src/gromacs/timing/cyclecounter.c

   1 /*
   2  * This file is part of the GROMACS molecular simulation package.
   3  *
   4  * Copyright (c) 1991-2006 David van der Spoel, Erik Lindahl, Berk Hess, University of Groningen.
   5  * Copyright (c) 2013,2014, by the GROMACS development team, led by
   6  * Mark Abraham, David van der Spoel, Berk Hess, and Erik Lindahl,
   7  * and including many others, as listed in the AUTHORS file in the
   8  * top-level source directory and at http://www.gromacs.org.
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  36 #include "gmxpre.h"
  37
  38 #include "cyclecounter.h"
  39
  40 #include "config.h"
  41
  42 #include <time.h>
  43
  44 #ifdef HAVE_SYS_TIME_H
  45 #include <sys/time.h>
  46 #endif
  47 #ifdef _MSC_VER
  48 #include <windows.h>
  49 #endif
  50
  51 /*! \brief Calculate number of seconds per cycle tick on host
  52  *
  53  *  This routine runs a timer loop to calibrate the number of
  54  *  seconds per the units returned fro gmx_cycles_read().
  55  *
  56  *  \param  sampletime Minimum real sample time. It takes some trial-and-error
  57  *          to find the correct delay loop size, so the total runtime of
  58  *          this routine is about twice this time.
  59  *  \return Number of seconds per cycle unit. If it is not possible to
  60  *          calculate on this system (for whatever reason) the return value
  61  *          will be -1, so check that it is positive before using it.
  62  */
  63 double
  64 gmx_cycles_calibrate(double sampletime)
  65 {
  66 #ifdef _MSC_VER
  67
  68     /* Windows does not have gettimeofday, but it provides a special
  69      * routine that returns the cycle counter frequency.
  70      */
  71     LARGE_INTEGER i;
  72
  73     QueryPerformanceFrequency(&i);
  74
  75     return 1.0/((double) i.QuadPart);
  76     /* end of MS Windows implementation */
  77
  78 #elif (defined HAVE_GETTIMEOFDAY)
  79
  80     /*  generic implementation with gettimeofday() */
  81     struct timeval t1, t2;
  82     gmx_cycles_t   c1, c2;
  83     double         timediff, cyclediff;
  84     double         d = 0.1; /* Dummy variable so we don't optimize away delay loop */
  85     int            i;
  86
  87     if (!gmx_cycles_have_counter())
  88     {
  89         return -1;
  90     }
  91
  92 #if (defined(__alpha__) || defined(__alpha))
  93     /* Alpha cannot count to more than 4e9, but I don't expect
  94      * that the architecture will go over 2GHz before it dies, so
  95      * up to 2.0 seconds of sampling should be safe.
  96      */
  97     if (sampletime > 2.0)
  98     {
  99         sampletime = 2.0;
 100     }
 101 #endif
 102
 103     /* Start a timing loop. We want this to be largely independent
 104      * of machine speed, so we need to start with a very small number
 105      * of iterations and repeat it until we reach the requested time.
 106      *
 107      * We call gettimeofday an extra time at the start to avoid cache misses.
 108      */
 109     gettimeofday(&t1, NULL);
 110     gettimeofday(&t1, NULL);
 111     c1 = gmx_cycles_read();
 112
 113     do
 114     {
 115         /* just a delay loop. To avoid optimizing it away, we calculate a number
 116          * that will underflow to zero in most cases. By conditionally adding it
 117          * to a result at the end it cannot be removed. n=10000 is arbitrary...
 118          */
 119         for (i = 0; i < 10000; i++)
 120         {
 121             d = d/(1.0+(double)i);
 122         }
 123         /* Read the time again */
 124         gettimeofday(&t2, NULL);
 125         c2       = gmx_cycles_read();
 126         timediff = (double)(t2.tv_sec-t1.tv_sec)+
 127             (double)(t2.tv_usec-t1.tv_usec)*1e-6;
 128     }
 129     while (timediff < sampletime);
 130
 131     cyclediff = c2-c1;
 132
 133     /* Add a very small result so the delay loop cannot be optimized away */
 134     if (d < 1e-30)
 135     {
 136         timediff += d;
 137     }
 138
 139     /* Return seconds per cycle */
 140     return timediff/cyclediff;
 141
 142 #else
 143     /* No timing function available */
 144     return -1;
 145 #endif
 146 }