I am writing a C extension for Python, which should release the Global Interpreter Lock while it operates on data. I think I have understood the mechanism of the GIL fairly well, but one question remains: Can I access data in a Python object while the thread does not own the GIL? For example, I want to read data from a (big) NumPy array in the C function while I still want to allow other threads to do other things on the other CPU cores. The C function should
- release the GIL with
Py_BEGIN_ALLOW_THREADS
- read and work on the data without using Python functions
- even write data to previously constructed NumPy arrays
- reacquire the GIL with
Py_END_ALLOW_THREADS
Is this safe? Of course, other threads are not supposed to change the variables which the C function uses. But maybe there is one hidden source for errors: could the Python interpreter move an object, eg. by some sort of garbage collection, while the C function works on it in a separate thread?
To illustrate the question with a minimal example, consider the (minimal but complete) code below. Compile it (on Linux) with
gcc -pthread -fno-strict-aliasing -DNDEBUG -g -fwrapv -fPIC -I/usr/lib/pymodules/python2.7/numpy/core/include -I/usr/include/python2.7 -c gilexample.c -o gilexample.o
gcc -pthread -shared gilexample.o -o gilexample.so
and test it in Python with
import gilexample
gilexample.sum([1,2,3])
Is the code between Py_BEGIN_ALLOW_THREADS
and Py_END_ALLOW_THREADS
safe? It accesses the contents of a Python object, and I do not want to duplicate the (possibly large) array in memory.
#include <Python.h>
#include <numpy/arrayobject.h>
// The relevant function
static PyObject * sum(PyObject * const self, PyObject * const args) {
PyObject * X;
PyArg_ParseTuple(args, "O", &X);
PyObject const * const X_double = PyArray_FROM_OTF(X, NPY_DOUBLE, NPY_ALIGNED);
npy_intp const size = PyArray_SIZE(X_double);
double * const data = (double *) PyArray_DATA(X_double);
double sum = 0;
Py_BEGIN_ALLOW_THREADS // IS THIS SAFE?
npy_intp i;
for (i=0; i<size; i++)
sum += data[i];
Py_END_ALLOW_THREADS
Py_DECREF(X_double);
return PyFloat_FromDouble(sum);
}
// Python interface code
// List the C methods that this extension provides.
static PyMethodDef gilexampleMethods[] = {
{"sum", sum, METH_VARARGS},
{NULL, NULL, 0, NULL} /* Sentinel - marks the end of this structure */
};
// Tell Python about these methods.
PyMODINIT_FUNC initgilexample(void) {
(void) Py_InitModule("gilexample", gilexampleMethods);
import_array(); // Must be present for NumPy.
}
See Question&Answers more detail:
os 与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…