python function(or a code block) runs much slower with a time interval in a loop

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.

I wonder the reason and a possible solution.

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 

result:

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

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 

another experiment: (we remove the for loop)

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 

result:

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

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; } 

result:

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