Filaments: Efficient Support for Fine-Grain Parallelism

It has long been thought that coarse-grain parallelism is much more efficient than fine-grain parallelism due to the overhead of process (thread) creation, context switching, and synchronization. On the other hand, there are several advantages to fine-grain parallelism: architecture independence, ease of programming, ease of use as a target for code generation, and load-balancing potential. This paper describes a portable threads package, Filaments, that supports efficient execution of fine-grain parallel programs on shared-memory multiprocessors. Filaments supports three kinds of threads—run-to-completion, barrier (iterative), and fork/join— which appear to be sufficient for scientific computations. Filaments employs a unique combination of techniques to achieve efficiency: stateless threads, very small thread descriptors, optimized barrier synchronization, scheduling that enhances data locality, and automatic pruning of fork/join threads. The gains in performance are such that on an application such as Jacobi iteration, the execution time for a fine-grain program with a worst-case granularity of a thread per point can be within 10% of that for a coarse-grain program with only one task per processor. Execution times for problems with more work per thread are usually indistinguishable from coarse-grain programs, and they can be faster when the amount of work per thread varies.