boost.python not supporting parallelism?

I am trying to wrap a piece of C++ code into python lib using boost.python, however, I found out that multiple instances cannot run at the same time:

code (C++):

class Foo{
public:
    Foo(){}
    void run(){
        int seconds = 2;
        clock_t endwait;
        endwait = clock () + seconds * CLOCKS_PER_SEC ;
        while (clock() < endwait) {}
    }   

};

BOOST_PYTHON_MODULE(run_test)
{
   using namespace boost::python;

   class_<Foo>("test", init<>())
      .def("run", &Foo::run)
      ;   

}

which is compile using CMake (CMake):

add_library(run_test SHARED run_test.cpp)
target_link_libraries(run_test boost_python python2.7)

and tested with the following code (Python):

class Dos(threading.Thread):
    def run(self):
        printl('performing DoS attack')

        proc = test()
        proc.run()

for i in range(5):
    t = Dos()
    t.start()

The output indicates that the code is parallelized in a very weird way. Each thread should take only 2 seconds and 4 threads should run simultaneously on my quadcore machine:

[2011-11-04 13:57:01] performing DoS attack
 [2011-11-04 13:57:01] performing DoS attack
[2011-11-04 13:57:05] performing DoS attack
 [2011-11-04 13:57:05] performing DoS attack
[2011-11-04 13:57:09] performing DoS attack

thank you for your help!