17.2.80. MPI_Comm_split_type — Open MPI 5.0.10 documentation

# 17.2.80. MPI_Comm_split_type

MPI_Comm_split_type — Creates new communicators based on colors and keys.

## 17.2.80.1. SYNTAX

### 17.2.80.1.1. C Syntax

    #include <mpi.h>

    int MPI_Comm_split_type(MPI_Comm comm, int split_type, int key,
         MPI_Info info, MPI_Comm *newcomm)

### 17.2.80.1.2. Fortran Syntax

    USE MPI
    ! or the older form: INCLUDE 'mpif.h'
    MPI_COMM_SPLIT_TYPE(COMM, SPLIT_TYPE, KEY, INFO, NEWCOMM, IERROR)
         INTEGER COMM, SPLIT_TYPE, KEY, INFO, NEWCOMM, IERROR

### 17.2.80.1.3. Fortran 2008 Syntax

    USE mpi_f08
    MPI_Comm_split_type(comm, split_type, key, info, newcomm, ierror)
         TYPE(MPI_Comm), INTENT(IN) :: comm
         INTEGER, INTENT(IN) :: split_type, key
         TYPE(MPI_Info), INTENT(IN) :: info
         TYPE(MPI_Comm), INTENT(OUT) :: newcomm
         INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 17.2.80.2. INPUT PARAMETERS

- `comm`: Communicator (handle).

- `split_type`: Type of processes to be grouped together (integer).

- `key`: Control of rank assignment (integer).

- `info`: Info argument (handle).

## 17.2.80.3. OUTPUT PARAMETERS

- `newcomm`: New communicator (handle).

- `ierror`: Fortran only: Error status (integer).

## 17.2.80.4. DESCRIPTION

This function partitions the group associated with comm into disjoint subgroups, based on the type specified by split_type. Each subgroup contains all processes of the same type. Within each subgroup, the processes are ranked in the order defined by the value of the argument key, with ties broken according to their rank in the old group. A new communicator is created for each subgroup and returned in newcomm. This is a collective call; all processes must provide the same split_type, but each process is permitted to provide different values for key. An exception to this rule is that a process may supply the type value MPI_UNDEFINED, in which case newcomm returns MPI_COMM_NULL.

## 17.2.80.5. SPLIT TYPES

MPI_COMM_TYPE_SHARED

  This type splits the communicator into subcommunicators, each of which can create a shared memory region.

OMPI_COMM_TYPE_NODE

  Synonym for MPI_COMM_TYPE_SHARED.

OMPI_COMM_TYPE_HWTHREAD

  This type splits the communicator into subcommunicators, each of which belongs to the same hardware thread.

OMPI_COMM_TYPE_CORE

  This type splits the communicator into subcommunicators, each of which belongs to the same core/processing unit.

OMPI_COMM_TYPE_L1CACHE

  This type splits the communicator into subcommunicators, each of which belongs to the same L1 cache.

OMPI_COMM_TYPE_L2CACHE

  This type splits the communicator into subcommunicators, each of which belongs to the same L2 cache.

OMPI_COMM_TYPE_L3CACHE

  This type splits the communicator into subcommunicators, each of which belongs to the same L3 cache.

OMPI_COMM_TYPE_SOCKET

  This type splits the communicator into subcommunicators, each of which belongs to the same socket.

OMPI_COMM_TYPE_NUMA

  This type splits the communicator into subcommunicators, each of which belongs to the same NUMA-node.

OMPI_COMM_TYPE_BOARD

  This type splits the communicator into subcommunicators, each of which belongs to the same board.

OMPI_COMM_TYPE_HOST

  This type splits the communicator into subcommunicators, each of which belongs to the same host.

OMPI_COMM_TYPE_CU

  This type splits the communicator into subcommunicators, each of which belongs to the same computational unit.

OMPI_COMM_TYPE_CLUSTER

  This type splits the communicator into subcommunicators, each of which belongs to the same cluster.

## 17.2.80.6. NOTES

The communicator keys denoted with an OMPI_ prefix instead of an MPI_ prefix are specific to Open MPI, and are not part of the MPI standard. Their use should be protected by the OPEN_MPI C preprocessor macro.

## 17.2.80.7. ERRORS

Almost all MPI routines return an error value; C routines as the return result of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions.

Open MPI includes three predefined error handlers that can be used:

- `MPI_ERRORS_ARE_FATAL` Causes the program to abort all connected MPI processes.

- `MPI_ERRORS_ABORT` An error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a window or file, acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. If called on a session, aborts only the local process.

- `MPI_ERRORS_RETURN` Returns an error code to the application.

MPI applications can also implement their own error handlers by calling:

- MPI_Comm_create_errhandler then MPI_Comm_set_errhandler

- MPI_File_create_errhandler then MPI_File_set_errhandler

- MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init

- MPI_Win_create_errhandler then MPI_Win_set_errhandler

Note that MPI does not guarantee that an MPI program can continue past an error.

See the MPI man page for a full list of MPI error codes.

See the Error Handling section of the MPI-3.1 standard for more information.

See also

- MPI_Comm_create

- MPI_Intercomm_create

- MPI_Comm_dup

- MPI_Comm_free

- MPI_Comm_split