为什么使用逻辑Sigmoid的tanh定义比scipy的expit更快? [英] Why is using tanh definition of logistic sigmoid faster than scipy's expit?
问题描述
我正在为应用程序使用逻辑Sigmoid.我比较了使用scipy.special
函数expit
和使用S型双曲线正切定义的时间.
I'm using a logistic sigmoid for an application. I compared the times using the scipy.special
function, expit
, versus using the hyperbolic tangent definition of the sigmoidal.
我发现双曲正切速度快了3倍.这里发生了什么?我还对排序数组上的时间进行了测试,以查看结果是否有所不同.
I found that the hyperbolic tangent was 3 times as fast. What is going on here? I also tested times on a sorted array to see if the result was any different.
以下是在IPython中运行的示例:
Here is an example that was run in IPython:
In [1]: from scipy.special import expit
In [2]: myexpit = lambda x: 0.5*tanh(0.5*x) + 0.5
In [3]: x = randn(100000)
In [4]: allclose(expit(x), myexpit(x))
Out[4]: True
In [5]: timeit expit(x)
100 loops, best of 3: 15.2 ms per loop
In [6]: timeit myexpit(x)
100 loops, best of 3: 4.94 ms per loop
In [7]: y = sort(x)
In [8]: timeit expit(y)
100 loops, best of 3: 15.3 ms per loop
In [9]: timeit myexpit(y)
100 loops, best of 3: 4.37 ms per loop
机器信息:
Machine info:
- Ubuntu 16.04
- RAM:7.4 GB
- Intel Core i7-3517U CPU @ 1.90GHz×4
Numpy/Scipy信息:
Numpy/Scipy info:
In [1]: np.__version__
Out[1]: '1.12.0'
In [2]: np.__config__.show()
lapack_opt_info:
libraries = ['openblas', 'openblas']
library_dirs = ['/usr/local/lib']
define_macros = [('HAVE_CBLAS', None)]
language = c
blas_opt_info:
libraries = ['openblas', 'openblas']
library_dirs = ['/usr/local/lib']
define_macros = [('HAVE_CBLAS', None)]
language = c
openblas_info:
libraries = ['openblas', 'openblas']
library_dirs = ['/usr/local/lib']
define_macros = [('HAVE_CBLAS', None)]
language = c
blis_info:
NOT AVAILABLE
openblas_lapack_info:
libraries = ['openblas', 'openblas']
library_dirs = ['/usr/local/lib']
define_macros = [('HAVE_CBLAS', None)]
language = c
lapack_mkl_info:
NOT AVAILABLE
blas_mkl_info:
NOT AVAILABLE
In [3]: import scipy
In [4]: scipy.__version__
Out[4]: '0.18.1'
推荐答案
我将把未来的人介绍给此问题.
总结有用评论的结果:
为什么使用逻辑Sigmoid的tanh定义比scipy的expit更快?"
"Why is using tanh definition of logistic sigmoid faster than scipy's expit?"
答案:不是;不是.在我的特定计算机上使用tanh
和exp
C函数会发生一些有趣的事情.
Answer: It's not; there's some funny business going on with the tanh
and exp
C functions on my specific machine.
事实证明,在我的机器上,tanh
的C函数比exp
快.为何如此的答案显然属于另一个问题.当我运行下面列出的C ++代码时,我看到了
It's turns out that on my machine, the C function for tanh
is faster than exp
. The answer to why this is the case obviously belongs to a different question. When I run the C++ code listed below, I see
tanh: 5.22203
exp: 14.9393
从Python调用时,
与tanh
函数中〜3x的增加相匹配.奇怪的是,当我在具有相同操作系统的另一台机器上运行相同的代码时,对于tanh
和exp
,我会得到相似的计时结果.
which matches the ~3x increase in the tanh
function when called from Python. The strange thing is that when I run the identical code on a separate machine that has the same OS, I get similar timing results for tanh
and exp
.
#include <iostream>
#include <cmath>
#include <ctime>
using namespace std;
int main() {
double a = -5;
double b = 5;
int N = 10001;
double x[10001];
double y[10001];
double h = (b-a) / (N-1);
clock_t begin, end;
for(int i=0; i < N; i++)
x[i] = a + i*h;
begin = clock();
for(int i=0; i < N; i++)
for(int j=0; j < N; j++)
y[i] = tanh(x[i]);
end = clock();
cout << "tanh: " << double(end - begin) / CLOCKS_PER_SEC << "\n";
begin = clock();
for(int i=0; i < N; i++)
for(int j=0; j < N; j++)
y[i] = exp(x[i]);
end = clock();
cout << "exp: " << double(end - begin) / CLOCKS_PER_SEC << "\n";
return 0;
}
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