Accidents always Come in Threes : A Case Study of Data-intensive Programs in Parallel Haskell

Accidents happen: “An invisible car came out of nowhere, struck my vehicle and vanished.” “I pulled away from the side of the road, glanced at my mother-in-law, and headed for the embankment.” “As I approached the intersection a sign suddenly appeared in a place where no stop sign had ever appeared before.” Luckily, we don’t normally have to deal with problems as bizarre as these. One interesting application that does arise at the Centre for Transport Studies consists of matching police reports of several accidents so as to locate accident blackspots. The application provides an interesting, data-intensive, test-bed for the persistent functional language PFL. We report here on an approach aimed at improving the performance of this application using Glasgow Parallel Haskell. The accident application is one of several large parallel Haskell programs under development at Glasgow. Our objective is to achieve wall-clock speedups over the best sequential implementations, and we report modest wallclock speedups for a demonstration program. From experience with these and other programs the group is developing a methodology for parallelising large functional programs. We have also developed strategies, a mechanism to separately specify a function’s algorithm and its dynamic behaviour.