MPI_IREDUCE_SCATTER_BLOCK(3)

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💅 MPI_IREDUCE_SCATTER_BLOCK(3) 💅

okay so this is a page from Open MPI's manual, and it's literally about three bestie functions at once — MPI_Reduce_scatter_block, MPI_Ireduce_scatter_block, and MPI_Reduce_scatter_block_init. they all do the same iconic thing: combine your values and scatter the results in blocks ✨

🎀 NAME 🎀

MPI_Reduce_scatter_block, MPI_Ireduce_scatter_block, MPI_Reduce_scatter_block_init — combines values and scatters the results in blocks. periodt 💅

✨ SYNTAX ✨

🌸 C Syntax

#include <mpi.h>

int MPI_Reduce_scatter_block(const void *sendbuf, void *recvbuf, int recvcount,
     MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)

int MPI_Ireduce_scatter_block(const void *sendbuf, void *recvbuf, int recvcount,
     MPI_Datatype datatype, MPI_Op op, MPI_Comm comm, MPI_Request *request)

int MPI_Reduce_scatter_block_init(const void *sendbuf, void *recvbuf, int recvcount,
     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_BLOCK(SENDBUF, RECVBUF, RECVCOUNT, DATATYPE, OP,
             COMM, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER RECVCOUNT, DATATYPE, OP, COMM, IERROR

MPI_IREDUCE_SCATTER_BLOCK(SENDBUF, RECVBUF, RECVCOUNT, DATATYPE, OP,
             COMM, REQUEST, IERROR)
     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER RECVCOUNT, DATATYPE, OP, COMM, REQUEST, IERROR

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

🧚 Fortran 2008 Syntax

USE mpi_f08
MPI_Ireduce_scatter_block(sendbuf, recvbuf, recvcount, datatype, op, comm,
             ierror)
     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN) :: recvcount
     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_block(sendbuf, recvbuf, recvcount, datatype, op, comm,
             request, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN) :: recvcount
     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_block_init(sendbuf, recvbuf, recvcount, datatype, op, comm,
             info, request, ierror)
     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN) :: recvcount
     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

ngl those Fortran blocks are a whole mood, but we keep them exactly as they are 🫶

💖 INPUT PARAMETERS 💖

  • sendbuf — starting address of send buffer (choice). it's giving "where the data lives" ✨
  • recvcount — element count per block (non-negative integer). no negatives, bestie, i'm serious 🙈
  • datatype — datatype of elements of input buffer (handle).
  • op — operation (handle).
  • comm — communicator (handle).
  • info — info (handle, persistent only). only the persistent one cares, iykyk 💅

🌈 OUTPUT PARAMETERS 🌈

  • recvbuf — starting address of receive buffer (choice).
  • request — request (handle, non-blocking only).
  • ierror — Fortran only: error status (integer).

👑 DESCRIPTION 👑

okay so here's the tea ☕ — MPI_Reduce_scatter_block first does an element-wise reduction on a vector of count = n * recvcount elements in the send buffer defined by sendbuf, count, and datatype, using the operation op, where n is the number of processes in the group of comm.

then — and this is the fun part — the resulting vector of results gets split into n disjoint segments, where n is the number of processes in the group. each segment contains recvcount elements. the ith segment is sent to process i and stored in the receive buffer defined by recvbuf, recvcount, and datatype. slay, that's it, that's the whole vibe 🦋

🍓 USE OF IN-PLACE OPTION 🍓

when the communicator is an intracommunicator, you can do a reduce-scatter operation in-place — meaning the output buffer is used as the input buffer. iconic, lowkey efficient 💅

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. and heads up — the area occupied by the input data may be either longer or shorter than the data filled by the output data. not me warning you twice 🫶

🧚 WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR 🧚

when the communicator is an inter-communicator, the reduce-scatter operation happens in two phases, bestie:

  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. match those sums or it's not gonna eat 😭

💀 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. so 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. scream. cry. it is what it is.

the reason for this is the performance problems in ensuring that all collective routines return the same error value. fr fr, it's a performance thing.

🔥 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. like, no cap, it might just be over 💀

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_Reduce_scatter

2003-2026, The Open MPI Community

and that's the whole page, bestie 🫶 this was MPI_IREDUCE_SCATTER_BLOCK(3) from Open MPI, dated Sep 11, 2026 — go forth and reduce-scatter responsibly, mwah 💖✨🎀

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