src/gmxlib/gmx_cyclecounter.c

   1  /* -*- mode: c; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4; c-file-style: "stroustrup"; -*-
   2  *
   3  *
   4  * This file is part of Gromacs        Copyright (c) 1991-2006
   5  * David van der Spoel, Erik Lindahl, Berk Hess, University of Groningen.
   6  *
   7  * This program is free software; you can redistribute it and/or
   8  * modify it under the terms of the GNU General Public License
   9  * as published by the Free Software Foundation; either version 2
  10  * of the License, or (at your option) any later version.
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  13  * the research papers on the package. Check out http://www.gromacs.org
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  17  */
  18 #ifdef HAVE_CONFIG_H
  19 #include <config.h>
  20 #endif
  21
  22 #include <time.h>
  23 #ifdef HAVE_SYS_TIME_H
  24 #include <sys/time.h>
  25 #endif
  26
  27 #ifdef _MSC_VER
  28 #include <windows.h>
  29 #endif
  30
  31 #include "gmx_cyclecounter.h"
  32
  33
  34
  35
  36
  37 /*! \brief Calculate number of seconds per cycle tick on host
  38 *
  39 *  This routine runs a timer loop to calibrate the number of
  40 *  seconds per the units returned fro gmx_cycles_read().
  41 *
  42 *  \param  sampletime Minimum real sample time. It takes some trial-and-error
  43 *          to find the correct delay loop size, so the total runtime of
  44 *          this routine is about twice this time.
  45 *  \return Number of seconds per cycle unit. If it is not possible to
  46 *          calculate on this system (for whatever reason) the return value
  47 *          will be -1, so check that it is positive before using it.
  48 */
  49 double
  50 gmx_cycles_calibrate(double sampletime)
  51 {
  52 #ifdef _MSC_VER
  53
  54   /* Windows does not have gettimeofday, but it provides a special
  55    * routine that returns the cycle counter frequency.
  56    */
  57   LARGE_INTEGER i;
  58
  59   QueryPerformanceFrequency(&i);
  60
  61   return 1.0/((double) i.QuadPart);
  62   /* end of MS Windows implementation */
  63
  64 #elif (defined HAVE_GETTIMEOFDAY)
  65
  66   /*  generic implementation with gettimeofday() */
  67     struct timeval t1,t2;
  68     struct timezone tz = { 0,0 };
  69     gmx_cycles_t c1,c2;
  70     double timediff,cyclediff;
  71     double d=0.1; /* Dummy variable so we don't optimize away delay loop */
  72     int i;
  73
  74     if(!gmx_cycles_have_counter())
  75     {
  76         return -1;
  77     }
  78
  79 #if (defined(__alpha__) || defined(__alpha))
  80     /* Alpha cannot count to more than 4e9, but I don't expect
  81         * that the architecture will go over 2GHz before it dies, so
  82         * up to 2.0 seconds of sampling should be safe.
  83         */
  84     if(sampletime>2.0)
  85     {
  86         sampletime=2.0;
  87     }
  88 #endif
  89
  90     /* Start a timing loop. We want this to be largely independent
  91      * of machine speed, so we need to start with a very small number
  92      * of iterations and repeat it until we reach the requested time.
  93      *
  94      * We call gettimeofday an extra time at the start to avoid cache misses.
  95      */
  96     gettimeofday(&t1,&tz);
  97     gettimeofday(&t1,&tz);
  98     c1=gmx_cycles_read();
  99
 100     do
 101     {
 102         /* just a delay loop. To avoid optimizing it away, we calculate a number
 103         * that will underflow to zero in most cases. By conditionally adding it
 104         * to a result at the end it cannot be removed. n=10000 is arbitrary...
 105         */
 106         for(i=0;i<10000;i++)
 107             d=d/(1.0+(double)i);
 108         /* Read the time again */
 109         gettimeofday(&t2,&tz);
 110         c2=gmx_cycles_read();
 111         timediff=(double)(t2.tv_sec-t1.tv_sec)+
 112             (double)(t2.tv_usec-t1.tv_usec)*1e-6;
 113     }
 114     while( timediff < sampletime );
 115
 116     cyclediff=c2-c1;
 117
 118     /* Add a very small result so the delay loop cannot be optimized away */
 119     if(d<1e-30)
 120     {
 121         timediff+=d;
 122     }
 123
 124     /* Return seconds per cycle */
 125     return timediff/cyclediff;
 126
 127 #else
 128     /* No timing function available */
 129     return -1;
 130 #endif
 131 }
 132
 133