So you want add your own collective.
Here's how you do it.
Step 1. Look at the collectives in the current ulm distribution. These should be in $ULMROOT/collective. Pay particular attention to the function prototypes. Your collective function must use exactly the same arguments.
Step 2. Write the new collective and place it in $ULMROOT/collective directory. We prefer that the file name and the function name be the same.
Step 3. Add the collective file name to libmpi.mk.
Step 4. Compile the library
Step 5. To use the new collective in an mpi/ulm programs. Declare the new collective as extern in the mpi program eg. extern int newtoall();
Step 6. Set the new collective function as the primary function for that type of collective operation. That is, suppose I have a new alltoall function called newalltoall. To use this function i call
ulm_set_coll_op("alltoall",newalltoall,0); This sets the newalltoall function as the primary alltoall collective function. So now whenever you use mpi_alltoall --the newalltoall func is used.