cpython vs cython vs numpy array performance

Question

I am doing some performance test on a variant of the prime numbers generator from http://docs.cython.org/src/tutorial/numpy.html. The below performance measures are with kma

Answer 1

cdef DTYPE_t [:] p_view = p

Using this instead of p in the calculations. reduced the runtime from 580 ms down to 2.8 ms for me. About the exact same runtime as the implementation using *int. And that's about the max you can expect from this.

DTYPE = np.int
ctypedef np.int_t DTYPE_t

@cython.boundscheck(False)
def primes(DTYPE_t kmax):
    cdef DTYPE_t n, k, i
    cdef np.ndarray p = np.empty(kmax, dtype=DTYPE)
    cdef DTYPE_t [:] p_view = p
    k = 0
    n = 2
    while k < kmax:
        i = 0
        while i < k and n % p_view[i] != 0:
            i = i + 1
        if i == k:
            p_view[k] = n
            k = k + 1
        n = n + 1
    return p

Answer 2

why is the numpy array so incredibly slower than a python list, when running on CPython?

Because you didn't fully type it. Use

cdef np.ndarray[dtype=np.int, ndim=1] p = np.empty(kmax, dtype=DTYPE)

how do I cast a numpy array to a int*?

By using np.intc as the dtype, not np.int (which is a C long). That's

cdef np.ndarray[dtype=int, ndim=1] p = np.empty(kmax, dtype=np.intc)

(But really, use a memoryview, they're much cleaner and the Cython folks want to get rid of the NumPy array syntax in the long run.)

Answer 3

Best syntax I found so far:

import numpy
cimport numpy
cimport cython

@cython.boundscheck(False)
@cython.wraparound(False)
def primes(int kmax):
    cdef int n, k, i
    cdef numpy.ndarray[int] p = numpy.empty(kmax, dtype=numpy.int32)
    k = 0
    n = 2
    while k < kmax:
        i = 0
        while i < k and n % p[i] != 0:
            i = i + 1
        if i == k:
            p[k] = n
            k = k + 1
        n = n + 1
    return p

Note where I used numpy.int32 instead of int. Anything on the left side of a cdef is a C type (thus int = int32 and float = float32), while anything on the RIGHT side of it (or outside of a cdef) is a python type (int = int64 and float = float64)