Coalition Threading: Combining traditional and non-traditional parallelism to maximize scalability

Non-traditional parallelism provides parallel speedup for a single thread without the need to manually divide and coordinate computation. This paper describes coalition threading, a technique that seeks the ideal combination of traditional and non-traditional threading to make the best use of available hardware parallelism. Coalition threading provides up to 2× gains over traditional parallel techniques on individual loops. However, deciding when and to what degree to apply either traditional or non-traditional threading is a difficult decision. This paper provides heuristics for identifying loops that benefit from a combination of traditional and nontraditional parallelism and those that will perform best with a single technique. Using this heuristic, coalition threading provides an average gain of 17% across all the loops and an average speedup of 16.7% for the full applications over traditional parallelism. This performance is within 0.7% of the speedup that an oracle heuristic could attain.