MPIX_Allreduce_enqueue(3)

💖 MPIX_Allreduce_enqueue — the MPI manual page, bestie edition 💖

okay so real quick: this page lives in section 3 of the MPI manual, and it's all about one lil' function that queues up an allreduce onto a GPU stream that's tied to the local stream. lowkey iconic. let's get into it ✨

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🌸 NAME 🌸

MPIX_Allreduce_enqueue — enqueue an allreduce operation to a GPU stream that is associated with the local stream 🫶

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🎀 SYNOPSIS 🎀

int MPIX_Allreduce_enqueue(const void *sendbuf, void *recvbuf, int count,
MPI_Datatype datatype, MPI_Op op, MPI_Comm comm)

int MPIX_Allreduce_enqueue_c(const void *sendbuf, void *recvbuf,
MPI_Count count, MPI_Datatype datatype, MPI_Op op,
MPI_Comm comm)

notice there's a _c twin up in here — that's the one where count is an MPI_Count instead of a plain int. two versions, same energy. 👑

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✨ INPUT PARAMETERS ✨

• sendbuf — the starting address of the send buffer (it's a choice, so you pick). no cap. 📤
• count — how many elements are chillin' in the send buffer (non-negative integer, so zero and up, bestie). 🔢
• datatype — the data type of the elements in the send buffer (handle). 💅
• op — the operation (handle). this is the actual reduce-y business. 🔥
• comm — the communicator (handle). the group chat of processes, fr fr. 🫂

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💜 OUTPUT PARAMETERS 💜

• recvbuf — the starting address of the receive buffer (choice). this is where the answer lands. 📥

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🧵 THREAD AND INTERRUPT SAFETY 🧵

good news first: this routine is thread-safe. that means multiple threads can use it at the same time and you do not need to bring your own thread locks. she's self-sufficient. 💅✨

sad news second: it is not interrupt safe. typically that's because it uses memory allocation routines like malloc or other non-MPICH runtime routines, and those routines are themselves not interrupt-safe. so, like, don't interrupt. 🙈

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🌈 NOTES FOR FORTRAN 🌈

okay so Fortran besties, listen up:

• every MPI routine in Fortran (except for MPI_WTIME and MPI_WTICK) has one extra argument called ierr at the very end of the argument list. 👀
• ierr is an integer, and it means the exact same thing as the return value of the routine in C.
• in Fortran, MPI routines are subroutines, and you invoke them with the call statement. periodt.
• all MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran. that's just the rules. 🎀

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💀 ERRORS 💀

so here's the deal: all MPI routines (except MPI_Wtime and MPI_Wtick) return an error value. in C, that's the value of the function itself; in Fortran, it's the last argument. before that value comes back to you, the current MPI error handler gets called.

by default, that error handler aborts the MPI job 😭 — like, full on. dramatic. but you can change it:

• MPI_Comm_set_errhandler — for communicators
• MPI_File_set_errhandler — for files
• MPI_Win_set_errhandler — for RMA windows

the MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated (aka, please don't, she's old 👵). and the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned instead.

⚠️ important caveat, do not skip this: MPI does not guarantee that an MPI program can continue past an error. however, MPI implementations will attempt to continue whenever possible. so like... fingers crossed, but no promises. 🙏

here's every error code you might catch, and what it's giving:

• MPI_SUCCESS — no error; the MPI routine completed successfully. the good ending. ✅
• MPI_ERR_ARG — invalid argument. some argument is invalid and isn't identified by a specific error class (e.g., MPI_ERR_RANK). 🤨
• MPI_ERR_BUFFER — invalid buffer pointer. usually a null buffer where one is not valid. 🕳️
• MPI_ERR_COMM — invalid communicator. a common error is using a null communicator in a call (not even allowed in MPI_Comm_rank, okay?? 💀).
• MPI_ERR_COUNT — invalid count argument. count arguments must be non-negative; a count of zero is often valid. 🔢
• MPI_ERR_OP — invalid operation. MPI operations (objects of type MPI_Op) must either be one of the predefined operations (e.g., MPI_SUM) or created with MPI_Op_create. ⚙️
• MPI_ERR_TYPE — invalid datatype argument. additionally, this error can occur if an uncommitted MPI_Datatype (see MPI_Type_commit) is used in a communication call. 🏷️
• MPI_ERR_OTHER — other error; use MPI_Error_string to get more information about this error code. the miscellaneous queen. 👑

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and that's the whole page, bestie! this was MPIX_Allreduce_enqueue from the MPI manual, section 3, dated 4/10/2026 — so go enqueue that allreduce on your GPU stream and slay 💅✨🦋💖

you got this. periodt. 🎀