python 函数(或代码块)在循环中的时间间隔内运行速度要慢得多

Question

我注意到Python中的一个情况，当嵌套在循环中的代码块连续运行时，它比使用某些.sleep()运行要快得多时间间隔。

我想知道原因和可能的解决方案。

我猜这与CPU缓存或cPython VM的某些机制有关。

'''
Created on Aug 22, 2015

@author: doge
'''

import numpy as np
import time
import gc
gc.disable()

t = np.arange(100000)

for i in xrange(100):

    #np.sum(t)
    time.sleep(1) #--> if you comment this line, the following lines will be much faster

    st = time.time()
    np.sum(t)
    print (time.time() - st)*1e6

结果:

without sleep in loop, time consumed:   50us
with  a sleep in loop, time consumed: >150us

.sleep() 的一些缺点是，它会释放 CPU，因此我提供了具有 C 代码的完全相同版本如下:

'''
Created on Aug 22, 2015

@author: doge
'''

import numpy as np
import time
import gc
gc.disable()

t = np.arange(100000)

count = 0
for i in xrange(100):

    count += 1
    if ( count % 1000000 != 0 ):
        continue
    #--> these three lines make the following lines much slower

    st = time.time()
    np.sum(t)
    print (time.time() - st)*1e6

另一个实验: (我们删除 for 循环)

st = time.time()
np.sum(t)
print (time.time() - st)*1e6

st = time.time()
np.sum(t)
print (time.time() - st)*1e6

st = time.time()
np.sum(t)
print (time.time() - st)*1e6

...

st = time.time()
np.sum(t)
print (time.time() - st)*1e6

结果:

execution time decreased from 150us -> 50us gradually.
and keep stable in 50us. 

为了查明这是否是CPU缓存的问题，我编写了一个C对应版本。并且发现这种现象不会发生。

#include <iostream>
#include <sys/time.h>

#define num 100000

using namespace std;

long gus()
{
    struct timeval tm;
    gettimeofday(&tm, NULL);
    return ( (tm.tv_sec % 86400 + 28800) % 86400 )*1000000 + tm.tv_usec;
}

double vec_sum(double *v, int n){
    double result = 0;
    for(int i = 0;i < n;++i){
         result += v[i];
    }
    return result;
}

int main(){

double a[num];

for(int i = 0; i < num; ++i){
    a[i] = (double)i;
}

//for(int i = 0; i < 1000; ++i){
// cout<<a[i]<<"\n";
//}

int count = 0;
long st;
while(1){
++count;

if(count%100000000 != 0){    //---> i use this line to create a delay, we can do the same way in python, result is the same
//if(count%1 != 0){
continue;
}

st = gus();
vec_sum(a,num);
cout<<gus() - st<<endl;

}


return 0;
}

结果:

time stable in 250us, no matter in "count%100000000" or "count%1"

Answer 1

(不是答案 - 但太长，无法作为评论发布)

我做了一些实验并通过timeit运行(稍微简单一些)。

from timeit import timeit
import time

n_loop = 15
n_timeit = 10
sleep_sec = 0.1

t = range(100000)

def with_sleep():
    for i in range(n_loop):
        s = sum(t)
        time.sleep(sleep_sec)

def without_sleep():
    for i in range(n_loop):
        s = sum(t)

def sleep_only():
     for i in range(n_loop):
        time.sleep(sleep_sec)

wo = timeit(setup='from __main__ import without_sleep',
           stmt='without_sleep()',
           number = n_timeit)
w = timeit(setup='from __main__ import with_sleep',
            stmt='with_sleep()',
            number = n_timeit)
so = timeit(setup='from __main__ import sleep_only',
            stmt='sleep_only()',
            number = n_timeit)

print(so - n_timeit*n_loop*sleep_sec, so)
print(w - n_timeit*n_loop*sleep_sec, w)
print(wo)

结果是:

0.031275457000447204 15.031275457000447
1.0220358229998965 16.022035822999896
0.41462676399987686

第一行只是检查 sleep 函数是否使用大约 n_timeit*n_loop*sleep_sec 秒。所以如果这个值小 - 应该没问题。

但正如您所看到的 - 您的发现仍然存在:具有 sleep 函数的循环(减去 sleep 使用的时间)比不带 sleep 的循环占用更多时间...

我认为 python 不会在没有 sleep 的情况下优化循环(c 编译器可能会；变量 s 从未使用过)。