Parallel Finger Search Structures

In this paper we present two versions of a parallel finger structure FS on p processors that supports searches, insertions and deletions, and has a finger at each end. This is to our knowledge the first implementation of a parallel search structure that is work-optimal with respect to the finger bound and yet has very good parallelism (within a factor of O( (log p)^2 ) of optimal). We utilize an extended implicit batching framework that transparently facilitates the use of FS by any parallel program P that is modelled by a dynamically generated DAG D where each node is either a unit-time instruction or a call to FS. The total work done by either version of FS is bounded by the finger bound F[L] (for some linearization L of D ), i.e. each operation on an item with finger distance r takes O( log r + 1 ) amortized work; it is cheaper for items closer to a finger. Running P using the simpler version takes O( ( T[1] + F[L] ) / p + T[inf] + d * ( (log p)^2 + log n ) ) time on a greedy scheduler, where T[1],T[inf] are the size and span of D respectively, and n is the maximum number of items in FS, and d is the maximum number of calls to FS along any path in D. Using the faster version, this is reduced to O( ( T[1] + F[L] ) / p + T[inf] + d * (log p)^2 + s[L] ) time, where s[L] is the weighted span of D where each call to FS is weighted by its cost according to F[L]. We also sketch how to extend FS to support a fixed number of movable fingers. The data structures in our paper fit into the dynamic multithreading paradigm, and their performance bounds are directly composable with other data structures given in the same paradigm. Also, the results can be translated to practical implementations using work-stealing schedulers.