Set default PnetCDF header alignment to 128 KiB

Increase the default Parallel NetCDF header alignment to 128 KiB when
creating new files with SMIOL. This increases the default header size,
providing additional header memory padding and reducing the likelihood
of header reallocations and associated performance degradation. The
alignment hint can be overridden via the PNETCDF_HINTS environment
variable.

Author: Andy Stokely

src/external/SMIOL/smiol.c

@@ -12,6 +12,7 @@
 #define PNETCDF_DEFINE_MODE 0
 #define PNETCDF_DATA_MODE 1
 #define MAX_REQS 256
+#define PNETCDF_HEADER_ALIGN_SIZE_STR "131072"
 #endif
 
 #define START_COUNT_READ 0
@@ -324,6 +325,7 @@ int SMIOL_open_file(struct SMIOL_context *context, const char *filename,
 			/*
 			 * Convert fformat to a PNetCDF file creation mode
 			 */
+			MPI_Info info = MPI_INFO_NULL;
 			int filecmode;
 			if (fformat == SMIOL_FORMAT_CDF2) {
 				filecmode = NC_64BIT_OFFSET;
@@ -336,11 +338,14 @@ int SMIOL_open_file(struct SMIOL_context *context, const char *filename,
 				MPI_Comm_free(&io_group_comm);
 				return SMIOL_INVALID_FORMAT;
 			}
-
+			MPI_Info_create(&info);
+			MPI_Info_set(info, "nc_header_align_size",
+			             PNETCDF_HEADER_ALIGN_SIZE_STR);
 			ierr = ncmpi_create(io_file_comm, filename,
 			                    (filecmode | NC_CLOBBER),
-			                    MPI_INFO_NULL,
+			                    info,
 			                    &((*file)->ncidp));
+			MPI_Info_free(&info);
 		}
 		(*file)->state = PNETCDF_DEFINE_MODE;
 #endif