3.5. MPI Functionality and Features

3.5.1. MPI Standard conformance

In the Open MPI v5.0.x series, all MPI-3.|mpi_standard_subversion| functionality is supported. Some MPI-4.0 functionality is supported, but not all of it.

As such, MPI_VERSION is set to 3 and MPI_SUBVERSION is set to 1.

For historical reference:

MPI standards conformance	Introduced in Open MPI version
MPI-2.0	Open MPI v1.2
MPI-2.1	Open MPI v1.3
MPI-3.0	Open MPI v1.8
MPI-3.1	Open MPI v2.0

3.5.2. Removed MPI APIs

Note that starting with Open MPI v4.0.0, prototypes for several legacy MPI-1 symbols that were deleted in the MPI-3.0 specification are no longer available by default in mpi.h. Specifically, several MPI-1 symbols were deprecated in the 1996 publishing of the MPI-2.0 specification. These deprecated symbols were eventually removed from the MPI-3.0 specification in 2012.

The symbols that now no longer appear by default in Open MPI’s mpi.h are:

MPI_Address (replaced by MPI_Get_address)
MPI_Errhandler_create (replaced by MPI_Comm_create_errhandler)
MPI_Errhandler_get (replaced by MPI_Comm_get_errhandler)
MPI_Errhandler_set (replaced by MPI_Comm_set_errhandler)
MPI_Type_extent (replaced by MPI_Type_get_extent)
MPI_Type_hindexed (replaced by MPI_Type_create_hindexed)
MPI_Type_hvector (replaced by MPI_Type_create_hvector)
MPI_Type_lb (replaced by MPI_Type_get_extent)
MPI_Type_struct (replaced by MPI_Type_create_struct)
MPI_Type_ub (replaced by MPI_Type_get_extent)
MPI_LB (replaced by MPI_Type_create_resized)
MPI_UB (replaced by MPI_Type_create_resized)
MPI_COMBINER_HINDEXED_INTEGER
MPI_COMBINER_HVECTOR_INTEGER
MPI_COMBINER_STRUCT_INTEGER
MPI_Handler_function (replaced by MPI_Comm_errhandler_function)

Although these symbols are no longer prototyped in mpi.h, they are still present in the MPI library in Open MPI v5.0.x. This enables legacy MPI applications to link and run successfully with Open MPI v5.0.x, even though they will fail to compile.

Warning

Future releases of Open MPI beyond the v5.0.x series may remove these symbols altogether.

Warning

The Open MPI team STRONGLY encourages all MPI application developers to stop using these constructs that were first deprecated over 20 years ago, and finally removed from the MPI specification in MPI-3.0 (in 2012).

Important

The “Removed MPI constructs” section contains examples of how to update legacy MPI applications using these deleted symbols to use the “new” symbols.

All that being said, if you are unable to immediately update your application to stop using these legacy MPI-1 symbols, you can re-enable them in mpi.h by configuring Open MPI with the --enable-mpi1-compatibility flag.

3.5.3. Other MPI features

Rank reordering support is available using the TreeMatch library. It is activated for the graph and dist_graph communicator topologies.

When using MPI deprecated functions, some compilers will emit warnings. For example:

shell$ cat deprecated_example.c
#include <mpi.h>
void foo(void) {
    MPI_Datatype type;
    MPI_Type_struct(1, NULL, NULL, NULL, &type);
}
shell$ mpicc -c deprecated_example.c
deprecated_example.c: In function 'foo':
deprecated_example.c:4: warning: 'MPI_Type_struct' is deprecated (declared at /opt/openmpi/include/mpi.h:1522)
shell$

MPI_THREAD_MULTIPLE is supported with some exceptions.

The following PMLs support MPI_THREAD_MULTIPLE:
1. cm, when used with the following MTLs:
  1. ofi (Libfabric)
  2. portals4
2. ob1, when used with the following BTLs:
  1. self
  2. sm
  3. smcuda
  4. tcp
  5. ugni
  6. usnic
3. ucx
Currently, MPI File operations are not thread safe even if MPI is initialized for MPI_THREAD_MULTIPLE support.
MPI_REAL16 and MPI_COMPLEX32 are only supported on platforms where a portable C datatype can be found that matches the Fortran type REAL*16, both in size and bit representation.
The “libompitrace” library is bundled in Open MPI and is installed by default (it can be disabled via the --disable-libompitrace flag). This library provides a simplistic tracing of select MPI function calls via the MPI profiling interface. Linking it in to your application via (e.g., via -lompitrace) will automatically output to stderr when some MPI functions are invoked:
```
shell$ cd examples/
shell$ mpicc hello_c.c -o hello_c -lompitrace
shell$ mpirun -n 1 hello_c
MPI_INIT: argc 1
Hello, world, I am 0 of 1
MPI_BARRIER[0]: comm MPI_COMM_WORLD
MPI_FINALIZE[0]
shell$
```
Keep in mind that the output from the trace library is going to stderr, so it may output in a slightly different order than the stdout from your application.

This library is being offered as a “proof of concept” / convenience from Open MPI. If there is interest, it is trivially easy to extend it to printf for other MPI functions. Pull requests on github.com would be greatly appreciated.