Effective fine-grain synchronization for automatically parallelized programs using optimistic synchronization primitives

Abstract

As shared-memory multiprocessors become the dominant commodity source of computation, parallelizing compilers must support mainstream computations that manipulate irregular, pointer-based data structures such as lists, trees and graphs, Our experience with a parallelizing compiler for this class of applications shows that their synchronization requirements differ significantly from those of traditional parallel computations. Instead of coarse-grain barrier synchronization, irregular computations require synchronization primitives that support efficient fine-grain atomic operations.The standard implementation mechanism for atomic operations uses mutual exclusion locks. But the overhead of acquiring and releasing locks can reduce the performance. Locks can also consume significant amourtts of memory. Optimistic synchronization primitives such as load linked/stor conditional are an attractive alternative. They require no additional memory and eliminate the use of heavyweight blocking synchronization constructs.This paper presents our experience using optimistic synchronization to implement fine-grain atomic operations in the context of a parallelizing compiler for irregular object-based programs. We have implemented two versions of the compiler. One version generates code that uses mutual exclusion locks to make operations execute atomically. The other version uses optimistic synchronization. This paper presents the first published algorithm that enables compilers to automatically generate optimistically synchronized parallel code. The presented experimental results indicate that optimistic synchronization is clearly the superior choice for our set of applications. Our results show that it can significantly reduce the memory consumption and improve the overall performance.