问题
This is the "algorithm", but when I want to measure the execution time it gives me zero. Why?
#define ARRAY_SIZE 10000
...
clock_t start, end;
start = clock();
for( i = 0; i < ARRAY_SIZE; i++)
{
non_parallel[i] = vec[i] * vec[i];
}
end = clock();
printf( "Number of seconds: %f\n", (end-start)/(double)CLOCKS_PER_SEC );
So What should i do to measure the time?
回答1:
Two things:
10000
is not a lot on a modern computer. Therefore that loop will run in probably less than a millisecond - less than the precision ofclock()
. Therefore it will return zero.If you aren't using the result of
non_parallel
its possible that the entire loop will be optimized out by the compiler.
Most likely, you just need a more expensive loop. Try increasing ARRAY_SIZE
to something much larger.
Here's a test on my machine with a larger array size:
#define ARRAY_SIZE 100000000
int main(){
clock_t start, end;
double *non_parallel = (double*)malloc(ARRAY_SIZE * sizeof(double));
double *vec = (double*)malloc(ARRAY_SIZE * sizeof(double));
start = clock();
for(int i = 0; i < ARRAY_SIZE; i++)
{
non_parallel[i] = vec[i] * vec[i];
}
end = clock();
printf( "Number of seconds: %f\n", (end-start)/(double)CLOCKS_PER_SEC );
free(non_parallel);
free(vec);
return 0;
}
Output:
Number of seconds: 0.446000
回答2:
This is an unreliable way to actually time number of seconds, since the clock()
function is pretty low precision, and your loop isn't doing a lot of work. You can either make your loop do more so that it runs longer, or use a better timing method.
The higher precision methods are platform specific. For Windows, see How to use QueryPerformanceCounter? and for linux see High resolution timer with C++ and Linux?
来源:https://stackoverflow.com/questions/10154962/c-how-to-measure-time-correctly