Advice to users.
This example shows how to write a collective communication operation
that uses caching to be more efficient after the first call.
The coding style assumes that MPI function results return only error statuses.
( End of advice to users.)
/* key for this module's stuff: */ static int gop_key = MPI_KEYVAL_INVALID; typedef struct { int ref_count; /* reference count */ /* other stuff, whatever else we want */ } gop_stuff_type; Efficient_Collective_Op (comm, ...) MPI_Comm comm; { gop_stuff_type *gop_stuff; MPI_Group group; int foundflag; MPI_Comm_group(comm, &group); if (gop_key == MPI_KEYVAL_INVALID) /* get a key on first call ever */ { if ( ! MPI_Comm_create_keyval( gop_stuff_copier, gop_stuff_destructor, &gop_key, (void *)0)); /* get the key while assigning its copy and delete callback behavior. */ MPI_Abort (comm, 99); } MPI_Comm_get_attr (comm, gop_key, &gop_stuff, &foundflag); if (foundflag) { /* This module has executed in this group before. We will use the cached information */ } else { /* This is a group that we have not yet cached anything in. We will now do so. */ /* First, allocate storage for the stuff we want, and initialize the reference count */ gop_stuff = (gop_stuff_type *) malloc (sizeof(gop_stuff_type)); if (gop_stuff == NULL) { /* abort on out-of-memory error */ } gop_stuff -> ref_count = 1; /* Second, fill in *gop_stuff with whatever we want. This part isn't shown here */ /* Third, store gop_stuff as the attribute value */ MPI_Comm_set_attr ( comm, gop_key, gop_stuff); } /* Then, in any case, use contents of *gop_stuff to do the global op ... */ } /* The following routine is called by MPI when a group is freed */ gop_stuff_destructor (comm, keyval, gop_stuff, extra) MPI_Comm comm; int keyval; gop_stuff_type *gop_stuff; void *extra; { if (keyval != gop_key) { /* abort -- programming error */ } /* The group's being freed removes one reference to gop_stuff */ gop_stuff -> ref_count -= 1; /* If no references remain, then free the storage */ if (gop_stuff -> ref_count == 0) { free((void *)gop_stuff); } } /* The following routine is called by MPI when a group is copied */ gop_stuff_copier (comm, keyval, extra, gop_stuff_in, gop_stuff_out, flag) MPI_Comm comm; int keyval; gop_stuff_type *gop_stuff_in, *gop_stuff_out; void *extra; { if (keyval != gop_key) { /* abort -- programming error */ } /* The new group adds one reference to this gop_stuff */ gop_stuff -> ref_count += 1; gop_stuff_out = gop_stuff_in; }