MPI_Op_commute(3)

💅 MPI_Op_commute(3) — the MPI manual, bestie edition

okay so this page lives in the MPI manual, and the filename is MPI_Op_commute(3) but the actual function you call is MPI_Op_commutative — yes, icon behavior, the title and the function don't match, we love that for us 🎀. Anyway. This one's short and sweet: it asks an MPI reduction operation "hey bestie, are you commutative?" and hands you back a yes/no. That's it, that's the whole vibe.

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✨ NAME ✨

MPI_Op_commute — queries an MPI reduction operation for its commutativity. 🔥

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🦋 SYNOPSIS 🦋

int MPI_Op_commutative(MPI_Op op, int *commute)

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🌸 INPUT PARAMETERS 🌸

• op — the operation, as a handle. this is the little guy you're interrogating 👑

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⭐ OUTPUT PARAMETERS ⭐

• commute — a logical flag that comes back true if op is a commutative operation. like, the answer to the question 💖

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💀 NULL HANDLES 💀

okay so this is the part where the standard gets very serious and we must too, no cap. the MPI 1.1 spec, in the section on opaque objects, explicitly disallows freeing a null communicator. here's the text straight from the standard, and yes it's giving law school:

A null handle argument is an erroneous IN argument in MPI calls, unless an exception is explicitly stated in the text that defines the function. Such exception is allowed for handles to request objects in Wait and Test calls (sections Communication Completion and Multiple Completions ). Otherwise, a null handle can only be passed to a function that allocates a new object and returns a reference to it in the handle.

so like. read it and weep 🫶 that's the rule.

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🧚 THREAD AND INTERRUPT SAFETY 🧚

this routine is thread-safe ✨ which means multiple threads can use it safely and you do not need to bring your own user-provided thread locks. self-sufficient queen behavior.

but — and this is the important part — the routine is not interrupt safe. typically that's because of memory allocation routines like malloc or other non-Intel(R) MPI Library runtime routines that are themselves not interrupt-safe. so like, interrupt it at your own risk 🙈

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🌈 NOTES FOR FORTRAN 🌈

• all MPI routines in Fortran (except MPI_WTIME and MPI_WTICK) have an extra argument ierr tacked on at the 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. slay, consistent 💅
• in Fortran, MPI routines are subroutines and you invoke them with the call statement.
• all MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran. that's just how it is 🎀

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🍓 ERRORS 🍓

all MPI routines (except MPI_Wtime and MPI_Wtick) return an error value — C routines as the value of the function, and Fortran routines in the last argument. before that value comes back, the current MPI error handler gets called. by default this error handler aborts the MPI job 😭 so like, heads up.

the error handler can be changed with:

• 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 also be used, but its use is deprecated (do not come for me, that's what the page says). the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned instead.

note that MPI does not guarantee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible. so like… best effort, no promises 💀

the specific ones for this routine:

• MPI_SUCCESS — no error; the MPI routine completed successfully. 🎉 we ate.
• MPI_ERR_ARG — invalid argument. some argument is invalid and isn't identified by a specific error class (e.g., MPI_ERR_RANK). something's wrong but the error is being mysterious about it 🙈

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🫶 SEE ALSO 🫶

• MPI_Op_create — go make an op first, then come back and vibe check it ✨

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👑 COPYRIGHT INFORMATION 👑

okay bestie, this is the long one, and every single word of it stays, so buckle in. 💜

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Intel may make changes to specifications and product descriptions at any time, without notice. designers must not rely on the absence or characteristics of any features or instructions marked "reserved" or "undefined." Intel reserves these for future definition and shall have no responsibility whatsoever for conflicts or incompatibilities arising from future changes to them. the information here is subject to change without notice. do not finalize a design with this information.

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okay that's the whole page, you made it, I'm so proud of you 🥹✨ this lil drop is from the MPI manual, page MPI_Op_commute(3), dated 11/9/2015 — go forth and ask your ops if they commute, bestie. mwah 💋🎀