HDFFV-10625 -- Implemented a process-0 read and then broadcast for collective read of full (HS_ALL), contiguous, atomic datasets by all the processes in the file communicator.

14 files changed, 869 insertions, 124 deletions

diff --git a/src/H5CX.c b/src/H5CX.c
index 1f91ee2..0d20132 100644
--- a/src/H5CX.c
+++ b/src/H5CX.c
@@ -198,6 +198,7 @@ typedef struct H5CX_t {
     MPI_Datatype btype;         /* MPI datatype for buffer, when using collective I/O */
     MPI_Datatype ftype;         /* MPI datatype for file, when using collective I/O */
     hbool_t mpi_file_flushing;  /* Whether an MPI-opened file is being flushed */
+    hbool_t rank0_bcast;        /* Whether a dataset meets read-with-rank0-and-bcast requirements */
 #endif /* H5_HAVE_PARALLEL */
 
     /* Cached DXPL properties */
@@ -261,6 +262,8 @@ typedef struct H5CX_t {
     hbool_t mpio_coll_chunk_multi_ratio_coll_set; /* Whether instrumented "collective chunk multi ratio coll" value is set */
     int mpio_coll_chunk_multi_ratio_ind;  /* Instrumented "collective chunk multi ratio ind" value (H5D_XFER_COLL_CHUNK_MULTI_RATIO_IND_NAME) */
     hbool_t mpio_coll_chunk_multi_ratio_ind_set; /* Whether instrumented "collective chunk multi ratio ind" value is set */
+    hbool_t mpio_coll_rank0_bcast;  /* Instrumented "collective chunk multi ratio ind" value (H5D_XFER_COLL_CHUNK_MULTI_RATIO_IND_NAME) */
+    hbool_t mpio_coll_rank0_bcast_set; /* Whether instrumented "collective chunk multi ratio ind" value is set */
 #endif /* H5_HAVE_INSTRUMENTED_LIBRARY */
 #endif /* H5_HAVE_PARALLEL */
 
@@ -1254,6 +1257,32 @@ H5CX_get_mpi_file_flushing(void)
 
     FUNC_LEAVE_NOAPI((*head)->ctx.mpi_file_flushing)
 } /* end H5CX_get_mpi_file_flushing() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5CX_get_mpio_rank0_bcast 
+ *
+ * Purpose:     Retrieves if the dataset meets read-with-rank0-and-bcast requirements for the current API call context.
+ *
+ * Return:      Non-negative on success / Negative on failure
+ *
+ * Programmer:  M. Breitenfeld
+ *              December 31, 2018
+ *
+ *-------------------------------------------------------------------------
+ */
+hbool_t
+H5CX_get_mpio_rank0_bcast(void)
+{
+    H5CX_node_t **head = H5CX_get_my_context();  /* Get the pointer to the head of the API context, for this thread */
+
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    /* Sanity check */
+    HDassert(head && *head);
+
+    FUNC_LEAVE_NOAPI((*head)->ctx.rank0_bcast)
+} /* end H5CX_get_mpio_rank0_bcast() */
 #endif /* H5_HAVE_PARALLEL */
 
 
@@ -2185,6 +2214,34 @@ H5CX_set_mpi_file_flushing(hbool_t flushing)
 
     FUNC_LEAVE_NOAPI_VOID
 } /* end H5CX_set_mpi_file_flushing() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5CX_set_mpio_rank0_bcast
+ *
+ * Purpose:     Sets the "dataset meets read-with-rank0-and-bcast requirements" flag for the current API call context.
+ *
+ * Return:      <none>
+ *
+ * Programmer:  M. Breitenfeld
+ *              December 31, 2018
+ *
+ *-------------------------------------------------------------------------
+ */
+void
+H5CX_set_mpio_rank0_bcast(hbool_t rank0_bcast)
+{
+    H5CX_node_t **head = H5CX_get_my_context();  /* Get the pointer to the head of the API context, for this thread */
+
+    FUNC_ENTER_NOAPI_NOINIT_NOERR
+
+    /* Sanity checks */
+    HDassert(head && *head);
+
+    (*head)->ctx.rank0_bcast = rank0_bcast;
+
+    FUNC_LEAVE_NOAPI_VOID
+} /* end H5CX_set_mpio_rank0_bcast() */
 #endif /* H5_HAVE_PARALLEL */
 
 
@@ -2596,6 +2653,40 @@ H5CX_test_set_mpio_coll_chunk_multi_ratio_ind(int mpio_coll_chunk_multi_ratio_in
 done:
     FUNC_LEAVE_NOAPI(ret_value)
 } /* end H5CX_test_set_mpio_coll_chunk_multi_ratio_ind() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5CX_test_set_mpio_coll_rank0_bcast
+ *
+ * Purpose:     Sets the instrumented "read-with-rank0-bcast" flag for the current API call context.
+ *
+ * Note:        Only sets value if property set in DXPL
+ *
+ * Return:      Non-negative on success / Negative on failure
+ *
+ * Programmer:  Quincey Koziol
+ *              January 2, 2019
+ *
+ *-------------------------------------------------------------------------
+ */
+herr_t
+H5CX_test_set_mpio_coll_rank0_bcast(hbool_t mpio_coll_rank0_bcast)
+{
+    H5CX_node_t **head = H5CX_get_my_context();  /* Get the pointer to the head of the API context, for this thread */
+    herr_t ret_value = SUCCEED;         /* Return value */
+
+    FUNC_ENTER_NOAPI_NOINIT
+
+    /* Sanity checks */
+    HDassert(head && *head);
+    HDassert(!((*head)->ctx.dxpl_id == H5P_DEFAULT || 
+            (*head)->ctx.dxpl_id == H5P_DATASET_XFER_DEFAULT));
+
+    H5CX_TEST_SET_PROP(H5D_XFER_COLL_RANK0_BCAST_NAME, mpio_coll_rank0_bcast)
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5CX_test_set_mpio_coll_rank0_bcast() */
 #endif /* H5_HAVE_INSTRUMENTED_LIBRARY */
 #endif /* H5_HAVE_PARALLEL */
 
@@ -2640,6 +2731,7 @@ H5CX__pop_common(void)
     H5CX_SET_PROP(H5D_XFER_COLL_CHUNK_LINK_NUM_FALSE_NAME, mpio_coll_chunk_link_num_false)
     H5CX_SET_PROP(H5D_XFER_COLL_CHUNK_MULTI_RATIO_COLL_NAME, mpio_coll_chunk_multi_ratio_coll)
     H5CX_SET_PROP(H5D_XFER_COLL_CHUNK_MULTI_RATIO_IND_NAME, mpio_coll_chunk_multi_ratio_ind)
+    H5CX_SET_PROP(H5D_XFER_COLL_RANK0_BCAST_NAME, mpio_coll_rank0_bcast)
 #endif /* H5_HAVE_INSTRUMENTED_LIBRARY */
 #endif /* H5_HAVE_PARALLEL */
 
diff --git a/src/H5CXprivate.h b/src/H5CXprivate.h
index 46289c4..46d25d0 100644
--- a/src/H5CXprivate.h
+++ b/src/H5CXprivate.h
@@ -77,6 +77,7 @@ H5_DLL H5AC_ring_t H5CX_get_ring(void);
 H5_DLL hbool_t H5CX_get_coll_metadata_read(void);
 H5_DLL herr_t H5CX_get_mpi_coll_datatypes(MPI_Datatype *btype, MPI_Datatype *ftype);
 H5_DLL hbool_t H5CX_get_mpi_file_flushing(void);
+H5_DLL hbool_t H5CX_get_mpio_rank0_bcast(void);
 #endif /* H5_HAVE_PARALLEL */
 
 /* "Getter" routines for DXPL properties cached in API context */
@@ -112,6 +113,7 @@ H5_DLL void H5CX_set_coll_metadata_read(hbool_t cmdr);
 H5_DLL herr_t H5CX_set_mpi_coll_datatypes(MPI_Datatype btype, MPI_Datatype ftype);
 H5_DLL herr_t H5CX_set_mpio_coll_opt(H5FD_mpio_collective_opt_t mpio_coll_opt);
 H5_DLL void H5CX_set_mpi_file_flushing(hbool_t flushing);
+H5_DLL void H5CX_set_mpio_rank0_bcast(hbool_t rank0_bcast);
 #endif /* H5_HAVE_PARALLEL */
 
 /* "Setter" routines for DXPL properties cached in API context */
@@ -137,6 +139,7 @@ H5_DLL herr_t H5CX_test_set_mpio_coll_chunk_link_num_true(int mpio_coll_chunk_li
 H5_DLL herr_t H5CX_test_set_mpio_coll_chunk_link_num_false(int mpio_coll_chunk_link_num_false);
 H5_DLL herr_t H5CX_test_set_mpio_coll_chunk_multi_ratio_coll(int mpio_coll_chunk_multi_ratio_coll);
 H5_DLL herr_t H5CX_test_set_mpio_coll_chunk_multi_ratio_ind(int mpio_coll_chunk_multi_ratio_ind);
+H5_DLL herr_t H5CX_test_set_mpio_coll_rank0_bcast(hbool_t rank0_bcast);
 #endif /* H5_HAVE_INSTRUMENTED_LIBRARY */
 #endif /* H5_HAVE_PARALLEL */
 
diff --git a/src/H5Dio.c b/src/H5Dio.c
index fe85d23..6062dff 100644
--- a/src/H5Dio.c
+++ b/src/H5Dio.c
@@ -657,22 +657,12 @@ H5D__write(H5D_t *dataset, hid_t mem_type_id, const H5S_t *mem_space,
 
     /* Various MPI based checks */
 #ifdef H5_HAVE_PARALLEL
-    if H5F_HAS_FEATURE(dataset->oloc.file, H5FD_FEAT_HAS_MPI) {
-        /* If MPI based VFD is used, no VL datatype support yet. */
+    if(H5F_HAS_FEATURE(dataset->oloc.file, H5FD_FEAT_HAS_MPI)) {
+        /* If MPI based VFD is used, no VL or region reference datatype support yet. */
         /* This is because they use the global heap in the file and we don't */
         /* support parallel access of that yet */
-        if(H5T_detect_class(type_info.mem_type, H5T_VLEN, FALSE) > 0)
-            HGOTO_ERROR(H5E_DATASET, H5E_UNSUPPORTED, FAIL, "Parallel IO does not support writing VL datatypes yet")
-
-        /* If MPI based VFD is used, no VL datatype support yet. */
-        /* This is because they use the global heap in the file and we don't */
-        /* support parallel access of that yet */
-        /* We should really use H5T_detect_class() here, but it will be difficult
-         * to detect the type of the reference if it is nested... -QAK
-         */
-        if(H5T_get_class(type_info.mem_type, TRUE) == H5T_REFERENCE &&
-                H5T_get_ref_type(type_info.mem_type) == H5R_DATASET_REGION)
-            HGOTO_ERROR(H5E_DATASET, H5E_UNSUPPORTED, FAIL, "Parallel IO does not support writing region reference datatypes yet")
+        if(H5T_is_vl_storage(type_info.mem_type) > 0)
+            HGOTO_ERROR(H5E_DATASET, H5E_UNSUPPORTED, FAIL, "Parallel IO does not support writing VL or region reference datatypes yet")
     } /* end if */
     else {
         H5FD_mpio_xfer_t io_xfer_mode;      /* MPI I/O transfer mode */
diff --git a/src/H5Dmpio.c b/src/H5Dmpio.c
index 2c06800..f5da33d 100644
--- a/src/H5Dmpio.c
+++ b/src/H5Dmpio.c
@@ -37,7 +37,6 @@
 #include "H5Eprivate.h"       /* Error handling    */
 #include "H5Fprivate.h"       /* File access       */
 #include "H5FDprivate.h"      /* File drivers      */
-#include "H5FDmpi.h"          /* MPI-based file drivers	*/
 #include "H5Iprivate.h"       /* IDs               */
 #include "H5MMprivate.h"      /* Memory management */
 #include "H5Oprivate.h"       /* Object headers    */
@@ -89,10 +88,20 @@
 /******************/
 /* Combine chunk address and chunk info into a struct for better performance. */
 typedef struct H5D_chunk_addr_info_t {
-  haddr_t chunk_addr;
-  H5D_chunk_info_t chunk_info;
+    haddr_t chunk_addr;
+    H5D_chunk_info_t chunk_info;
 } H5D_chunk_addr_info_t;
 
+/* Rank 0 Bcast values */
+typedef enum H5D_mpio_no_rank0_bcast_cause_t {
+    H5D_MPIO_RANK0_BCAST = 0x00,
+    H5D_MPIO_RANK0_NOT_H5S_ALL = 0x01,
+    H5D_MPIO_RANK0_NOT_CONTIGUOUS = 0x02,
+    H5D_MPIO_RANK0_NOT_FIXED_SIZE = 0x04,
+    H5D_MPIO_RANK0_GREATER_THAN_2GB = 0x08
+} H5D_mpio_no_rank0_bcast_cause_t;
+
+
 /*
  * Information about a single chunk when performing collective filtered I/O. All
  * of the fields of one of these structs are initialized at the start of collective
@@ -281,8 +290,10 @@ H5D__mpio_opt_possible(const H5D_io_info_t *io_info, const H5S_t *file_space,
     const H5S_t *mem_space, const H5D_type_info_t *type_info)
 {
     H5FD_mpio_xfer_t io_xfer_mode;      /* MPI I/O transfer mode */
-    unsigned local_cause = 0;                /* Local reason(s) for breaking collective mode */
-    unsigned global_cause = 0;               /* Global reason(s) for breaking collective mode */
+    unsigned local_cause[2] = {0,0};    /* [0] Local reason(s) for breaking collective mode */
+                                        /* [1] Flag if dataset is both: H5S_ALL and small */
+    unsigned global_cause[2] = {0,0};   /* Global reason(s) for breaking collective mode */
+    htri_t is_vl_storage;               /* Whether the dataset's datatype is stored in a variable-length form */
     htri_t ret_value = SUCCEED;         /* Return value */
 
     FUNC_ENTER_PACKAGE
@@ -296,36 +307,37 @@ H5D__mpio_opt_possible(const H5D_io_info_t *io_info, const H5S_t *file_space,
 
     /* For independent I/O, get out quickly and don't try to form consensus */
     if(H5CX_get_io_xfer_mode(&io_xfer_mode) < 0)
-        HGOTO_ERROR(H5E_DATASET, H5E_CANTGET, FAIL, "can't get MPI-I/O transfer mode")
+        /* Set error flag, but keep going */
+        local_cause[0] |= H5D_MPIO_ERROR_WHILE_CHECKING_COLLECTIVE_POSSIBLE;
     if(io_xfer_mode == H5FD_MPIO_INDEPENDENT)
-        local_cause |= H5D_MPIO_SET_INDEPENDENT;
+        local_cause[0] |= H5D_MPIO_SET_INDEPENDENT;
 
     /* Optimized MPI types flag must be set */
     /* (based on 'HDF5_MPI_OPT_TYPES' environment variable) */
     if(!H5FD_mpi_opt_types_g)
-        local_cause |= H5D_MPIO_MPI_OPT_TYPES_ENV_VAR_DISABLED;
+        local_cause[0] |= H5D_MPIO_MPI_OPT_TYPES_ENV_VAR_DISABLED;
 
     /* Don't allow collective operations if datatype conversions need to happen */
     if(!type_info->is_conv_noop)
-        local_cause |= H5D_MPIO_DATATYPE_CONVERSION;
+        local_cause[0] |= H5D_MPIO_DATATYPE_CONVERSION;
 
     /* Don't allow collective operations if data transform operations should occur */
     if(!type_info->is_xform_noop)
-        local_cause |= H5D_MPIO_DATA_TRANSFORMS;
+        local_cause[0] |= H5D_MPIO_DATA_TRANSFORMS;
 
     /* Check whether these are both simple or scalar dataspaces */
     if(!((H5S_SIMPLE == H5S_GET_EXTENT_TYPE(mem_space) || H5S_SCALAR == H5S_GET_EXTENT_TYPE(mem_space))
             && (H5S_SIMPLE == H5S_GET_EXTENT_TYPE(file_space) || H5S_SCALAR == H5S_GET_EXTENT_TYPE(file_space))))
-        local_cause |= H5D_MPIO_NOT_SIMPLE_OR_SCALAR_DATASPACES;
+        local_cause[0] |= H5D_MPIO_NOT_SIMPLE_OR_SCALAR_DATASPACES;
 
     /* Dataset storage must be contiguous or chunked */
     if(!(io_info->dset->shared->layout.type == H5D_CONTIGUOUS ||
             io_info->dset->shared->layout.type == H5D_CHUNKED))
-        local_cause |= H5D_MPIO_NOT_CONTIGUOUS_OR_CHUNKED_DATASET;
+        local_cause[0] |= H5D_MPIO_NOT_CONTIGUOUS_OR_CHUNKED_DATASET;
 
     /* check if external-file storage is used */
     if(io_info->dset->shared->dcpl_cache.efl.nused > 0)
-        local_cause |= H5D_MPIO_NOT_CONTIGUOUS_OR_CHUNKED_DATASET;
+        local_cause[0] |= H5D_MPIO_NOT_CONTIGUOUS_OR_CHUNKED_DATASET;
 
     /* The handling of memory space is different for chunking and contiguous
      *  storage.  For contiguous storage, mem_space and file_space won't change
@@ -340,31 +352,84 @@ H5D__mpio_opt_possible(const H5D_io_info_t *io_info, const H5S_t *file_space,
      * is less than 3. The functions needed (MPI_Mprobe and MPI_Imrecv) will
      * not be available.
      */
-    if (io_info->op_type == H5D_IO_OP_WRITE &&
-        io_info->dset->shared->layout.type == H5D_CHUNKED &&
-        io_info->dset->shared->dcpl_cache.pline.nused > 0)
-        local_cause |= H5D_MPIO_PARALLEL_FILTERED_WRITES_DISABLED;
+    if(io_info->op_type == H5D_IO_OP_WRITE &&
+            io_info->dset->shared->layout.type == H5D_CHUNKED &&
+            io_info->dset->shared->dcpl_cache.pline.nused > 0)
+        local_cause[0] |= H5D_MPIO_PARALLEL_FILTERED_WRITES_DISABLED;
 #endif
 
+    /* Check if we are able to do a MPI_Bcast of the data from one rank
+     * instead of having all the processes involved in the collective I/O call.
+     */
+
+    /* Check to see if the process is reading the entire dataset */
+    if(H5S_GET_SELECT_TYPE(file_space) != H5S_SEL_ALL)
+        local_cause[1] |= H5D_MPIO_RANK0_NOT_H5S_ALL; 
+    /* Only perform this optimization for contigous datasets, currently */
+    else if(H5D_CONTIGUOUS != io_info->dset->shared->layout.type)
+        /* Flag to do a MPI_Bcast of the data from one proc instead of 
+         * having all the processes involved in the collective I/O.
+         */
+        local_cause[1] |= H5D_MPIO_RANK0_NOT_CONTIGUOUS; 
+    else if((is_vl_storage = H5T_is_vl_storage(type_info->dset_type)) < 0)
+        local_cause[0] |= H5D_MPIO_ERROR_WHILE_CHECKING_COLLECTIVE_POSSIBLE;
+    else if(is_vl_storage)
+        local_cause[1] |= H5D_MPIO_RANK0_NOT_FIXED_SIZE;
+    else {
+        size_t type_size;       /* Size of dataset's datatype */
+
+        /* Retrieve the size of the dataset's datatype */
+        if(0 == (type_size = H5T_GET_SIZE(type_info->dset_type)))
+            local_cause[0] |= H5D_MPIO_ERROR_WHILE_CHECKING_COLLECTIVE_POSSIBLE;
+        else {
+            hssize_t snelmts;   /* [Signed] # of elements in dataset's dataspace */
+
+            /* Retrieve the size of the dataset's datatype */
+            if((snelmts = H5S_GET_EXTENT_NPOINTS(file_space)) < 0)
+                local_cause[0] |= H5D_MPIO_ERROR_WHILE_CHECKING_COLLECTIVE_POSSIBLE;
+            else {
+                hsize_t dset_size;
+
+                /* Determine dataset size */
+                dset_size = ((hsize_t)snelmts) * type_size;
+
+               /* If the size of the dataset is less than 2GB then do an MPI_Bcast
+                * of the data from one process instead of having all the processes 
+                * involved in the collective I/O.
+                */
+                if(dset_size > ((hsize_t)(2.0F * H5_GB) - 1))
+                    local_cause[1] |= H5D_MPIO_RANK0_GREATER_THAN_2GB;
+            } /* end else */
+        } /* end else */
+    } /* end else */
+    
     /* Check for independent I/O */
-    if(local_cause & H5D_MPIO_SET_INDEPENDENT)
-        global_cause = local_cause;
+    if(local_cause[0] & H5D_MPIO_SET_INDEPENDENT)
+        global_cause[0] = local_cause[0];
     else {
         int mpi_code;               /* MPI error code */
 
         /* Form consensus opinion among all processes about whether to perform
          * collective I/O
          */
-        if(MPI_SUCCESS != (mpi_code = MPI_Allreduce(&local_cause, &global_cause, 1, MPI_UNSIGNED, MPI_BOR, io_info->comm)))
+        if(MPI_SUCCESS != (mpi_code = MPI_Allreduce(&local_cause, &global_cause, 2, MPI_UNSIGNED, MPI_BOR, io_info->comm)))
             HMPI_GOTO_ERROR(FAIL, "MPI_Allreduce failed", mpi_code)
     } /* end else */
 
     /* Set the local & global values of no-collective-cause in the API context */
-    H5CX_set_mpio_local_no_coll_cause(local_cause);
-    H5CX_set_mpio_global_no_coll_cause(global_cause);
+    H5CX_set_mpio_local_no_coll_cause(local_cause[0]);
+    H5CX_set_mpio_global_no_coll_cause(global_cause[0]);
+
+    /* Set read-with-rank0-and-bcast flag if possible */
+    if(global_cause[0] == 0 && global_cause[1] == 0) {
+        H5CX_set_mpio_rank0_bcast(TRUE);
+#ifdef H5_HAVE_INSTRUMENTED_LIBRARY
+        H5CX_test_set_mpio_coll_rank0_bcast(TRUE);
+#endif /* H5_HAVE_INSTRUMENTED_LIBRARY */
+    } /* end if */
 
     /* Set the return value, based on the global cause */
-    ret_value = global_cause > 0 ? FALSE : TRUE;
+    ret_value = global_cause[0] > 0 ? FALSE : TRUE;
 
 done:
     FUNC_LEAVE_NOAPI(ret_value)
@@ -3069,8 +3134,8 @@ H5D__filtered_collective_chunk_entry_io(H5D_filtered_collective_io_info_t *chunk
         chunk_entry->chunk_states.new_chunk.length = chunk_entry->chunk_states.chunk_current.length;
 
         /* Currently, these chunk reads are done independently and will likely
-	 * cause issues with collective metadata reads enabled. In the future,
-	 * this should be refactored to use collective chunk reads - JTH */
+         * cause issues with collective metadata reads enabled. In the future,
+         * this should be refactored to use collective chunk reads - JTH */
 
         /* Get the original state of parallel I/O transfer mode */
         if(H5CX_get_io_xfer_mode(&xfer_mode) < 0)
diff --git a/src/H5Dprivate.h b/src/H5Dprivate.h
index aaa3db2..6fb7889 100644
--- a/src/H5Dprivate.h
+++ b/src/H5Dprivate.h
@@ -95,7 +95,13 @@
 /* Definitions for all collective chunk instrumentation properties */
 #define H5D_XFER_COLL_CHUNK_SIZE        sizeof(unsigned)
 #define H5D_XFER_COLL_CHUNK_DEF         1
-#define H5D_XFER_COLL_CHUNK_FIX         0
+
+/* General collective I/O instrumentation properties */
+#define H5D_XFER_COLL_RANK0_BCAST_NAME  "coll_rank0_bcast"
+
+/* Definitions for general collective I/O instrumentation properties */
+#define H5D_XFER_COLL_RANK0_BCAST_SIZE  sizeof(hbool_t)
+#define H5D_XFER_COLL_RANK0_BCAST_DEF   FALSE
 #endif /* H5_HAVE_INSTRUMENTED_LIBRARY */
 
 /* Default temporary buffer size */
diff --git a/src/H5FDmpio.c b/src/H5FDmpio.c
index d160858..3ab90aa 100644
--- a/src/H5FDmpio.c
+++ b/src/H5FDmpio.c
@@ -1354,6 +1354,7 @@ H5FD__mpio_read(H5FD_t *_file, H5FD_mem_t H5_ATTR_UNUSED type,
     int         n;
 #endif
     hbool_t     use_view_this_time = FALSE;
+    hbool_t     rank0_bcast = FALSE; /* If read-with-rank0-and-bcast flag was used */
     herr_t      ret_value = SUCCEED;
 
     FUNC_ENTER_STATIC
@@ -1437,8 +1438,25 @@ H5FD__mpio_read(H5FD_t *_file, H5FD_mem_t H5_ATTR_UNUSED type,
             if(H5FD_mpio_Debug[(int)'r'])
                 HDfprintf(stdout, "%s: doing MPI collective IO\n", FUNC);
 #endif
-            if(MPI_SUCCESS != (mpi_code = MPI_File_read_at_all(file->f, mpi_off, buf, size_i, buf_type, &mpi_stat)))
-                HMPI_GOTO_ERROR(FAIL, "MPI_File_read_at_all failed", mpi_code)
+            /* Check whether we should read from rank 0 and broadcast to other ranks */
+            if(H5CX_get_mpio_rank0_bcast()) {
+#ifdef H5FDmpio_DEBUG
+                if(H5FD_mpio_Debug[(int)'r'])
+                    HDfprintf(stdout, "%s: doing read-rank0-and-MPI_Bcast\n", FUNC);
+#endif
+                /* Indicate path we've taken */
+                rank0_bcast = TRUE;
+
+                /* Read on rank 0 Bcast to other ranks */
+                if(file->mpi_rank == 0)
+                    if(MPI_SUCCESS != (mpi_code = MPI_File_read_at(file->f, mpi_off, buf, size_i, buf_type, &mpi_stat)))
+                        HMPI_GOTO_ERROR(FAIL, "MPI_File_read_at failed", mpi_code)
+                if(MPI_SUCCESS != (mpi_code = MPI_Bcast(buf, size_i, buf_type, 0, file->comm)))
+                    HMPI_GOTO_ERROR(FAIL, "MPI_Bcast failed", mpi_code)
+            } /* end if */
+            else
+                if(MPI_SUCCESS != (mpi_code = MPI_File_read_at_all(file->f, mpi_off, buf, size_i, buf_type, &mpi_stat)))
+                    HMPI_GOTO_ERROR(FAIL, "MPI_File_read_at_all failed", mpi_code)
         } /* end if */
         else {
 #ifdef H5FDmpio_DEBUG
@@ -1460,13 +1478,26 @@ H5FD__mpio_read(H5FD_t *_file, H5FD_mem_t H5_ATTR_UNUSED type,
         if(MPI_SUCCESS != (mpi_code = MPI_File_read_at(file->f, mpi_off, buf, size_i, buf_type, &mpi_stat)))
             HMPI_GOTO_ERROR(FAIL, "MPI_File_read_at failed", mpi_code)
 
-    /* How many bytes were actually read? */
+    /* Only retrieve bytes read if this rank _actually_ participated in I/O */
+    if(!rank0_bcast || (rank0_bcast && file->mpi_rank == 0) ) {
+        /* How many bytes were actually read? */
 #if MPI_VERSION >= 3
-    if(MPI_SUCCESS != (mpi_code = MPI_Get_elements_x(&mpi_stat, buf_type, &bytes_read)))
+        if(MPI_SUCCESS != (mpi_code = MPI_Get_elements_x(&mpi_stat, buf_type, &bytes_read)))
 #else
-    if(MPI_SUCCESS != (mpi_code = MPI_Get_elements(&mpi_stat, MPI_BYTE, &bytes_read)))
+        if(MPI_SUCCESS != (mpi_code = MPI_Get_elements(&mpi_stat, MPI_BYTE, &bytes_read)))
 #endif
-        HMPI_GOTO_ERROR(FAIL, "MPI_Get_elements failed", mpi_code)
+            HMPI_GOTO_ERROR(FAIL, "MPI_Get_elements failed", mpi_code)
+    } /* end if */
+ 
+    /* If the rank0-bcast feature was used, broadcast the # of bytes read to
+     * other ranks, which didn't perform any I/O.
+     */
+    /* NOTE: This could be optimized further to be combined with the broadcast
+     *          of the data.  (QAK - 2019/1/2)
+     */
+    if(rank0_bcast)
+        if(MPI_SUCCESS != MPI_Bcast(&bytes_read, 1, MPI_LONG_LONG, 0, file->comm))
+            HMPI_GOTO_ERROR(FAIL, "MPI_Bcast failed", 0)
 
     /* Get the type's size */
 #if MPI_VERSION >= 3
diff --git a/src/H5Pdxpl.c b/src/H5Pdxpl.c
index bfc1d93..8338d84 100644
--- a/src/H5Pdxpl.c
+++ b/src/H5Pdxpl.c
@@ -2017,6 +2017,7 @@ done:
     FUNC_LEAVE_API(ret_value)
 } /* end H5Pget_mpio_actual_io_mode() */
 
+
 /*-------------------------------------------------------------------------
  * Function:	H5Pget_mpio_no_collective_cause
  *
@@ -2053,8 +2054,6 @@ H5Pget_mpio_no_collective_cause(hid_t plist_id, uint32_t *local_no_collective_ca
 done:
     FUNC_LEAVE_API(ret_value)
 } /* end H5Pget_mpio_no_collective_cause() */
-
-
 #endif /* H5_HAVE_PARALLEL */
 
 
diff --git a/src/H5Ppublic.h b/src/H5Ppublic.h
index 2f094ea..078fe74 100644
--- a/src/H5Ppublic.h
+++ b/src/H5Ppublic.h
@@ -167,7 +167,8 @@ typedef enum H5D_mpio_no_collective_cause_t {
     H5D_MPIO_NOT_SIMPLE_OR_SCALAR_DATASPACES = 0x10,
     H5D_MPIO_NOT_CONTIGUOUS_OR_CHUNKED_DATASET = 0x20,
     H5D_MPIO_PARALLEL_FILTERED_WRITES_DISABLED = 0x40,
-    H5D_MPIO_NO_COLLECTIVE_MAX_CAUSE = 0x80
+    H5D_MPIO_ERROR_WHILE_CHECKING_COLLECTIVE_POSSIBLE = 0x80,
+    H5D_MPIO_NO_COLLECTIVE_MAX_CAUSE = 0x100
 } H5D_mpio_no_collective_cause_t;
 
 /********************/
diff --git a/src/H5T.c b/src/H5T.c
index 01ace87..9544488 100644
--- a/src/H5T.c
+++ b/src/H5T.c
@@ -295,6 +295,7 @@ static htri_t H5T__compiler_conv(H5T_t *src, H5T_t *dst);
 static herr_t H5T__set_size(H5T_t *dt, size_t size);
 static herr_t H5T__close_cb(H5T_t *dt);
 static H5T_path_t *H5T__path_find_real(const H5T_t *src, const H5T_t *dst, const char *name, H5T_conv_func_t *conv);
+static hbool_t H5T__detect_reg_ref(const H5T_t *dt);
 
 
 /*****************************/
@@ -5506,6 +5507,111 @@ done:
 
 
 /*-------------------------------------------------------------------------
+ * Function:   H5T_detect_reg_ref
+ *
+ * Purpose:    Check whether a datatype contains (or is) a region reference
+ *             datatype.
+ *
+ * Return:      TRUE (1) or FALSE (0) on success
+ *		(Can't fail)
+ *
+ * Programmer:  Quincey Koziol
+ *              Saturday, January 5, 2019
+ *
+ *-------------------------------------------------------------------------
+ */
+static hbool_t
+H5T__detect_reg_ref(const H5T_t *dt)
+{
+    unsigned    u;                      /* Local index variable */
+    hbool_t     ret_value = FALSE;      /* Return value */
+
+    FUNC_ENTER_STATIC_NOERR
+
+    /* Sanity checks */
+    HDassert(dt);
+
+    /* Check if this datatype is a region reference */
+    if(H5T_REFERENCE == dt->shared->type && H5R_DATASET_REGION == dt->shared->u.atomic.u.r.rtype)
+        HGOTO_DONE(TRUE);
+
+    /* Check for types that might have the correct type as a component */
+    switch(dt->shared->type) {
+        case H5T_COMPOUND:
+            /* Iterate over all the compound datatype's fields */
+            for(u = 0; u < dt->shared->u.compnd.nmembs; u++)
+                /* Recurse on field's datatype */
+                if(H5T__detect_reg_ref(dt->shared->u.compnd.memb[u].type))
+                    HGOTO_DONE(TRUE);
+            break;
+
+        case H5T_ARRAY:
+        case H5T_VLEN:
+        case H5T_ENUM:
+            HGOTO_DONE(H5T__detect_reg_ref(dt->shared->parent));
+            break;
+
+        case H5T_NO_CLASS:
+        case H5T_INTEGER:
+        case H5T_FLOAT:
+        case H5T_TIME:
+        case H5T_STRING:
+        case H5T_BITFIELD:
+        case H5T_OPAQUE:
+        case H5T_REFERENCE:
+        case H5T_NCLASSES:
+        default:
+            break;
+    } /* end if */
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5T__detect_reg_ref() */
+
+
+/*-------------------------------------------------------------------------
+ * Function:    H5T_is_vl_storage
+ *
+ * Purpose:     Check if a datatype will be stored in a variable-length form.
+ *
+ * Notes:       Currently, only variable-length string & sequences and region
+ *              references are stored in a variable-length form.
+ *
+ * Return:
+ *  One of two values on success:
+ *      TRUE - If the datatype will be stored in a variable-length form
+ *      FALSE - If the datatype will NOT be stored in a variable-length form
+ *  <0 is returned on failure
+ *
+ * Programmer:  Quincey Koziol
+ *              Saturday, January 5, 2019
+ *
+ *-------------------------------------------------------------------------
+ */
+htri_t
+H5T_is_vl_storage(const H5T_t *dt)
+{
+    htri_t ret_value = FALSE;
+
+    FUNC_ENTER_NOAPI(FAIL)
+
+    /* Sanity check */
+    HDassert(dt);
+
+    /* VL and region reference datatypes are stored in variable-length form */
+    if(H5T_detect_class(dt, H5T_VLEN, FALSE))
+        ret_value = TRUE;
+    else if(H5T_detect_class(dt, H5T_REFERENCE, FALSE))
+        ret_value = H5T__detect_reg_ref(dt);
+    else
+        ret_value = FALSE;
+
+done:
+    FUNC_LEAVE_NOAPI(ret_value)
+} /* end H5T_is_vl_storage() */
+
+
+/*-------------------------------------------------------------------------
  * Function:    H5T_upgrade_version_cb
  *
  * Purpose:     H5T__visit callback to Upgrade the version of a datatype
diff --git a/src/H5Tprivate.h b/src/H5Tprivate.h
index 6b6446f..3dcbb2c 100644
--- a/src/H5Tprivate.h
+++ b/src/H5Tprivate.h
@@ -145,6 +145,7 @@ H5_DLL H5T_t *H5T_get_actual_type(H5T_t *dt);
 H5_DLL herr_t H5T_save_refresh_state(hid_t tid, struct H5O_shared_t *cached_H5O_shared);
 H5_DLL herr_t H5T_restore_refresh_state(hid_t tid, struct H5O_shared_t *cached_H5O_shared);
 H5_DLL hbool_t H5T_already_vol_managed(const H5T_t *dt);
+H5_DLL htri_t H5T_is_vl_storage(const H5T_t *dt);
 
 /* Reference specific functions */
 H5_DLL H5R_type_t H5T_get_ref_type(const H5T_t *dt);
diff --git a/testpar/t_bigio.c b/testpar/t_bigio.c
index fdd3488..1d882b8 100644
--- a/testpar/t_bigio.c
+++ b/testpar/t_bigio.c
@@ -671,7 +671,7 @@ dataset_big_write(void)
     /* create a memory dataspace independently */
     mem_dataspace = H5Screate_simple (RANK, dims, NULL);
     VRFY((mem_dataspace >= 0), "");
-    if(!mpi_rank == 0) {
+    if(mpi_rank != 0) {
         ret = H5Sselect_none(mem_dataspace);
         VRFY((ret >= 0), "H5Sset_none succeeded");
     }
@@ -980,7 +980,7 @@ dataset_big_read(void)
     /* create a memory dataspace independently */
     mem_dataspace = H5Screate_simple (RANK, dims, NULL);
     VRFY((mem_dataspace >= 0), "");
-    if(!mpi_rank == 0) {
+    if(mpi_rank != 0) {
         ret = H5Sselect_none(mem_dataspace);
         VRFY((ret >= 0), "H5Sset_none succeeded");
     }
diff --git a/testpar/t_dset.c b/testpar/t_dset.c
index 281d027..35501d8 100644
--- a/testpar/t_dset.c
+++ b/testpar/t_dset.c
@@ -2649,7 +2649,7 @@ compress_readAll(void)
 
             /* Try reading the data */
             ret = H5Dread(dataset, H5T_NATIVE_INT, H5S_ALL, H5S_ALL, xfer_plist, data_read);
-            VRFY((ret >= 0), "H5Pset_dxpl_mpio succeeded");
+            VRFY((ret >= 0), "H5Dread succeeded");
 
             /* Verify data read */
             for(u=0; u<dim; u++)
diff --git a/testpar/t_mdset.c b/testpar/t_mdset.c
index 5d989bb..16eb13c 100644
--- a/testpar/t_mdset.c
+++ b/testpar/t_mdset.c
@@ -12,6 +12,7 @@
  * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
 
 #include "testphdf5.h"
+#include "H5Dprivate.h"
 
 #define DIM  2
 #define SIZE 32
@@ -311,13 +312,27 @@ void compact_dataset(void)
      VRFY((ret>= 0),"set independent IO collectively succeeded");
     }
 
-
     dataset = H5Dopen2(iof, dname, H5P_DEFAULT);
     VRFY((dataset >= 0), "H5Dopen2 succeeded");
 
+#ifdef H5_HAVE_INSTRUMENTED_LIBRARY
+    hbool_t prop_value;
+    prop_value = H5D_XFER_COLL_RANK0_BCAST_DEF;
+    ret = H5Pinsert2(dxpl, H5D_XFER_COLL_RANK0_BCAST_NAME, H5D_XFER_COLL_RANK0_BCAST_SIZE, &prop_value,
+                     NULL, NULL, NULL, NULL, NULL, NULL);
+    VRFY((ret >= 0), "H5Pinsert2() succeeded");
+#endif /* H5_HAVE_INSTRUMENTED_LIBRARY */
+
     ret = H5Dread(dataset, H5T_NATIVE_DOUBLE, H5S_ALL, H5S_ALL, dxpl, inme);
     VRFY((ret >= 0), "H5Dread succeeded");
 
+#ifdef H5_HAVE_INSTRUMENTED_LIBRARY
+    prop_value = FALSE;
+    ret = H5Pget(dxpl, H5D_XFER_COLL_RANK0_BCAST_NAME, &prop_value);
+    VRFY((ret >= 0), "H5Pget succeeded");
+    VRFY((prop_value == FALSE && dxfer_coll_type == DXFER_COLLECTIVE_IO),"rank 0 Bcast optimization was performed for a compact dataset");
+#endif /* H5_HAVE_INSTRUMENTED_LIBRARY */
+
     /* Verify data value */
     for(i = 0; i < size; i++)
         for(j = 0; j < size; j++)
@@ -603,8 +618,8 @@ void dataset_fillvalue(void)
     hsize_t     req_count[4] = {1, 6, 7, 8};
     hsize_t     dset_size;      /* Dataset size */
     int *rdata, *wdata;         /* Buffers for data to read and write */
-    int *twdata, *trdata;        /* Temporary pointer into buffer */
-    int acc, i, j, k, l;        /* Local index variables */
+    int *twdata, *trdata;       /* Temporary pointer into buffer */
+    int acc, i, j, k, l, ii;    /* Local index variables */
     herr_t ret;                 /* Generic return value */
     const char *filename;
 
@@ -645,27 +660,60 @@ void dataset_fillvalue(void)
     /*
      * Read dataset before any data is written.
      */
-    /* set entire read buffer with the constant 2 */
-    HDmemset(rdata,2,(size_t)(dset_size*sizeof(int)));
-    /* Independently read the entire dataset back */
-    ret = H5Dread(dataset, H5T_NATIVE_INT, H5S_ALL, H5S_ALL, H5P_DEFAULT, rdata);
-    VRFY((ret >= 0), "H5Dread succeeded");
 
-    /* Verify all data read are the fill value 0 */
-    trdata = rdata;
-    err_num = 0;
-    for(i = 0; i < (int)dset_dims[0]; i++)
+    /* Create DXPL for I/O */
+    dxpl = H5Pcreate(H5P_DATASET_XFER);
+    VRFY((dxpl >= 0), "H5Pcreate succeeded");
+
+#ifdef H5_HAVE_INSTRUMENTED_LIBRARY
+      hbool_t prop_value;
+      prop_value = H5D_XFER_COLL_RANK0_BCAST_DEF;
+      ret = H5Pinsert2(dxpl, H5D_XFER_COLL_RANK0_BCAST_NAME, H5D_XFER_COLL_RANK0_BCAST_SIZE, &prop_value,
+                   NULL, NULL, NULL, NULL, NULL, NULL);
+      VRFY((ret >= 0),"testing property list inserted succeeded");
+#endif /* H5_HAVE_INSTRUMENTED_LIBRARY */
+
+    for(ii = 0; ii < 2; ii++) {
+
+      if(ii == 0)
+        ret = H5Pset_dxpl_mpio(dxpl, H5FD_MPIO_INDEPENDENT);
+      else
+        ret = H5Pset_dxpl_mpio(dxpl, H5FD_MPIO_COLLECTIVE);
+      VRFY((ret >= 0), "H5Pset_dxpl_mpio succeeded");
+
+      /* set entire read buffer with the constant 2 */
+      HDmemset(rdata,2,(size_t)(dset_size*sizeof(int)));
+
+      /* Read the entire dataset back */
+      ret = H5Dread(dataset, H5T_NATIVE_INT, H5S_ALL, H5S_ALL, dxpl, rdata);
+      VRFY((ret >= 0), "H5Dread succeeded");
+
+#ifdef H5_HAVE_INSTRUMENTED_LIBRARY
+      prop_value = FALSE;
+      ret = H5Pget(dxpl, H5D_XFER_COLL_RANK0_BCAST_NAME, &prop_value);
+      VRFY((ret >= 0), "testing property list get succeeded");
+      if(ii == 0)
+        VRFY((prop_value == FALSE), "correctly handled rank 0 Bcast");
+      else
+        VRFY((prop_value == TRUE), "correctly handled rank 0 Bcast");
+#endif /* H5_HAVE_INSTRUMENTED_LIBRARY */
+
+      /* Verify all data read are the fill value 0 */
+      trdata = rdata;
+      err_num = 0;
+      for(i = 0; i < (int)dset_dims[0]; i++)
         for(j = 0; j < (int)dset_dims[1]; j++)
-            for(k = 0; k < (int)dset_dims[2]; k++)
-                for(l = 0; l < (int)dset_dims[3]; l++, twdata++, trdata++)
-		    if(*trdata != 0)
-			if(err_num++ < MAX_ERR_REPORT || VERBOSE_MED)
-			    printf("Dataset Verify failed at [%d][%d][%d][%d]: expect 0, got %d\n", i, j, k, l, *trdata);
-    if(err_num > MAX_ERR_REPORT && !VERBOSE_MED)
+          for(k = 0; k < (int)dset_dims[2]; k++)
+            for(l = 0; l < (int)dset_dims[3]; l++, twdata++, trdata++)
+              if(*trdata != 0)
+                if(err_num++ < MAX_ERR_REPORT || VERBOSE_MED)
+                  printf("Dataset Verify failed at [%d][%d][%d][%d]: expect 0, got %d\n", i, j, k, l, *trdata);
+      if(err_num > MAX_ERR_REPORT && !VERBOSE_MED)
         printf("[more errors ...]\n");
-    if(err_num){
+      if(err_num) {
         printf("%d errors found in check_value\n", err_num);
-	nerrors++;
+        nerrors++;
+      }
     }
 
     /* Barrier to ensure all processes have completed the above test. */
@@ -681,10 +729,6 @@ void dataset_fillvalue(void)
     ret = H5Sselect_hyperslab(memspace, H5S_SELECT_SET, req_start, NULL, req_count, NULL);
     VRFY((ret >= 0), "H5Sselect_hyperslab succeeded on memory dataspace");
 
-    /* Create DXPL for collective I/O */
-    dxpl = H5Pcreate(H5P_DATASET_XFER);
-    VRFY((dxpl >= 0), "H5Pcreate succeeded");
-
     ret = H5Pset_dxpl_mpio(dxpl, H5FD_MPIO_COLLECTIVE);
     VRFY((ret >= 0), "H5Pset_dxpl_mpio succeeded");
     if(dxfer_coll_type == DXFER_INDEPENDENT_IO) {
@@ -711,37 +755,64 @@ void dataset_fillvalue(void)
     /*
      * Read dataset after partial write.
      */
-    /* set entire read buffer with the constant 2 */
-    HDmemset(rdata,2,(size_t)(dset_size*sizeof(int)));
-    /* Independently read the entire dataset back */
-    ret = H5Dread(dataset, H5T_NATIVE_INT, H5S_ALL, H5S_ALL, H5P_DEFAULT, rdata);
-    VRFY((ret >= 0), "H5Dread succeeded");
 
-    /* Verify correct data read */
-    twdata=wdata;
-    trdata=rdata;
-    err_num=0;
-    for(i=0; i<(int)dset_dims[0]; i++)
+#ifdef H5_HAVE_INSTRUMENTED_LIBRARY
+      prop_value = H5D_XFER_COLL_RANK0_BCAST_DEF;
+      ret = H5Pset(dxpl, H5D_XFER_COLL_RANK0_BCAST_NAME, &prop_value);
+      VRFY((ret >= 0), " H5Pset succeeded");
+#endif /* H5_HAVE_INSTRUMENTED_LIBRARY */
+
+    for(ii = 0; ii < 2; ii++) {
+
+      if(ii == 0)
+        ret = H5Pset_dxpl_mpio(dxpl, H5FD_MPIO_INDEPENDENT);
+      else
+        ret = H5Pset_dxpl_mpio(dxpl, H5FD_MPIO_COLLECTIVE);
+      VRFY((ret >= 0), "H5Pset_dxpl_mpio succeeded");
+
+      /* set entire read buffer with the constant 2 */
+      HDmemset(rdata,2,(size_t)(dset_size*sizeof(int)));
+
+      /* Read the entire dataset back */
+      ret = H5Dread(dataset, H5T_NATIVE_INT, H5S_ALL, H5S_ALL, dxpl, rdata);
+      VRFY((ret >= 0), "H5Dread succeeded");
+
+#ifdef H5_HAVE_INSTRUMENTED_LIBRARY
+      prop_value = FALSE;
+      ret = H5Pget(dxpl, H5D_XFER_COLL_RANK0_BCAST_NAME, &prop_value);
+      VRFY((ret >= 0), "testing property list get succeeded");
+      if(ii == 0)
+        VRFY((prop_value == FALSE), "correctly handled rank 0 Bcast");
+      else
+        VRFY((prop_value == TRUE), "correctly handled rank 0 Bcast");
+#endif /* H5_HAVE_INSTRUMENTED_LIBRARY */
+
+      /* Verify correct data read */
+      twdata=wdata;
+      trdata=rdata;
+      err_num=0;
+      for(i=0; i<(int)dset_dims[0]; i++)
         for(j=0; j<(int)dset_dims[1]; j++)
-            for(k=0; k<(int)dset_dims[2]; k++)
-                for(l=0; l<(int)dset_dims[3]; l++, twdata++, trdata++)
-                    if(i<mpi_size) {
-                        if(*twdata != *trdata )
-                            if(err_num++ < MAX_ERR_REPORT || VERBOSE_MED)
-                                printf("Dataset Verify failed at [%d][%d][%d][%d]: expect %d, got %d\n", i,j,k,l, *twdata, *trdata);
-                    } /* end if */
-                    else {
-                        if(*trdata != 0)
-                            if(err_num++ < MAX_ERR_REPORT || VERBOSE_MED)
-                                printf("Dataset Verify failed at [%d][%d][%d][%d]: expect 0, got %d\n", i,j,k,l, *trdata);
-                    } /* end else */
-    if(err_num > MAX_ERR_REPORT && !VERBOSE_MED)
+          for(k=0; k<(int)dset_dims[2]; k++)
+            for(l=0; l<(int)dset_dims[3]; l++, twdata++, trdata++)
+              if(i<mpi_size) {
+                if(*twdata != *trdata )
+                  if(err_num++ < MAX_ERR_REPORT || VERBOSE_MED)
+                    printf("Dataset Verify failed at [%d][%d][%d][%d]: expect %d, got %d\n", i,j,k,l, *twdata, *trdata);
+              } /* end if */
+              else {
+                if(*trdata != 0)
+                  if(err_num++ < MAX_ERR_REPORT || VERBOSE_MED)
+                    printf("Dataset Verify failed at [%d][%d][%d][%d]: expect 0, got %d\n", i,j,k,l, *trdata);
+              } /* end else */
+      if(err_num > MAX_ERR_REPORT && !VERBOSE_MED)
         printf("[more errors ...]\n");
-    if(err_num){
+      if(err_num){
         printf("%d errors found in check_value\n", err_num);
-	nerrors++;
+        nerrors++;
+      }
     }
-
+    
     /* Close all file objects */
     ret = H5Dclose(dataset);
     VRFY((ret >= 0), "H5Dclose succeeded");
@@ -856,7 +927,7 @@ void collective_group_write(void)
         if(!((m+1) % 10)) {
             printf("created %d groups\n", m+1);
             MPI_Barrier(MPI_COMM_WORLD);
-	}
+        }
 #endif /* BARRIER_CHECKS */
     }
 
diff --git a/testpar/t_pread.c b/testpar/t_pread.c
index 0905d44..74feeb6 100644
--- a/testpar/t_pread.c
+++ b/testpar/t_pread.c
@@ -17,6 +17,7 @@
  */
 
 #include "testpar.h"
+#include "H5Dprivate.h"
 
 /* The collection of files is included below to aid
  * an external "cleanup" process if required.
@@ -34,6 +35,8 @@ const char *FILENAMES[NFILENAME + 1]={"reloc_t_pread_data_file",
 
 #define COUNT 1000
 
+#define LIMIT_NPROC 6
+
 hbool_t pass = true;
 static const char *random_hdf5_text =
 "Now is the time for all first-time-users of HDF5 to read their \
@@ -46,7 +49,7 @@ completely foolproof is to underestimate the ingenuity of complete\n\
 fools.\n";
 
 static int generate_test_file(MPI_Comm comm, int mpi_rank, int group);
-static int test_parallel_read(MPI_Comm comm, int mpi_rank, int group);
+static int test_parallel_read(MPI_Comm comm, int mpi_rank, int mpi_size, int group);
 
 static char *test_argv0 = NULL;
 
@@ -108,6 +111,9 @@ generate_test_file( MPI_Comm comm, int mpi_rank, int group_id )
     hid_t fapl_id   = -1;
     hid_t dxpl_id   = -1;
     hid_t dset_id   = -1;
+    hid_t dset_id_ch = -1;
+    hid_t dcpl_id = H5P_DEFAULT;
+    hsize_t chunk[1];
     float nextValue;
     float *data_slice = NULL;
 
@@ -272,6 +278,55 @@ generate_test_file( MPI_Comm comm, int mpi_rank, int group_id )
         }
     }
 
+
+    /* create a chunked dataset */
+    chunk[0] = COUNT/8;
+    
+    if ( pass ) {
+      if ( (dcpl_id = H5Pcreate (H5P_DATASET_CREATE)) < 0 ) {
+        pass = false;
+          failure_mssg = "H5Pcreate() failed.\n";
+      }
+    }
+
+    if ( pass ) {
+      if ( (H5Pset_chunk (dcpl_id, 1, chunk) ) < 0 ) {
+        pass = false;
+        failure_mssg = "H5Pset_chunk() failed.\n";
+      }
+    }
+    
+    if ( pass ) {
+      
+      if ( (dset_id_ch = H5Dcreate2(file_id, "dataset0_chunked", H5T_NATIVE_FLOAT,
+                                    filespace, H5P_DEFAULT, dcpl_id,
+                                    H5P_DEFAULT)) < 0 ) {
+        pass = false;
+        failure_mssg = "H5Dcreate2() failed.\n";
+      }
+    }
+
+    if ( pass ) {
+      if ( (H5Dwrite(dset_id_ch, H5T_NATIVE_FLOAT, memspace,
+                     filespace, dxpl_id, data_slice)) < 0 ) {
+        pass = false;
+        failure_mssg = "H5Dwrite() failed.\n";
+      }
+    }
+    if ( pass || (dcpl_id != -1)) {
+      if ( H5Pclose(dcpl_id) < 0 ) {
+        pass = false;
+        failure_mssg = "H5Pclose(dcpl_id) failed.\n";
+      }
+    }
+    
+    if ( pass || (dset_id_ch != -1)) {
+      if ( H5Dclose(dset_id_ch) < 0 ) {
+        pass = false;
+        failure_mssg = "H5Dclose(dset_id_ch) failed.\n";
+      }
+    }
+
     /* close file, etc. */
     if ( pass || (dset_id != -1)) {
         if ( H5Dclose(dset_id) < 0 ) {
@@ -413,7 +468,7 @@ generate_test_file( MPI_Comm comm, int mpi_rank, int group_id )
  * Function:    test_parallel_read
  *
  * Purpose:     This actually tests the superblock optimization
- *              and covers the two primary cases we're interested in.
+ *              and covers the three primary cases we're interested in.
  *              1). That HDF5 files can be opened in parallel by
  *                  the rank 0 process and that the superblock
  *                  offset is correctly broadcast to the other
@@ -423,6 +478,10 @@ generate_test_file( MPI_Comm comm, int mpi_rank, int group_id )
  *                  subgroups of MPI_COMM_WORLD and that each
  *                  subgroup operates as described in (1) to
  *                  collectively read the data.
+ *              3). Testing proc0-read-and-MPI_Bcast using
+ *                  sub-communicators, and reading into
+ *                  a memory space that is different from the
+ *                  file space, and chunked datasets.
  *
  *              The global MPI rank is used for reading and
  *              writing data for process specific data in the
@@ -444,7 +503,7 @@ generate_test_file( MPI_Comm comm, int mpi_rank, int group_id )
  *-------------------------------------------------------------------------
  */
 static int
-test_parallel_read(MPI_Comm comm, int mpi_rank, int group_id)
+test_parallel_read(MPI_Comm comm, int mpi_rank, int mpi_size, int group_id)
 {
     const char *failure_mssg;
     const char *fcn_name = "test_parallel_read()";
@@ -457,8 +516,13 @@ test_parallel_read(MPI_Comm comm, int mpi_rank, int group_id)
     hid_t fapl_id   = -1;
     hid_t file_id   = -1;
     hid_t dset_id   = -1;
+    hid_t dset_id_ch = -1;
+    hid_t dxpl_id   = H5P_DEFAULT;
     hid_t memspace  = -1;
     hid_t filespace = -1;
+    hid_t filetype  = -1;
+    size_t filetype_size;
+    hssize_t dset_size;
     hsize_t i;
     hsize_t offset;
     hsize_t count = COUNT;
@@ -552,6 +616,14 @@ test_parallel_read(MPI_Comm comm, int mpi_rank, int group_id)
         }
     }
 
+    /* open the chunked data set */
+    if ( pass ) {
+      if ( (dset_id_ch = H5Dopen2(file_id, "dataset0_chunked", H5P_DEFAULT)) < 0 ) {
+        pass = FALSE;
+        failure_mssg = "H5Dopen2() failed\n";
+      }
+    }
+
     /* setup memspace */
     if ( pass ) {
         dims[0] = count;
@@ -606,14 +678,6 @@ test_parallel_read(MPI_Comm comm, int mpi_rank, int group_id)
         }
     }
 
-    /* close file, etc. */
-    if ( pass || (dset_id != -1) ) {
-        if ( H5Dclose(dset_id) < 0 ) {
-            pass = false;
-            failure_mssg = "H5Dclose(dset_id) failed.\n";
-        }
-    }
-
     if ( pass || (memspace != -1) ) {
         if ( H5Sclose(memspace) < 0 ) {
             pass = false;
@@ -628,6 +692,330 @@ test_parallel_read(MPI_Comm comm, int mpi_rank, int group_id)
         }
     }
 
+    /* free data_slice if it has been allocated */
+    if ( data_slice != NULL ) {
+        HDfree(data_slice);
+        data_slice = NULL;
+    }
+
+    /* 
+     * Test reading proc0-read-and-bcast with sub-communicators 
+     */
+
+    /* Don't test with more than LIMIT_NPROC processes to avoid memory issues */
+
+    if( group_size <= LIMIT_NPROC ) {
+#ifdef H5_HAVE_INSTRUMENTED_LIBRARY
+      hbool_t prop_value;
+#endif /* H5_HAVE_INSTRUMENTED_LIBRARY */
+
+      if ( (filespace = H5Dget_space(dset_id )) < 0 ) {
+        pass = FALSE;
+        failure_mssg = "H5Dget_space failed.\n";
+      }
+
+      if ( (dset_size = H5Sget_simple_extent_npoints(filespace)) < 0 ) {
+        pass = FALSE;
+        failure_mssg = "H5Sget_simple_extent_npoints failed.\n";
+      }
+
+      if ( (filetype = H5Dget_type(dset_id)) < 0 ) {
+        pass = FALSE;
+        failure_mssg = "H5Dget_type failed.\n";
+      }
+
+      if ( (filetype_size = H5Tget_size(filetype)) == 0 ) {
+        pass = FALSE;
+        failure_mssg = "H5Tget_size failed.\n";
+      }
+
+      if ( H5Tclose(filetype) < 0 ) {
+        pass = FALSE;
+        failure_mssg = "H5Tclose failed.\n";
+      };
+
+      if ( (data_slice = (float *)HDmalloc((size_t)dset_size*filetype_size)) == NULL ) {
+        pass = FALSE;
+        failure_mssg = "malloc of data_slice failed.\n";
+      }
+
+      if ( pass ) {
+        if ( (dxpl_id = H5Pcreate(H5P_DATASET_XFER)) < 0 ) {
+          pass = FALSE;
+          failure_mssg = "H5Pcreate(H5P_DATASET_XFER) failed.\n";
+        }
+      }
+
+      if ( pass ) {
+        if ( (H5Pset_dxpl_mpio(dxpl_id, H5FD_MPIO_COLLECTIVE)) < 0 ) {
+          pass = FALSE;
+          failure_mssg = "H5Pset_dxpl_mpio() failed.\n";
+        }
+      }
+
+#ifdef H5_HAVE_INSTRUMENTED_LIBRARY
+      if ( pass ) {
+        prop_value = H5D_XFER_COLL_RANK0_BCAST_DEF;
+        if(H5Pinsert2(dxpl_id, H5D_XFER_COLL_RANK0_BCAST_NAME, H5D_XFER_COLL_RANK0_BCAST_SIZE, &prop_value,
+                   NULL, NULL, NULL, NULL, NULL, NULL) < 0) {
+            pass = FALSE;
+            failure_mssg = "H5Pinsert2() failed\n";
+        }
+      }
+#endif /* H5_HAVE_INSTRUMENTED_LIBRARY */
+
+      /* read H5S_ALL section */
+      if ( pass ) {
+        if ( (H5Dread(dset_id, H5T_NATIVE_FLOAT, H5S_ALL,
+                      H5S_ALL, dxpl_id, data_slice)) < 0 ) {
+          pass = FALSE;
+          failure_mssg = "H5Dread() failed\n";
+        }
+      }
+
+#ifdef H5_HAVE_INSTRUMENTED_LIBRARY
+      if ( pass ) {
+        prop_value = FALSE;
+        if(H5Pget(dxpl_id, H5D_XFER_COLL_RANK0_BCAST_NAME, &prop_value) < 0) {
+            pass = FALSE;
+            failure_mssg = "H5Pget() failed\n";
+        }
+        if (pass) {
+          if(prop_value != TRUE) {
+            pass = FALSE;
+            failure_mssg = "rank 0 Bcast optimization was mistakenly not performed\n";
+          }
+        }
+      }
+#endif /* H5_HAVE_INSTRUMENTED_LIBRARY */
+
+      /* verify the data */
+      if ( pass ) {
+
+        if ( comm == MPI_COMM_WORLD )       /* test 1 */
+          nextValue = 0;
+        else if ( group_id == 0 )           /* test 2 group 0 */
+          nextValue = 0;
+        else                                /* test 2 group 1 */
+          nextValue = (float)((hsize_t)( mpi_size / 2 )*count);
+        
+        i = 0;
+        while ( ( pass ) && ( i < (hsize_t)dset_size ) ) {
+          /* what we really want is data_slice[i] != nextValue --
+           * the following is a circumlocution to shut up the
+           * the compiler.
+           */
+          if ( ( data_slice[i] > nextValue ) ||
+               ( data_slice[i] < nextValue ) ) {
+            pass = FALSE;
+            failure_mssg = "Unexpected dset contents.\n";
+          }
+          nextValue += 1;
+          i++;
+        }
+      }
+
+      /* read H5S_ALL section for the chunked dataset */
+
+#ifdef H5_HAVE_INSTRUMENTED_LIBRARY
+      if ( pass ) {
+        prop_value = H5D_XFER_COLL_RANK0_BCAST_DEF;
+        if(H5Pset(dxpl_id, H5D_XFER_COLL_RANK0_BCAST_NAME, &prop_value) < 0) {
+            pass = FALSE;
+            failure_mssg = "H5Pset() failed\n";
+        }
+      }
+#endif /* H5_HAVE_INSTRUMENTED_LIBRARY */
+
+      for ( i = 0; i < (hsize_t)dset_size; i++) {
+        data_slice[i] = 0;
+      }
+      if ( pass ) {
+        if ( (H5Dread(dset_id_ch, H5T_NATIVE_FLOAT, H5S_ALL,
+                      H5S_ALL, dxpl_id, data_slice)) < 0 ) {
+          pass = FALSE;
+          failure_mssg = "H5Dread() failed\n";
+        }
+      }
+
+#ifdef H5_HAVE_INSTRUMENTED_LIBRARY
+      if ( pass ) {
+        prop_value = FALSE;
+        if(H5Pget(dxpl_id, H5D_XFER_COLL_RANK0_BCAST_NAME, &prop_value) < 0) {
+            pass = FALSE;
+            failure_mssg = "H5Pget() failed\n";
+        }
+        if (pass) {
+          if(prop_value == TRUE) {
+            pass = FALSE;
+            failure_mssg = "rank 0 Bcast optimization was mistakenly performed for chunked dataset\n";
+          }
+        }
+      }
+#endif /* H5_HAVE_INSTRUMENTED_LIBRARY */
+
+      /* verify the data */
+      if ( pass ) {
+
+        if ( comm == MPI_COMM_WORLD )       /* test 1 */
+          nextValue = 0;
+        else if ( group_id == 0 )           /* test 2 group 0 */
+          nextValue = 0;
+        else                                /* test 2 group 1 */
+          nextValue = (float)((hsize_t)( mpi_size / 2 )*count);
+        
+        i = 0;
+        while ( ( pass ) && ( i < (hsize_t)dset_size ) ) {
+          /* what we really want is data_slice[i] != nextValue --
+           * the following is a circumlocution to shut up the
+           * the compiler.
+           */
+          if ( ( data_slice[i] > nextValue ) ||
+               ( data_slice[i] < nextValue ) ) {
+            pass = FALSE;
+            failure_mssg = "Unexpected chunked dset contents.\n";
+          }
+          nextValue += 1;
+          i++;
+        }
+      }
+
+      if ( pass || (filespace != -1) ) {
+        if ( H5Sclose(filespace) < 0 ) {
+          pass = false;
+          failure_mssg = "H5Sclose(filespace) failed.\n";
+        }
+      }
+
+      /* free data_slice if it has been allocated */
+      if ( data_slice != NULL ) {
+        HDfree(data_slice);
+        data_slice = NULL;
+      }
+
+      /* 
+       * Read an H5S_ALL filespace into a hyperslab defined memory space 
+       */
+
+      if ( (data_slice = (float *)HDmalloc((size_t)(dset_size*2)*filetype_size)) == NULL ) {
+        pass = FALSE;
+        failure_mssg = "malloc of data_slice failed.\n";
+      }
+
+      /* setup memspace */
+      if ( pass ) {
+        dims[0] = (hsize_t)dset_size*2;
+        if ( (memspace = H5Screate_simple(1, dims, NULL)) < 0 ) {
+          pass = FALSE;
+          failure_mssg = "H5Screate_simple(1, dims, NULL) failed\n";
+        }
+      }
+      if ( pass ) {
+        offset = (hsize_t)dset_size;
+        if ( (H5Sselect_hyperslab(memspace, H5S_SELECT_SET,
+                                  &offset, NULL, &offset, NULL)) < 0 ) {
+          pass = FALSE;
+          failure_mssg = "H5Sselect_hyperslab() failed\n";
+        }
+      }
+
+#ifdef H5_HAVE_INSTRUMENTED_LIBRARY
+      if ( pass ) {
+        prop_value = H5D_XFER_COLL_RANK0_BCAST_DEF;
+        if(H5Pset(dxpl_id, H5D_XFER_COLL_RANK0_BCAST_NAME, &prop_value) < 0) {
+            pass = FALSE;
+            failure_mssg = "H5Pset() failed\n";
+        }
+      }
+#endif /* H5_HAVE_INSTRUMENTED_LIBRARY */
+
+      /* read this processes section of the data */
+      if ( pass ) {
+        if ( (H5Dread(dset_id, H5T_NATIVE_FLOAT, memspace,
+                      H5S_ALL, dxpl_id, data_slice)) < 0 ) {
+          pass = FALSE;
+          failure_mssg = "H5Dread() failed\n";
+        }
+      }
+
+#ifdef H5_HAVE_INSTRUMENTED_LIBRARY
+      if ( pass ) {
+        prop_value = FALSE;
+        if(H5Pget(dxpl_id, H5D_XFER_COLL_RANK0_BCAST_NAME, &prop_value) < 0) {
+            pass = FALSE;
+            failure_mssg = "H5Pget() failed\n";
+        }
+        if (pass) {
+          if(prop_value != TRUE) {
+            pass = FALSE;
+            failure_mssg = "rank 0 Bcast optimization was mistakenly not performed\n";
+          }
+        }
+      }
+#endif /* H5_HAVE_INSTRUMENTED_LIBRARY */
+
+      /* verify the data */
+      if ( pass ) {
+
+        if ( comm == MPI_COMM_WORLD )       /* test 1 */
+          nextValue = 0;
+        else if ( group_id == 0 )           /* test 2 group 0 */
+          nextValue = 0;
+        else                                /* test 2 group 1 */
+          nextValue = (float)((hsize_t)(mpi_size / 2)*count);
+
+        i = (hsize_t)dset_size;
+        while ( ( pass ) && ( i < (hsize_t)dset_size ) ) {
+          /* what we really want is data_slice[i] != nextValue --
+           * the following is a circumlocution to shut up the
+           * the compiler.
+           */
+          if ( ( data_slice[i] > nextValue ) ||
+               ( data_slice[i] < nextValue ) ) {
+            pass = FALSE;
+            failure_mssg = "Unexpected dset contents.\n";
+          }
+          nextValue += 1;
+          i++;
+        }
+      }
+      
+      if ( pass || (memspace != -1) ) {
+        if ( H5Sclose(memspace) < 0 ) {
+          pass = false;
+          failure_mssg = "H5Sclose(memspace) failed.\n";
+        }
+      }
+      
+      /* free data_slice if it has been allocated */
+      if ( data_slice != NULL ) {
+        HDfree(data_slice);
+        data_slice = NULL;
+      }
+
+      if ( pass || (dxpl_id != -1) ) {
+        if ( H5Pclose(dxpl_id) < 0 ) {
+          pass = false;
+          failure_mssg = "H5Pclose(dxpl_id) failed.\n";
+        }
+      }
+    }
+
+    /* close file, etc. */
+    if ( pass || (dset_id != -1) ) {
+        if ( H5Dclose(dset_id) < 0 ) {
+            pass = false;
+            failure_mssg = "H5Dclose(dset_id) failed.\n";
+        }
+    }
+
+    if ( pass || (dset_id_ch != -1) ) {
+        if ( H5Dclose(dset_id_ch) < 0 ) {
+            pass = false;
+            failure_mssg = "H5Dclose(dset_id_ch) failed.\n";
+        }
+    }
+
     if ( pass || (file_id != -1) ) {
         if ( H5Fclose(file_id) < 0 ) {
             pass = false;
@@ -668,17 +1056,9 @@ test_parallel_read(MPI_Comm comm, int mpi_rank, int group_id)
             HDfprintf(stdout, "%s: failure_mssg = \"%s\"\n",
                       fcn_name, failure_mssg);
         }
-
         HDremove(reloc_data_filename);
     }
 
-    /* free data_slice if it has been allocated */
-    if ( data_slice != NULL ) {
-        HDfree(data_slice);
-        data_slice = NULL;
-    }
-
-
     return( ! pass );
 
 } /* test_parallel_read() */
@@ -803,7 +1183,7 @@ main( int argc, char **argv)
     }
 
     /* Now read the generated test file (stil using MPI_COMM_WORLD) */
-    nerrs += test_parallel_read( MPI_COMM_WORLD, mpi_rank, which_group);
+    nerrs += test_parallel_read( MPI_COMM_WORLD, mpi_rank, mpi_size, which_group);
 
     if ( nerrs > 0 ) {
         if ( mpi_rank == 0 ) {
@@ -819,7 +1199,7 @@ main( int argc, char **argv)
     }
 
     /* run the 2nd set of tests */
-    nerrs += test_parallel_read(group_comm, mpi_rank, which_group);
+    nerrs += test_parallel_read(group_comm, mpi_rank, mpi_size, which_group);
 
     if ( nerrs > 0 ) {
         if ( mpi_rank == 0 ) {