python函数(或代码块)以一定的时间间隔在循环中运行慢得多 [英] python function(or a code block) runs much slower with a time interval in a loop

问题描述

我注意到python中的一种情况，当嵌套在循环中的代码块连续运行时，它比以某些 .sleep() 时间间隔运行要快得多.

I notice a case in python, when a block of code, nested in a loop, runs continuously, it is much faster than running with some .sleep() time interval.

我想知道原因和可能的解决方案.

我想这与CPU缓存或cPython VM的某种机制有关.

I guess it's related to CPU-cache or some mechanism of cPython VM.

'''
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，因此我在下面提供了与 C 代码完全相同的版本:

some disadvantage of the .sleep() is, that it releases CPU, thus I provide the exactly same version with a C code below:

'''
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: (我们删除了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. 

为了找出这是否是CPU缓存问题，我写了一个 C 副本.并且发现这种现象不会发生.

to find out whether this is problem of CPU-cache, I wrote a C counterpart. And have found out that this kind of phenomenon does not happen.

#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"

推荐答案

(不是答案-太长而无法发表评论)

(not an answer - but too long to post as comment)

我做了一些实验，并通过(c4>进行了一些简化).

i did some experimentation and ran (something slightly simpler) through timeit.

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

第一行只是检查睡眠功能是否使用了大约n_timeit*n_loop*sleep_sec秒.因此，如果此值为 small -应该没问题.

the first line is just to check that the the sleep function uses about n_timeit*n_loop*sleep_sec seconds. so if this value is small - that should be ok.

但是，正如您所看到的-您的发现仍然存在:具有睡眠功能的循环(减去睡眠所用的时间)比没有睡眠的循环要花更多的时间...

but as you see - your findings remain: the loop with the sleep function (subtracting the time sleep uses) takes up more time than the loop without sleep...

我不认为python可以在不休眠的情况下优化循环(c编译器可能会使用该变量；从不使用变量s).

i don't think that python optimizes the loop without sleep (a c compiler might; the variable s is never used).

这篇关于python函数(或代码块)以一定的时间间隔在循环中运行慢得多的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！