BFDir: A Space-Efficient Coherence Directory Based on Bloom Filter

Directory-based coherence is widely used in modern CMP systems. As the number of cores increases, it is increasingly deemed as the only candidate for on-chip cache coherence maintaining. However, limitations of traditional coherence directory pose serious challenges to deal with the ever-increasing size of the system. The hardware overhead and redundant message broadcasting problems dramatically degrade the scalability and performance of the system. In this paper, a space-efficient coherence directory BFDir is proposed. The directory dramatically reduces the directory size as the share list is shortened by Bloom filter. Also, it does not incur message broadcasting as that in limited directories. The evaluation results show, for 32-core CMP systems, compared to full-map directory, 59% overhead of share list can be avoided at the expense of 2.77% performance loss on average; compared to 16-bit coarse directory, 22% overhead of share list can be avoided at the expense of 0.16% average performance loss on average; compared to 8-bit coarse directory, 48% invalid messages are saved and the performance is improved by 2.31%.