MPI_IREDUCE_SCATTER(3)

💅 MPI_IREDUCE_SCATTER — Open MPI manual page 💅

okay bestie, welcome to the Open MPI page for MPI_IREDUCE_SCATTER(3) (and its two iconic siblings, no cap). This is the one where everybody's numbers get smushed together and then handed back out like friendship bracelets. Let's get into it 🎀

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✨ NAME ✨

MPI_Reduce_scatter, MPI_Ireduce_scatter, MPI_Reduce_scatter_init — Combines values and scatters the results.

like… it reduces, and then it scatters. it's giving exactly what it says on the tin 💖

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✨ SYNTAX ✨

🌸 C Syntax

#include <mpi.h>

int MPI_Reduce_scatter(const void *sendbuf, void *recvbuf, const int recvcounts[],
     MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)

int MPI_Ireduce_scatter(const void *sendbuf, void *recvbuf, const int recvcounts[],
     MPI_Datatype datatype, MPI_Op op, MPI_Comm comm, MPI_Request *request)

int MPI_Reduce_scatter_init(const void *sendbuf, void *recvbuf,
     const int recvcounts[], MPI_Datatype datatype, MPI_Op op,
     MPI_Comm comm, MPI_Info info, MPI_Request *request)

🌸 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_REDUCE_SCATTER(SENDBUF, RECVBUF, RECVCOUNTS, DATATYPE, OP,
             COMM, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER RECVCOUNTS(*), DATATYPE, OP, COMM, IERROR

MPI_IREDUCE_SCATTER(SENDBUF, RECVBUF, RECVCOUNTS, DATATYPE, OP,
             COMM, REQUEST, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER RECVCOUNTS(*), DATATYPE, OP, COMM, REQUEST, IERROR

MPI_REDUCE_SCATTER_INIT(SENDBUF, RECVBUF, RECVCOUNTS, DATATYPE, OP,
             COMM, INFO, REQUEST, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER RECVCOUNTS(*), DATATYPE, OP, COMM, INFO, REQUEST, IERROR

🌸 Fortran 2008 Syntax

USE mpi_f08
MPI_Reduce_scatter(sendbuf, recvbuf, recvcounts, datatype, op, comm,
             ierror)
     TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
     TYPE(*), DIMENSION(..) :: recvbuf
     INTEGER, INTENT(IN) :: recvcounts(*)
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     TYPE(MPI_Op), INTENT(IN) :: op
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_Ireduce_scatter(sendbuf, recvbuf, recvcounts, datatype, op, comm,
             request, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN), ASYNCHRONOUS :: recvcounts(*)
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     TYPE(MPI_Op), INTENT(IN) :: op
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Request), INTENT(OUT) :: request
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_Reduce_scatter_init(sendbuf, recvbuf, recvcounts, datatype, op, comm,
             info, request, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN), ASYNCHRONOUS :: recvcounts(*)
     TYPE(MPI_Datatype), INTENT(IN) :: datatype
     TYPE(MPI_Op), INTENT(IN) :: op
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Info), INTENT(IN) :: info
     TYPE(MPI_Request), INTENT(OUT) :: request
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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✨ INPUT PARAMETERS ✨

• sendbuf: Starting address of send buffer (choice).
• recvcounts: Integer array specifying the number of elements in result distributed to each process. Array must be identical on all calling processes. IDENTICAL. ON. ALL. CALLING. PROCESSES. fr fr, everyone has to agree or it's chaos 😭
• datatype: Datatype of elements of input buffer (handle).
• op: Operation (handle).
• comm: Communicator (handle).
• info: Info (handle, persistent).

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✨ OUTPUT PARAMETERS ✨

• recvbuf: Starting address of receive buffer (choice).
• request: Request (handle, non-blocking only).
• ierror: Fortran only: Error status (integer).

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✨ DESCRIPTION ✨

So MPI_Reduce_scatter first does an element-wise reduction on vector of count = S(i)recvcounts[i] elements in the send buffer defined by sendbuf, count, and datatype. Iconic.

Then — and this is the part that eats — the resulting vector of results is split into n disjoint segments, where n is the number of processes in the group. Segment i contains recvcounts[i] elements. The ith segment is sent to process i and stored in the receive buffer defined by recvbuf, recvcounts[i], and datatype.

so basically: everybody dumps their stuff in, it all gets combined, and then each process walks away with its own lil slice. main character energy for every single rank 💅✨

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✨ USE OF IN-PLACE OPTION ✨

When the communicator is an intracommunicator, you can perform a reduce-scatter operation in-place (the output buffer is used as the input buffer). Use the variable MPI_IN_PLACE as the value of the sendbuf. In this case, the input data is taken from the top of the receive buffer. The area occupied by the input data may be either longer or shorter than the data filled by the output data.

ngl in-place is the thrifty queen move 🫶 no extra buffer, we reuse what we got.

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✨ WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR ✨

When the communicator is an inter-communicator, the reduce-scatter operation occurs in two phases.

1. First, the result of the reduction performed on the data provided by the processes in the first group is scattered among the processes in the second group.
2. Then the reverse occurs: the reduction performed on the data provided by the processes in the second group is scattered among the processes in the first group.

For each group, all processes provide the same recvcounts argument, and the sum of the recvcounts values should be the same for both groups.

two groups, two phases, one very specific set of rules. everybody plays by the same recvcounts or nobody plays at all, bestie 😤

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✨ NOTES ON COLLECTIVE OPERATIONS ✨

The reduction functions ( MPI_Op ) do not return an error value. As a result, if the functions detect an error, all they can do is either call MPI_Abort or silently skip the problem. Thus, if you change the error handler from MPI_ERRORS_ARE_FATAL to something else, for example, MPI_ERRORS_RETURN , then no error may be indicated.

The reason for this is the performance problems in ensuring that all collective routines return the same error value.

💀 so like… errors might just. not show up. that's not a bug, that's a performance tradeoff. screaming, crying, throwing up, but make it collective.

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✨ 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.

⚠️ real talk: MPI does NOT promise your program survives an error. build your own handler if you want a say in it, queen 👑

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✨ Copyright ✨

2003-2026, The Open MPI Community

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and that's the whole page, bestie 🦋 go forth and scatter responsibly. this cutie was brought to you by the Open MPI squad, dated Sep 10, 2026, filed under MPI_IREDUCE_SCATTER(3) — iykyk 💖✨🎀