Fix too small GPU pair count estimates

For triclinic unit-cells with DD the non-local cluster pair count
estimate was too high, especially for thin local domains, due to an
incorrect estimate of the cluster size. Since the pair count estimate
for the local pair-list was determined as a total minus a non-local
estimate, the local estimate could get negative and cause exceptions.
Fixed the cluster size estimate and added a lower limit for the local
size estimate.

Fixes #1762.

Change-Id: I3489550968f66bc03ba4e6056017a58eba37f7cc

diff --git a/src/gromacs/mdlib/nbnxn_search.c b/src/gromacs/mdlib/nbnxn_search.c
index d10ddf70b32739badc6e19dd84afa57fdb7aee76..16fd3b010127634444485d6a7b183a2192331ad9 100644 (file)
--- a/src/gromacs/mdlib/nbnxn_search.c
+++ b/src/gromacs/mdlib/nbnxn_search.c
@@ -4380,7 +4380,7 @@ static void get_nsubpair_target(const nbnxn_search_t  nbs,
 
     ls[XX] = (grid->c1[XX] - grid->c0[XX])/(grid->ncx*GPU_NSUBCELL_X);
     ls[YY] = (grid->c1[YY] - grid->c0[YY])/(grid->ncy*GPU_NSUBCELL_Y);
-    ls[ZZ] = (grid->c1[ZZ] - grid->c0[ZZ])*grid->ncx*grid->ncy/(grid->nc*GPU_NSUBCELL_Z);
+    ls[ZZ] = grid->na_c/(grid->atom_density*ls[XX]*ls[YY]);
 
     /* The average squared length of the diagonal of a sub cell */
     xy_diag2 = ls[XX]*ls[XX] + ls[YY]*ls[YY] + ls[ZZ]*ls[ZZ];
@@ -4410,11 +4410,25 @@ static void get_nsubpair_target(const nbnxn_search_t  nbs,
         /* 4 octants of a sphere */
         vol_est += 0.5*4.0/3.0*M_PI*pow(r_eff_sup, 3);
 
+        /* Estimate the number of cluster pairs as the local number of
+         * clusters times the volume they interact with times the density.
+         */
         nsp_est = grid->nsubc_tot*vol_est*grid->atom_density/grid->na_c;
 
         /* Subtract the non-local pair count */
         nsp_est -= nsp_est_nl;
 
+        /* For small cut-offs nsp_est will be an underesimate.
+         * With DD nsp_est_nl is an overestimate so nsp_est can get negative.
+         * So to avoid too small or negative nsp_est we set a minimum of
+         * all cells interacting with all 3^3 direct neighbors (3^3-1)/2+1=14.
+         * This might be a slight overestimate for small non-periodic groups of
+         * atoms as will occur for a local domain with DD, but for small
+         * groups of atoms we'll anyhow be limited by nsubpair_target_min,
+         * so this overestimation will not matter.
+         */
+        nsp_est = max(nsp_est, grid->nsubc_tot*14.0);
+
         if (debug)
         {
             fprintf(debug, "nsp_est local %5.1f non-local %5.1f\n",