并行调用 boost.python

我正在尝试从使用 ThreadPoolExecutor 并行化的 python 代码中调用 C++ 代码。似乎在进入一个 C++ 函数的那一刻，所有的 Python 线程都被冻结了。例子：

#include <boost/python.hpp>
#include <iostream>
#include <iomanip>
#include <thread>
#include <chrono>

using namespace std;
using namespace std::chrono;
using namespace std::this_thread;
using namespace boost::python;

void foo(int i)
{
    cout<<"Enter " << " foo("<<i<<") (c++) thread: " << get_id() << endl;
    sleep_for(  seconds(2) );
    cout<<"Exit  " << " foo("<<i<<") (c++) thread: " << get_id() << endl;
}

BOOST_PYTHON_MODULE( test_py_async )
{
    def("foo", &foo);
}

Python：

import test_py_async
import concurrent.futures

N=8

def foo(i):
    print( f"Enter  foo({i}) (py)")
    test_py_async.foo(i)
    print( f"Exit   foo({i}) (py)")
    return i

with concurrent.futures.ThreadPoolExecutor(max_workers=N) as executor:

    print("submit tasks:")
    futures=[ executor.submit( foo, i ) for i in range(N) ]

    print("wait for tasks:")
    results=[ future.result() for future in futures ]

    print(f"results: {results}")

print(f"exit ThreadPoolExecutor")
assert( results == [ i for i in range(N) ] )

结论：

submit tasks:
Enter  foo(0) (py)
Enter  foo(0) (c++) thread: 11796
Exit   foo(0) (c++) thread: 11796
Enter  foo(1) (py)
Enter  foo(1) (c++) thread: 16580
Exit   foo(1) (c++) thread: 16580
Exit   foo(0) (py)
Enter  foo(2) (py)
Enter  foo(2) (c++) thread: 20240
Exit   foo(2) (c++) thread: 20240
Enter  foo(3) (py)
Exit   foo(1) (py)
Enter  foo(3) (c++) thread: 11796
Exit   foo(3) (c++) thread: 11796
Exit   foo(2) (py)
Enter  foo(4) (py)
Enter  foo(4) (c++) thread: 17568
Exit   foo(4) (c++) thread: 17568
Enter  foo(6) (py)
Enter  foo(6) (c++) thread: 19672
Exit   foo(6) (c++) thread: 19672
Enter  foo(5) (py)
Enter  foo(7) (py)
Enter  foo(7) (c++) thread: 16580
Exit   foo(7) (c++) thread: 16580
wait for tasks:
Exit   foo(3) (py)
Exit   foo(6) (py)
Exit   foo(7) (py)
Exit   foo(4) (py)
Exit   foo(5) (c++) thread: 16804
Exit   foo(5) (py)
results: [0, 1, 2, 3, 4, 5, 6, 7]
exit ThreadPoolExecutor

从日志中可以看出，c++ 调用在执行时间上不会与其他 c++ 调用或 Python 代码重叠，尽管这些调用确实来自不同的线程。我究竟做错了什么？