Parallelising a large functional program or: Keeping LOLITA busy

Abstract

In this paper we report on the ongoing parallelisation of LOLITA, a natural language engineering system. Although LOLITA currently exhibits only modest parallelism, we believe that it is the largest parallel functional program ever, comprising more than 47,000 lines of Haskell. LOLITA has the following interesting features common to real world applications of lazy languages:

• the code was not specifically designed for parallelism;

• laziness is essential for efficiency in LOLITA;

• LOLITA interfaces to data structures outside the Haskell heap, using a foreign language interface;

• LOLITA was not written by those most closely involved in the parallelisation.

Our expectations in parallelising the program were to achieve moderate speedups with small changes in the code. To date speedups of up to 2.4 have been achieved for LOLITA running under a realistic simulation of our 4 processor shared-memory target machine. Most notably, the parallelism is achieved with a very small number of changes to, and without requiring an understanding of most of the application. On the Sun SPARCserver target machine wall-clock speedup is currently limited by physical memory availability.