Pipelining with Futures

Abstract.

Pipelining has been used in the design of many PRAM algorithms to reduce their asymptotic running time. Paul, Vishkin, and Wagener (PVW) used the approach in a parallel implementation of 2-3 trees. The approach was later used by Cole in the first O( lg n) time sorting algorithm on the PRAM not based on the AKS sorting network, and has since been used to improve the time of several other algorithms. Although the approach has improved the asymptotic time of many algorithms, there are two practical problems: maintaining the pipeline is quite complicated for the programmer, and the pipelining forces highly synchronous code execution. Synchronous execution is less practical on asynchronous machines and makes it difficult to modify a schedule to use less memory or to take better advantage of locality.

In this paper we show how futures (a parallel language construct) can be used to implement pipelining without requiring the user to code it explicitly, allowing for much simpler code and more asynchronous execution. A runtime system manages the pipelining implicitly. As with user-managed pipelining, we show how the technique reduces the depth of many algorithms by a logarithmic factor over the nonpipelined version. We describe and analyze four algorithms for which this is the case: a parallel merging algorithm on trees, parallel algorithms for finding the union and difference of two randomized balanced trees (treaps), and insertion into a variant of the PVW 2-3 trees. For three of these, the pipeline delays are data dependent making them particularly difficult to pipeline by hand. To determine the runtime of algorithms we first analyze the algorithms in a language-based cost model in terms of the work w and depth d of the computations, and then show universal bounds for implementing the language on various machine models.