Question

我在cython代码中看到了一些奇怪的行为。我正在编写代码来计算前向卡尔曼滤波器，但我有一个状态转换模型，其中有许多0 s，所以能够只计算协方差矩阵的某些元素会很好。

因此，为了测试这一点，我想使用cython填充单个数组元素。令我惊讶的是，我找到了

将输出写入特定数组位置的速度非常慢(function fill(...))，而每次(function nofill(...))只是将其分配给标量变量（基本上忘记了结果），并且
设置C=0.1或31，同时不会影响nofill(...)运行的时间，后者选择C使fill(...)运行速度慢2倍。这让我感到困惑。任何人都可以解释为什么我会看到这个吗？

代码： -

#################  file way_too_slow.pyx
from libc.math cimport sin

#  Setting C=0.1 or 31 doesn't change affect performance of calling nofill(...), but it makes the fill(...) slower.  I have no clue why.
cdef double C = 0.1

#  This function just throws away its output.

def nofill(double[::1] x, double[::1] y, long N):
    cdef int i
    cdef double *p_x = &x[0]
    cdef double *p_y = &y[0]
    cdef double d

    with nogil:
        for 0 <= i < N:
            d = ((p_x[i] + p_y[i])*3 + p_x[i] - p_y[i]) + sin(p_x[i]*C)  #  C appears here

#  Same function keeps its output.
#  However:   #1 - MUCH slower than 
def fill(double[::1] x, double[::1] y, double[::1] out, long N):
    cdef int i
    cdef double *p_x = &x[0]
    cdef double *p_y = &y[0]
    cdef double *p_o = &out[0]
    cdef double d

    with nogil:
        for 0 <= i < N:
            p_o[i] = ((p_x[i] + p_y[i])*3 + p_x[i] - p_y[i]) + sin(p_x[i]*C)    # C appears here

以上代码由python程序调用

####################  run_way_too_slow.py
import way_too_slow as _wts
import time as _tm

N = 80000
x = _N.random.randn(N)
y = _N.random.randn(N)
out  = _N.empty(N)

t1 = _tm.time()
_wts.nofill(x, y, N)
t2 = _tm.time()
_wts.fill(x, y, out, N)
t3 = _tm.time()

print "nofill() ET: %.3e" % (t2-t1)
print "fill()   ET: %.3e" % (t3-t2)

print "fill() is slower by factor %.3f" % ((t3-t2)/(t2-t1))

使用setup.py文件编译cython

#################  setup.py
from distutils.core import setup, Extension
from distutils.sysconfig import get_python_inc
from distutils.extension import Extension
from Cython.Distutils import build_ext

incdir=[get_python_inc(plat_specific=1)]
libdir = ['/usr/local/lib']

cmdclass = {'build_ext' : build_ext}

ext_modules = Extension("way_too_slow",
                        ["way_too_slow.pyx"],
                        include_dirs=incdir,   #  include_dirs for Mac
                        library_dirs=libdir)

setup(
    name="way_too_slow",
    cmdclass = cmdclass,
    ext_modules = [ext_modules]
)

以下是跑步的典型输出＆＃34; run_way_too_slow.py＆＃34;使用C = 0.1

>>> exf("run_way_too_slow.py")
nofill() ET: 6.700e-05
fill()   ET: 6.409e-04
fill() is slower by factor 9.566

C = 31的典型运行。

>>> exf("run_way_too_slow.py")
nofill() ET: 6.795e-05
fill()   ET: 1.566e-03
fill() is slower by factor 23.046

我们可以看到

与分配给double相比，分配到指定的数组位置相当慢。
出于某种原因，分配速度似乎取决于计算中的操作 - 这对我来说毫无意义。

非常感谢任何见解。

使用cython分配到任意数组位置。分配速度取决于值？

0 个答案: