Calling python from C with OpenMP

[MRAB]

On 2016-05-13 17:22, Øystein Schønning-Johansen wrote:
> On Friday, May 13, 2016 at 2:04:53 AM UTC+2, Sturla Molden wrote:
>> You must own the GIL before you can safely use the Python C API, object
>> creation and refcounting in particular. Use the "Simplified GIL API" to
>> grab the GIL and release it when you are done.
>
> I've now read about the GIL and it looks like I am in deep problems.
>
> I've added the GILState lock to the threaded loop like this:
>
> #pragma omp parallel for
>     for( int i = 0; i < 10; i++ ){
>         PyGILState_STATE gstate;
>         gstate = PyGILState_Ensure();
>         PyObject *ret = PyObject_CallMethod( mult_obj, "do_multiply", "i", i );
>         if( !ret ){
>             printf("Cannot call 'do_multiply'\n");
>             continue;
>         }
>         printf("The value calculated in Python was: %3d\n", (int) PyLong_AsLong(ret));
>         Py_DECREF(ret);
>         PyGILState_Release(gstate);
>     }
>
> .... but still no success. Have I done it right?
>
> regs,
> -Øystein
>
Every PyGILState_Ensure call must be matched with a PyGILState_Release 
call. The way it's currently written, it won't call PyGILState_Release 
if ret is NULL.

However, I don't think you'll gain much here because you can gain from 
multi-threading only if the threads can run in parallel. You need to 
hold the GIL while making Python calls, and only 1 thread can hold the 
GIL at any time.