src/gromacs/domdec/ga2la.cpp

   1 /*
   2  * This file is part of the GROMACS molecular simulation package.
   3  *
   4  * Copyright (c) 2018, by the GROMACS development team, led by
   5  * Mark Abraham, David van der Spoel, Berk Hess, and Erik Lindahl,
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  35 /*! \internal \file
  36  * \brief
  37  * Implements functions for mapping from global to local atom indices.
  38  *
  39  * \ingroup module_domdec
  40  *
  41  * \author Berk Hess <hess@kth.se>
  42  */
  43
  44 #include "gmxpre.h"
  45
  46 #include "ga2la.h"
  47
  48 /*! \brief Returns whether to use a direct list only
  49  *
  50  * There are two methods implemented for finding the local atom number
  51  * belonging to a global atom number:
  52  * 1) a simple, direct array
  53  * 2) a hash table consisting of list of linked lists indexed with
  54  *    the global number modulo mod.
  55  * Memory requirements:
  56  * 1) numAtomsTotal*2 ints
  57  * 2) numAtomsLocal*(2+1-2(1-e^-1/2))*4 ints
  58  * where numAtomsLocal is the number of atoms in the home + communicated zones.
  59  * Method 1 is faster for low parallelization, 2 for high parallelization.
  60  * We switch to method 2 when it uses less than half the memory method 1.
  61  */
  62 static bool directListIsFaster(int numAtomsTotal,
  63                                int numAtomsLocal)
  64 {
  65     constexpr int c_numAtomsSmallRelativeToCache  = 1024;
  66     constexpr int c_memoryRatioHashedVersusDirect = 9;
  67
  68     return (numAtomsTotal <= c_numAtomsSmallRelativeToCache ||
  69             numAtomsTotal <= numAtomsLocal*c_memoryRatioHashedVersusDirect);
  70 }
  71
  72 gmx_ga2la_t::gmx_ga2la_t(int numAtomsTotal,
  73                          int numAtomsLocal) :
  74     usingDirect_(directListIsFaster(numAtomsTotal, numAtomsLocal))
  75 {
  76     if (usingDirect_)
  77     {
  78         new(&(data_.direct)) std::vector<Entry>(numAtomsTotal, {-1, -1});
  79     }
  80     else
  81     {
  82         new(&(data_.hashed)) gmx::HashedMap<Entry>(numAtomsLocal);
  83     }
  84 }