src/gromacs/mdlib/gmx_omp_nthreads.h

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  35
  36 #ifndef GMX_OMP_NTHREADS_H
  37 #define GMX_OMP_NTHREADS_H
  38
  39 #include <stdio.h>
  40
  41 #include "gromacs/utility/basedefinitions.h"
  42
  43 struct t_commrec;
  44
  45 namespace gmx
  46 {
  47 class MDLogger;
  48 }
  49
  50 /** Enum values corresponding to multithreaded algorithmic modules. */
  51 typedef enum module_nth
  52 {
  53     /* Default is meant to be used in OMP regions outside the named
  54      * algorithmic modules listed below. */
  55     emntDefault, emntDomdec, emntPairsearch, emntNonbonded,
  56     emntBonded, emntPME,  emntUpdate, emntVSITE, emntLINCS, emntSETTLE,
  57     emntNR
  58 } module_nth_t;
  59
  60 /*! \brief
  61  * Initializes the per-module thread count.
  62  *
  63  * It is compatible with tMPI, thread-safety is ensured (for the features
  64  * available with tMPI).
  65  * This function should caled only once during the initialization of mdrun. */
  66 void gmx_omp_nthreads_init(const gmx::MDLogger &fplog, t_commrec *cr,
  67                            int nthreads_hw_avail,
  68                            int omp_nthreads_req,
  69                            int omp_nthreads_pme_req,
  70                            gmx_bool bCurrNodePMEOnly,
  71                            gmx_bool bFullOmpSupport);
  72
  73 /*! \brief
  74  * Returns the number of threads to be used in the given module \p mod. */
  75 int gmx_omp_nthreads_get(int mod);
  76
  77 /*! \brief
  78  * Returns the number of threads to be used in the given module \p mod for simple rvec operations.
  79  *
  80  * When the, potentially, parallel task only consists of a loop of clear_rvec
  81  * or rvec_inc for nrvec elements, the OpenMP overhead might be higher than
  82  * the reduction in computional cost due to parallelization. This routine
  83  * returns 1 when the overhead is expected to be higher than the gain.
  84  */
  85 static inline int gmx_omp_nthreads_get_simple_rvec_task(int mod, int nrvec)
  86 {
  87     /* There can be a relatively large overhead to an OpenMP parallel for loop.
  88      * This overhead increases, slowly, with the numbe of threads used.
  89      * The computational gain goes as 1/#threads. The two effects combined
  90      * lead to a cross-over point for a (non-)parallel loop at loop count
  91      * that is not strongly dependent on the thread count.
  92      * Note that a (non-)parallel loop can have benefit later in the code
  93      * due to generating more cache hits, depending on how the next lask
  94      * that accesses the same data is (not) parallelized over threads.
  95      *
  96      * A value of 2000 is the switch-over point for Haswell without
  97      * hyper-threading. With hyper-threading it is about a factor 1.5 higher.
  98      */
  99     const int nrvec_omp = 2000;
 100
 101     if (nrvec < nrvec_omp)
 102     {
 103         return 1;
 104     }
 105     else
 106     {
 107         return gmx_omp_nthreads_get(mod);
 108     }
 109 }
 110
 111 /*! \brief Sets the number of threads to be used in module.
 112  *
 113  * Intended for use in testing. */
 114 void gmx_omp_nthreads_set(int mod, int nthreads);
 115
 116 /*! \brief
 117  * Read the OMP_NUM_THREADS env. var. and check against the value set on the
 118  * command line. */
 119 void gmx_omp_nthreads_read_env(int     *nthreads_omp,
 120                                gmx_bool bIsSimMaster);
 121
 122 #endif