MPIX_COMM_ACK_FAILED(3)

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💅 MPIX_COMM_ACK_FAILED(3) — Open MPI, but make it make sense

okay so bestie, this is the page for MPIX_COMM_ACK_FAILED(3) from Open MPI, and it's lowkey one of the most ULFM-coded pages in the whole set. Buckle up 🎀

🌸 NAME

MPIX_Comm_get_failed — acknowledge failed processes in a communicator.

(yes, that's the exact line the page gives, bestie — I'm just the messenger 🙈)

This is part of the User Level Fault Mitigation ULFM extension. iconic behavior, honestly.

✨ SYNTAX

C Syntax

#include <mpi.h>
#include <mpi-ext.h>

int MPIX_Comm_ack_failed(MPI_Comm comm, int num_to_ack, int *num_acked)

Fortran Syntax

USE MPI
USE MPI_EXT
! or the older form: INCLUDE 'mpif.h'

MPIX_COMM_ACK_FAILED(COMM, NUM_TO_ACK, NUM_ACKED, IERROR)
     INTEGER COMM, NUM_TO_ACK, NUM_ACKED, IERROR

Fortran 2008 Syntax

USE mpi_f08
USE mpi_ext_f08

MPIX_Comm_ack_failed(comm, num_to_ack, num_acked, ierror)
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, INTENT(IN) :: num_to_ack
     INTEGER, INTENT(OUT) :: num_acked
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

🫶 INPUT PARAMETERS

  • comm — the communicator handle. the whole vibe lives here ✨
  • num_to_ack — maximum number of process failures to acknowledge in comm, as an integer. think of it as your "how many ghosts am I willing to accept" budget 👻

💖 OUTPUT PARAMETERS

  • num_acked — number of acknowledged failures in comm, integer.
  • ierror — Fortran only: error status, integer.

🦋 DESCRIPTION

so this is a local operation, no cap — it gives users a way to acknowledge locally notified failures on comm.

The operation acknowledges the first num_to_ack process failures on comm — meaning it acknowledges the failure of members with a rank lower than num_to_ack in the group that would be produced by a concurrent call to MPIX_Comm_get_failed on the same comm. the way that's a whole mouthful 😭

It also sets the value of num_acked to the current number of acknowledged process failures in comm. and like, a process failure has been acknowledged on comm if and only if the rank of the process is lower than num_acked in the group that would be produced by a subsequent call to MPIX_Comm_get_failed on the same comm. iykyk.

ngl, num_acked can be larger than num_to_ack when process failures have already been acknowledged in a prior call to MPIX_Comm_ack_failed. it's giving "you had it before I even asked" 💅

🔥 EFFECT OF ACKNOWLEDGING FAILURES

After an MPI process failure is acknowledged on comm, unmatched MPI_ANY_SOURCE receive operations on the same comm that would have raised an error of class MPIX_ERR_PROC_FAILED_PENDING proceed without further raising errors due to this acknowledged failure. slay, no more drama 🎀

Also, MPIX_Comm_agree on the same comm will not raise an error of class MPI_ERR_PROC_FAILED due to this acknowledged failure. bestie behavior fr fr.

💜 USAGE PATTERNS

  • You can query, without side effect, for the number of currently acknowledged process failures in comm by supplying 0 in num_to_ack. free info, no strings attached ✨
  • Conversely, you may unconditionally acknowledge all currently known process failures in comm by supplying the size of the group of comm in num_to_ack. go big or go home 👑

note that the number of acknowledged processes, as returned in num_acked, can be smaller or larger than the value supplied in num_to_ack — but it is however never larger than the size of the group returned by a subsequent call to MPIX_Comm_get_failed. so like, chaotic but not that chaotic 💀

🌈 EFFECT ON COLLECTIVE OPERATIONS

Calling MPIX_Comm_ack_failed on a communicator with failed MPI processes has no effect on collective operations — except for MPIX_Comm_agree, which is the exception, as always 🙈

If a collective operation would raise an error due to the communicator containing a failed process, it will continue to raise an error even after the failure has been acknowledged. so don't come for collectives like that's gonna work.

In order to use collective operations between MPI processes of a communicator that contains failed MPI processes, users should create a new communicator — e.g., by calling MPIX_Comm_shrink. fresh comm, fresh me, periodt 🌸

🌟 WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR

When the communicator is an inter-communicator, the failures of members in both the local and the remote groups of comm are acknowledged. both sides, no favorites 🫶

😭 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. That 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
  • the 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. rude but okay 😤

Open MPI includes three predefined error handlers that can be used:

  • MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. the nuclear option 💀
  • 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, it 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, it aborts only the local process.
  • MPI_ERRORS_RETURN — returns an error code to the application. the "just tell me what happened" icon 👑

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, it might just be over 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.

👀 SEE ALSO

  • MPIX_Comm_get_failed
  • MPIX_Comm_agree

2003-2026, The Open MPI Community

and that's the whole page, bestie 🦋 this came to you straight from the Open MPI corner of the universe, dated May 14, 2026 — MPIX_COMM_ACK_FAILED(3), signed, sealed, and fully slayed 💅✨💖

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