MPI_ALLTOALLV_INIT(3)

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version 5.0.10 · 4 versions

💅 MPI_ALLTOALLV_INIT(3) — the Open MPI manual, bestie edition ✨

okay so. you opened the Open MPI manual, section 3, and the star of the show today is the family of all-to-all-but-everyone-gets-something-different routines:

MPI_Alltoallv, MPI_Ialltoallv, and MPI_Alltoallv_init 🎀

Everyone sends a different amount of data to, and receives a different amount of data from, everybody else. it's giving ✨no uniformity✨. it's giving ✨bespoke buffet✨.

🌸 SYNTAX 🌸

✨ C Syntax

#include <mpi.h>

int MPI_Alltoallv(const void *sendbuf, const int sendcounts[],
     const int sdispls[], MPI_Datatype sendtype,
     void *recvbuf, const int recvcounts[],
     const int rdispls[], MPI_Datatype recvtype, MPI_Comm comm)

int MPI_Ialltoallv(const void *sendbuf, const int sendcounts[],
     const int sdispls[], MPI_Datatype sendtype,
     void *recvbuf, const int recvcounts[],
     const int rdispls[], MPI_Datatype recvtype, MPI_Comm comm,
     MPI_Request *request)

int MPI_Alltoallv_init(const void *sendbuf, const int sendcounts[],
     const int sdispls[], MPI_Datatype sendtype,
     void *recvbuf, const int recvcounts[],
     const int rdispls[], MPI_Datatype recvtype, MPI_Comm comm,
     MPI_Info info, MPI_Request *request)

✨ Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_ALLTOALLV(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
     RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, IERROR)

     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPE
     INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPE
     INTEGER COMM, IERROR

MPI_IALLTOALLV(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
     RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, REQUEST, IERROR)

     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPE
     INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPE
     INTEGER COMM, REQUEST, IERROR

MPI_ALLTOALLV_INIT(SENDBUF, SENDCOUNTS, SDISPLS, SENDTYPE,
     RECVBUF, RECVCOUNTS, RDISPLS, RECVTYPE, COMM, INFO, REQUEST, IERROR)

     <type>  SENDBUF(*), RECVBUF(*)
     INTEGER SENDCOUNTS(*), SDISPLS(*), SENDTYPE
     INTEGER RECVCOUNTS(*), RDISPLS(*), RECVTYPE
     INTEGER COMM, INFO, REQUEST, IERROR

✨ Fortran 2008 Syntax

USE mpi_f08
MPI_Alltoallv(sendbuf, sendcounts, sdispls, sendtype, recvbuf, recvcounts,
             rdispls, recvtype, comm, ierror)

     TYPE(*), DIMENSION(..), INTENT(IN) :: sendbuf
     TYPE(*), DIMENSION(..) :: recvbuf
     INTEGER, INTENT(IN) :: sendcounts(*), sdispls(*), recvcounts(*),
     rdispls(*)
     TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_Ialltoallv(sendbuf, sendcounts, sdispls, sendtype, recvbuf, recvcounts,
             rdispls, recvtype, comm, request, ierror)

     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), sdispls(*),
     recvcounts(*), rdispls(*)
     TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Request), INTENT(OUT) :: request
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_Alltoallv_init(sendbuf, sendcounts, sdispls, sendtype, recvbuf, recvcounts,
                     rdispls, recvtype, comm, info, request, ierror)

     TYPE(*), DIMENSION(..), INTENT(IN), ASYNCHRONOUS :: sendbuf
     TYPE(*), DIMENSION(..), ASYNCHRONOUS :: recvbuf
     INTEGER, INTENT(IN), ASYNCHRONOUS :: sendcounts(*), sdispls(*),
     recvcounts(*), rdispls(*)
     TYPE(MPI_Datatype), INTENT(IN) :: sendtype, recvtype
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(MPI_Info), INTENT(IN) :: info
     TYPE(MPI_Request), INTENT(OUT) :: request
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

🫶 INPUT PARAMETERS 🫶

sendbuf — the starting address of your send buffer. like, where the goodies live. 💖
sendcounts — an integer array, and entry i says how many elements you're sending to rank i. yes, per rank, custom-fitted, iconic. 🌸
sdispls — an integer array, and entry i says the displacement (offset from sendbuf, measured in units of sendtype 👀) from which to send data to rank i. in units of sendtype, not bytes, don't play yourself.
sendtype — the datatype of the send buffer elements.
recvcounts — an integer array, and entry j says how many elements you're receiving from rank j.
rdispls — an integer array, and entry j says the displacement (offset from recvbuf, in units of recvtype) where data from rank j should land.
recvtype — the datatype of the receive buffer elements.
comm — the communicator over which all this data is getting swapped. ✨
info — Info (handle, persistent only). so if you're not doing the persistent thing, this lil guy isn't invited. 🙈

🎀 OUTPUT PARAMETERS 🎀

recvbuf — address of the receive buffer. your inbox, bestie. 🍓
request — Request (handle, non-blocking only).
ierror — Fortran only: error status. 💀

🌈 DESCRIPTION 🌈

MPI_Alltoallv is a generalized collective operation where every process sends data to, and receives data from, all other processes. it's the group chat. everyone talking to everyone. no cap. 💅

it takes MPI_Alltoall and adds ✨flexibility✨ by letting you specify data to send and receive vector-style — via a displacement and an element count. so you're not stuck sending the same amount to everybody like some boring routine. 🦋

here's the vibe of what it's actually doing, where each process performs 2n independent point-to-point communications (n being the number of processes in communicator comm), including communication with itself. yes, it talks to itself. relatable. 😌

MPI_Comm_size(comm, &n);
for (i = 0, i < n; i++)
    MPI_Send(sendbuf + sdispls[i] * extent(sendtype),
        sendcounts[i], sendtype, i, ..., comm);
for (i = 0, i < n; i++)
    MPI_Recv(recvbuf + rdispls[i] * extent(recvtype),
        recvcounts[i], recvtype, i, ..., comm);

so process j sends the k-th block of its local sendbuf to process k, and process k drops that data into the j-th block of its local recvbuf. ✨matching✨

when a pair of processes exchanges data, each one can pass different element count and datatype arguments — as long as the sender specifies the same amount of data to send (in bytes) as the receiver expects to receive. byte count matching, that's the whole game. 🎯

and note, okay: process i may send a different amount of data to process j than it receives from process j. also, a process may send entirely different amounts of data to different processes in the communicator. no symmetry required. main character energy for real. 👑

🌸 WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR 🌸

When the communicator is an inter-communicator, the gather operation occurs in two phases. The data is gathered from all the members of the first group and received by all the members of the second group. Then the data is gathered from all the members of the second group and received by all the members of the first. The operation exhibits a symmetric, full-duplex behavior. 🔁

The first group defines the root process. The root process uses MPI_ROOT as the value of root. All other processes in the first group use MPI_PROC_NULL as the value of root. All processes in the second group use the rank of the root process in the first group as the value of root. 🧚

When the communicator is an intra-communicator, these groups are the same, and the operation occurs in a single phase. one and done. ✨

💖 USE OF IN-PLACE OPTION 💖

When the communicator is an intracommunicator, you can perform an all-to-all operation in-place — which means the output buffer is used as the input buffer. 😭 iconic, honestly.

Use the variable MPI_IN_PLACE as the value of sendbuf. In this case, sendcounts, sdispls, and sendtype are ignored. The input data of each process is assumed to be in the area where that process would receive its own contribution to the receive buffer. so it already knows where its own stuff lives. 🫶

🎀 NOTES 🎀

The specification of counts and displacements should not cause any location to be written more than once. seriously. do not do that. 🙈

All arguments on all processes are significant. The comm argument, in particular, must describe the same communicator on all processes. no sneaky mismatched comms, bestie. that's a vibe check fail. 💀

The offsets of sdispls and rdispls are measured in units of sendtype and recvtype, respectively. Compare this to MPI_Alltoallw, where these offsets are measured in bytes. different units, different routine, don't mix them up. 👑

💅 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. 💀 all of them. gone. periodt.
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. the chill one. 🫶

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. danger danger, bestie. 😭

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.