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Comparing the predictive models, the last successor and PCM models saw reasonable improvements, but the increased lead time of EPCM's prefetching enables significantly greater improvements in read latencies and in total elapsed times. Although last successor based prefetching can be effective in reducing I/O latencies, its limitations are that it cannot predict more than the next event and it provides no confidence estimates for the predictions. While PCM based prefetching provides a measure of likelihood with each prediction, this method cannot predict more than the next event, limiting its ability to reduce I/O latencies. We have seen that EPCM based prefetching can greatly reduce I/O latencies by predicting further ahead than PCM based prefetching.

While these tests have clearly shown that predictive prefetching can greatly reduce I/O latencies, we note that these tests are limited representations of any common computer workload, and that several key issues still need to be addressed to implement a system that could be widely used. These tests run the same benchmark repeating the same patterns. While our simulations with file system traces clearly show a strong degree of correlation between file access events, our repetition of the same benchmark numerous times has artificially increased this correlation. Additionally this experimental implementation does not store any of the predictive data to disk. We envision that this would affect our system by slightly increasing the I/O activity and significantly decreasing memory overhead. How this would affect performance is unclear; we have not studied the effects such changes would have on performance, and can only speculate based on experiences with this implementation. Any practical implementation of predictive prefetching would need to handle these issues.

These results show that predictive prefetching can significantly reduce I/O latencies and shows useful reductions in total runtime of our benchmarks. Our prototype focused on using predictive prefetching to reduce the latencies of read system calls, and this is exactly what we have seen. From the reductions in elapsed time, we have shown that a predictive prefetching system as a whole offers potential for valuable performance improvements. In the best case, such a system performs almost as well as when all of the data is already available in RAM. However, care must be taken in the design of a predictive prefetching system to ensure that the prefetching uses resources wisely and does not hinder demand driven requests. Nevertheless these test have shown that, for the workloads studied, predictive prefetching has the potential to remove significant portions of I/O latencies.

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Next: Acknowledgments Up: Design and Implementation of Previous: Future Work
Tom M. Kroeger