gmx_bool wallcycle_have_counter(void)
{
return gmx_cycles_have_counter();
}
/*! \brief Calculate number of seconds per cycle tick on host
*
* This routine runs a timer loop to calibrate the number of
* seconds per the units returned fro gmx_cycles_read().
*
* \param sampletime Minimum real sample time. It takes some trial-and-error
* to find the correct delay loop size, so the total runtime of
* this routine is about twice this time.
* \return Number of seconds per cycle unit. If it is not possible to
* calculate on this system (for whatever reason) the return value
* will be -1, so check that it is positive before using it.
*/
double
gmx_cycles_calibrate(double sampletime)
{
#ifdef _MSC_VER
/* Windows does not have gettimeofday, but it provides a special
* routine that returns the cycle counter frequency.
*/
LARGE_INTEGER i;
QueryPerformanceFrequency(&i);
return 1.0/static_cast<double>(i.QuadPart);
/* end of MS Windows implementation */
#elif HAVE_GETTIMEOFDAY
/* generic implementation with gettimeofday() */
struct timeval t1, t2;
gmx_cycles_t c1, c2;
double timediff, cyclediff;
double d = 0.1; /* Dummy variable so we don't optimize away delay loop */
if (!gmx_cycles_have_counter())
{
return -1;
}
#if (defined(__alpha__) || defined(__alpha))
/* Alpha cannot count to more than 4e9, but I don't expect
* that the architecture will go over 2GHz before it dies, so
* up to 2.0 seconds of sampling should be safe.
*/
if (sampletime > 2.0)
{
sampletime = 2.0;
}
#endif
/* Start a timing loop. We want this to be largely independent
* of machine speed, so we need to start with a very small number
* of iterations and repeat it until we reach the requested time.
*
* We call gettimeofday an extra time at the start to avoid cache misses.
*/
gettimeofday(&t1, nullptr);
gettimeofday(&t1, nullptr);
c1 = gmx_cycles_read();
do
{
/* just a delay loop. To avoid optimizing it away, we calculate a number
* that will underflow to zero in most cases. By conditionally adding it
* to a result at the end it cannot be removed. n=10000 is arbitrary...
*/
for (int i = 0; i < 10000; i++)
{
d = d/(1.0+static_cast<double>(i));
}
/* Read the time again */
gettimeofday(&t2, nullptr);
c2 = gmx_cycles_read();
timediff = static_cast<double>(t2.tv_sec-t1.tv_sec)+(t2.tv_usec-t1.tv_usec)*1e-6;
}
while (timediff < sampletime);
cyclediff = c2-c1;
/* Add a very small result so the delay loop cannot be optimized away */
if (d < 1e-30)
{
timediff += d;
}
/* Return seconds per cycle */
return timediff/cyclediff;
#else
/* No timing function available */
return -1;
#endif
}
