Implementation of the mpio driver with selection I/O.

src/H5Dio.c

@@ -415,8 +415,14 @@ H5D__read(size_t count, H5D_dset_io_info_t *dset_info)
                 HGOTO_ERROR(H5E_DATASET, H5E_CANTGET, FAIL, "can't get MPI-I/O transfer mode");
 
             /* Only report the collective I/O mode if we're actually performing collective I/O */
-            if (xfer_mode == H5FD_MPIO_COLLECTIVE)
+            if (xfer_mode == H5FD_MPIO_COLLECTIVE) {
                 H5CX_set_mpio_actual_io_mode(io_info.actual_io_mode);
+
+                /* If we did selection I/O, report that we used "link chunk" mode, since that's the most
+                 * analogous to what selection I/O does */
+                if (io_info.use_select_io == H5D_SELECTION_IO_MODE_ON)
+                    H5CX_set_mpio_actual_chunk_opt(H5D_MPIO_LINK_CHUNK);
+            }
         }
 #endif /* H5_HAVE_PARALLEL */
     }
@@ -1131,20 +1137,16 @@ H5D__typeinfo_init_phase2(H5D_io_info_t *io_info)
     assert(io_info);
 
     /* If selection I/O mode is default (auto), enable it here if the VFD supports it (it will be turned off
-     * later if something else conflicts), otherwise disable it.  If we're using the MPIO VFD, the automatic
-     * selection will happen in H5D__mpio_opt_possible() inside H5D__ioinfo_adjust(). */
-#ifdef H5_HAVE_PARALLEL
-    if (!io_info->using_mpi_vfd)
-#endif /* H5_HAVE_PARALLEL */
-        if (io_info->use_select_io == H5D_SELECTION_IO_MODE_DEFAULT) {
-            if (H5F_has_vector_select_io(io_info->dsets_info[0].dset->oloc.file,
-                                         io_info->op_type == H5D_IO_OP_WRITE))
-                io_info->use_select_io = H5D_SELECTION_IO_MODE_ON;
-            else {
-                io_info->use_select_io = H5D_SELECTION_IO_MODE_OFF;
-                io_info->no_selection_io_cause |= H5D_SEL_IO_DEFAULT_OFF;
-            }
+     * later if something else conflicts), otherwise disable it */
+    if (io_info->use_select_io == H5D_SELECTION_IO_MODE_DEFAULT) {
+        if (H5F_has_vector_select_io(io_info->dsets_info[0].dset->oloc.file,
+                                     io_info->op_type == H5D_IO_OP_WRITE))
+            io_info->use_select_io = H5D_SELECTION_IO_MODE_ON;
+        else {
+            io_info->use_select_io = H5D_SELECTION_IO_MODE_OFF;
+            io_info->no_selection_io_cause |= H5D_SEL_IO_DEFAULT_OFF;
         }
+    }
 
     /* If we're doing type conversion and we might be doing selection I/O, check if the buffers are large
      * enough to handle the whole I/O */

src/H5Dmpio.c

@@ -622,6 +622,9 @@ H5D__mpio_opt_possible(H5D_io_info_t *io_info)
         if (!H5FD_mpi_opt_types_g)
             local_cause[0] |= H5D_MPIO_MPI_OPT_TYPES_ENV_VAR_DISABLED;
 
+        /* Decision on whether to use selection I/O should have been made by now */
+        assert(io_info->use_select_io != H5D_SELECTION_IO_MODE_DEFAULT);
+
         /* Datatype conversions and transformations are allowed with selection I/O.  If the selection I/O mode
          * is auto (default), disable collective for now and re-enable later if we can */
         if (io_info->use_select_io != H5D_SELECTION_IO_MODE_ON) {
@@ -731,25 +734,6 @@ H5D__mpio_opt_possible(H5D_io_info_t *io_info)
             HMPI_GOTO_ERROR(FAIL, "MPI_Allreduce failed", mpi_code)
     } /* end else */
 
-    /* If the selection I/O mode is default (auto), decide here whether it should be on or off */
-    if (io_info->use_select_io == H5D_SELECTION_IO_MODE_DEFAULT) {
-        /* If the only reason(s) we've disabled collective are type conversions and/or transforms, enable
-         * selection I/O and re-enable collective I/O since it's supported by selection I/O */
-        if (global_cause[0] && !(global_cause[0] & ~((unsigned)H5D_MPIO_DATATYPE_CONVERSION |
-                                                     (unsigned)H5D_MPIO_DATA_TRANSFORMS))) {
-            assert(!(local_cause[0] &
-                     ~((unsigned)H5D_MPIO_DATATYPE_CONVERSION | (unsigned)H5D_MPIO_DATA_TRANSFORMS)));
-            local_cause[0]         = 0;
-            global_cause[0]        = 0;
-            io_info->use_select_io = H5D_SELECTION_IO_MODE_ON;
-        }
-        else {
-            /* Otherwise, there's currently no benefit to selection I/O, so leave it off */
-            io_info->use_select_io = H5D_SELECTION_IO_MODE_OFF;
-            io_info->no_selection_io_cause |= H5D_SEL_IO_DEFAULT_OFF;
-        }
-    }
-
     /* Set the local & global values of no-collective-cause in the API context */
     H5CX_set_mpio_local_no_coll_cause(local_cause[0]);
     H5CX_set_mpio_global_no_coll_cause(global_cause[0]);