function
  ...
  gil.restore();                        // restore gil (release)

  // long calculation
  ...
}                                       // restore gil (acquire)

---

這是一個完整的示例， 演示了使用這些輔助類的GIL管理：

#include <cassert>
#include <iostream> // std::cout, std::endl
#include <memory>   // std::shared_ptr
#include <thread>   // std::this_thread
#include <stack>    // std::stack
#include <boost/python.hpp>

/// @brief Guard that will acquire the GIL upon construction, and
///        restore its state upon destruction.
class with_gil
{
public:
  with_gil()  { state_ = PyGILState_Ensure(); }
  ~with_gil() { PyGILState_Release(state_);   }

  with_gil(const with_gil&)            = delete;
  with_gil& operator=(const with_gil&) = delete;
private:
  PyGILState_STATE state_;
};

/// @brief Guard that will unlock the GIL upon construction, and
///        restore its staet upon destruction.
class without_gil
{
public:
  without_gil()  { state_ = PyEval_SaveThread(); }
  ~without_gil() { PyEval_RestoreThread(state_); }

  without_gil(const without_gil&)            = delete;
  without_gil& operator=(const without_gil&) = delete;
private:
  PyThreadState* state_;
};

/// @brief Guard that provides higher-level GIL controls.
class gil_guard
{
public:
  struct no_acquire_t {} // tag type used for gil acquire strategy
  static no_acquire;

  gil_guard()             { acquire(); }
  gil_guard(no_acquire_t) { release(); }
  ~gil_guard()            { while (!stack_.empty()) { restore(); } }

  void acquire()          { stack_.emplace(new with_gil); }
  void release()          { stack_.emplace(new without_gil); }
  void restore()          { stack_.pop(); }

  static bool owns_gil()
  {
    // For Python 3.4+, one can use `PyGILState_Check()`.
    return _PyThreadState_Current == PyGILState_GetThisThreadState();
  }

  gil_guard(const gil_guard&)            = delete;
  gil_guard& operator=(const gil_guard&) = delete;

private:
  // Use std::shared_ptr<void> for type erasure.
  std::stack<std::shared_ptr<void>> stack_;
};

void f()
{
  std::cout << "in f()" << std::endl;

  // long calculation
  gil_guard gil(gil_guard::no_acquire);
  assert(!gil.owns_gil());
  std::this_thread::sleep_for(std::chrono::milliseconds(500));
  std::cout << "calculating without gil..." << std::endl;

  // call python function
  gil.acquire();
  assert(gil.owns_gil());
  namespace python = boost::python;
  python::object print =
  python::import("__main__").attr("__builtins__").attr("print");
    print(python::str("calling a python function"));
  gil.restore();

  // long calculation
  assert(!gil.owns_gil());
  std::cout << "calculating without gil..." << std::endl;
}

BOOST_PYTHON_MODULE(example)
{
  // Force the GIL to be created and initialized.  The current caller will
  // own the GIL.
  PyEval_InitThreads();

  namespace python = boost::python;
  python::def("f", +[] {
    // For exposition, assert caller owns GIL before and after
    // invoking function `f()`.
    assert(gil_guard::owns_gil());
    f();
    assert(gil_guard::owns_gil());
  });
}

互動用法：

>>> import threading
>>> import example
>>> t1 = threading.Thread(target=example.f)
>>> t1.start(); print "Still running"
in f()
Still running
calculating without gil...
calling a python function
calculating without gil...
>>> t1.join()