Limiting factors of join performance on parallel processors

The effectiveness of parallel processing of relational join operations is examined. The skew in the distribution of join attribute values and the stochastic nature of the task processing times are identified as the major factors that can affect the effective utilization of parallelism. When many small processors are used in the parallel architecture, the skew can result in some processors becoming sources of bottleneck while other processors are being under utilized. Even in the absence of skew, the variations in the processing times of the parallel tasks belonging to a query can lead to high task synchronization delay and impact the maximum speedup achievable through parallel execution. Analytic expressions for join execution time are developed for different task time distributions with or without skew.