The problem was a lack of memory to execute the join function of bx.intervals.operations.join.
I tested the joining dataset in several machines with different memory sizes. It was only succeeded on the 16gb memory system.
Also I observed that the joining two datasets, galaxy-dist/tools/new_operations/gops_join.py, used around 10gb memory for the computation.
I used 37 mega bytes exons and 185 mega bytes repeats.
Other than 16gb memory system, it was terminated like 'Killed' when it was running of memory. I suspect the join function might have a memory 'leak'.
I just wanted to update what I found out.
Thanks,
Hyungro