SHMEM_WAIT(3)

💖 SHMEM_WAIT(3) — Open MPI 💖

okay bestie, welcome to the Open MPI manual page for SHMEM_WAIT(3) — the whole wait-until-your-PE-friend-changes-something situation. It’s giving group-chat roll call for the wait family, and every single one is invited. 💅

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✨ NAME ✨

shmem_int_wait(3), shmem_int_wait(3)_until, shmem_int4_wait(3), shmem_int4_wait(3)_until, shmem_int8_wait(3), shmem_int8_wait(3)_until, shmem_long_wait(3), shmem_long_wait(3)_until, shmem_longlong_wait(3), shmem_longlong_wait(3)_until, shmem_short_wait(3), shmem_short_wait(3)_until, shmem_wait(3), shmem_wait(3)_until — Waits for a variable on the local processing element (PE) to change. periodt. 🫶

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🌸 SYNOPSIS 🌸

C or C++:

C or C++:
#include <mpp/shmem.h>

void shmem_int_wait(volatile int *var, int value);

void shmem_int_wait_until(volatile int *var, int cond, int value);

void shmem_long_wait(volatile long *var, long value);

void shmem_long_wait_until(volatile long *var, int cond, long value);

void shmem_longlong_wait(volatile long long *var, long long value);

void shmem_longlong_wait_until(volatile long long *var, int cond,
  long long value);

void shmem_short_wait(volatile short *var, short value);

void shmem_short_wait_until(volatile short *var, int cond,
  short value);

void shmem_wait(volatile long *ivar, long cmp_value);

void shmem_wait_until(volatile long *ivar, int cmp, long value);

Fortran:

Fortran:
INCLUDE "mpp/shmem.fh"

CALL SHMEM_INT4_WAIT(ivar, cmp_value)

CALL SHMEM_INT4_WAIT_UNTIL(ivar, cmp, cmp_value)

CALL SHMEM_INT8_WAIT(ivar, cmp_value)

CALL SHMEM_INT8_WAIT_UNTIL(ivar, cmp, cmp_value)

CALL SHMEM_WAIT(ivar, cmp_value)

CALL SHMEM_WAIT_UNTIL(ivar, cmp, cmp_value)

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🦋 DESCRIPTION 🦋

so shmem_wait and shmem_wait_until are literally waiting for ivar to be changed by a remote write or atomic swap issued by a different processor. They’re for point-to-point directed synchronization, iykyk. 💫

A call to shmem_wait does not return until some other processor writes a value, not equal to cmp_value, into ivar on the waiting processor. A call to shmem_wait_until does not return until some other processor changes ivar to satisfy the condition implied by cmp and cmp_value. This mechanism is useful when a processor needs to tell another processor that it has completed some action. slay. 👑

🧸 The arguments, bestie 🧸

The arguments are as follows:

• target — the remotely accessible integer data object to be updated on the remote PE. If you’re using C/C++, the type of target should match what’s implied in the SYNOPSIS section. If you’re using the Fortran compiler, it must be of type integer with an element size of 4 bytes for SHMEM_INT4_ADD and 8 bytes for SHMEM_INT8_ADD.
• value — the value to be atomically added to target. If you’re using C/C++, the type of value should match what’s implied in the SYNOPSIS section. If you’re using Fortran, it must be of type integer with an element size of target.
• pe — an integer that indicates the PE number upon which target is to be updated. If you’re using Fortran, it must be a default integer value.
• ivar — a remotely accessible integer variable that is being updated by another PE. If you’re using C/C++, the type of ivar should match what’s implied in the SYNOPSIS section. If you’re using Fortran, ivar must be a specific sized integer type according to the function being called, as follows:
  • shmem_wait, shmem_wait_until: default INTEGER
  • shmem_int4_wait, shmem_int4_wait_until: INTEGER*4
  • shmem_int8_wait, shmem_int8_wait_until: INTEGER*8
• cmp — the compare operator that compares ivar with cmp_value. cmp must be of type integer. If you’re using Fortran, it must be of default kind. If you’re using C/C++, the type of cmp should match what’s implied in the SYNOPSIS section. The following cmp values are supported:
  • SHMEM_CMP_EQ — Equal
  • SHMEM_CMP_NE — Not equal
  • SHMEM_CMP_GT — Greater than
  • SHMEM_CMP_LE — Less than or equal to
  • SHMEM_CMP_LT — Less than
  • SHMEM_CMP_GE — Greater than or equal to
• cmp_value — must be of type integer. If you’re using C/C++, the type of cmp_value should match what’s implied in the SYNOPSIS section. If you’re using Fortran, cmp_value must be an integer of the same size and kind as ivar. The shmem_wait routines return when ivar is no longer equal to cmp_value. The shmem_wait_until routines return when the compare condition is true. The compare condition is defined by the ivar argument compared with the cmp_value using the comparison operator, cmp. 💖

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🌈 EXAMPLES 🌈

Example 1: the following call returns when variable ivar is not equal to 100:

INTEGER*8 IVAR

CALL SHMEM_INT8_WAIT(IVAR, INT8(100))

Example 2: the following call to SHMEM_INT8_WAIT_UNTIL is equivalent to the call to SHMEM_INT8_WAIT in example 1:

INTEGER*8 IVAR

CALL SHMEM_INT8_WAIT_UNTIL(IVAR, SHMEM_CMP_NE, INT8(100))

Example 3: the following C/C++ call waits until the sign bit in ivar is set by a transfer from a remote PE:

int ivar;

shmem_int_wait_until(&ivar, SHMEM_CMP_LT, 0);

Example 4: the following Fortran example is in the context of a subroutine:

SUBROUTINE EXAMPLE()
  INTEGER FLAG_VAR
  COMMON/FLAG/FLAG_VAR
  . . .
  FLAG_VAR = FLAG_VALUE ! initialize the event variable
  . . .
  IF (FLAG_VAR .EQ. FLAG_VALUE) THEN
    CALL SHMEM_WAIT(FLAG_VAR, FLAG_VALUE)
  ENDIF
  FLAG_VAR = FLAG_VALUE ! reset the event variable for next time
  . . .
END

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👑 SEE ALSO 👑

intro_shmem(3), shmem_put(3)

go read those next, bestie. no cap. 🦋

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