Hello again,
My time function was imprecise.
double getTime()
{
timeval tv ;
gettimeofday ( & tv, NULL ) ;
return (double) tv.tv_sec + (double) tv.tv_usec / 1000000.0 ;
}
Using the function above gives times of around 0.13 ms, which is much
better than what I've seen in my project. I saw times that were more
than 2 ms. What could slow memcpy down?
Thanks,
Mike
On Tue, 2008-10-07 at 12:50 -0500, Michael.Schmidt@xxxxxxxxxx wrote:
> Hello,
>
> I apologize in advance if this is the wrong message board. I've noticed
> that memcpy seems to take a while (sometimes) for large chunks of memory
> (about 1 MB).
>
> Here's a small sample application:
>
> #include<iostream>
> #include<time.h>
> #include<cmath>
> #include<iomanip>
> #include <sys/timeb.h>
>
> using namespace std;
>
> double getTime()
> {
> struct timeb tb ;
>
> ftime ( &tb );
> return (double) tb.time + (double) tb.millitm / 1000.0 ;
> }
>
> int main()
> {
> char src[1048576] = {0};
> char dst[1048576];
>
> for(unsigned int i = 0; i < 25; ++i)
> {
> double start_time = getTime();
> memcpy(dst, src, 1048576);
> double end_time = getTime();
> cout << "Time: " << 1000.0*(end_time - start_time) << endl;
> }
>
> return 0;
> }
>
> Here's the output:
>
> Time: 0.999928
> Time: 0
> Time: 0
> Time: 0
> Time: 0
> Time: 0.999928
> Time: 0
> Time: 0
> Time: 0
> Time: 0
> Time: 0
> Time: 0
> Time: 0
> Time: 0.999928
> Time: 0
> Time: 0
> Time: 0
> Time: 0
> Time: 0
> Time: 0
> Time: 1.00017
> Time: 0
> Time: 0
> Time: 0
> Time: 0
>
> I would be eternally grateful if anyone could explain what's happening.
> Run on a Core 2 Duo E6550, 4 GB RAM.
>
> Thanks,
> Mike
>
