Improving multiprocessor performance with fine-grain coherence bypass

Abstract

Efficient and scalable cache coherence protocol is crucial to high-performance servers with shared-memory. The directory-based cache coherence protocol is more desirable than the snooping-based protocol with respect to the scalability. However, even for the former protocol, scaling to a large number of cores is also challenging due to the additional area requirements of the directories. We observed that a significant percentage of the referenced memory blocks were only accessed by a single core (even in parallel applications) which could be considered as private memory blocks. An intuitive motivation from this observation is that memory blocks accessed by a single core do not require coherence maintenance. The issue is to identify the private block and track the change of its access pattern. We propose a novel hardware approach to (1) dynamically identify the shared memory blocks at the cache block level, and (2) bypass the coherence procedure for the private memory blocks. This approach increases the effectiveness of the directory-based approach and therefore improves the system performance. Experimental results showed that, our approach can on an average (1) avoid the coherence tracking of about 54% referenced memory blocks, (2) reduce the coherence overhead by 77%, (3) avoid 8% L2 cache misses, and (4) shorten the execution time of parallel applications by 13%.

创新点

通过比较数据归属的内核ID区分共享和私有数据,并避免对私有数据进行一致性检验.可扩展的一致性协议是实现片上众核的关键因素之一.基于目录的一致性Cache受芯片面积约束,处理速度难以随核数扩展.通过实验发现,即便是并行程序,也只有少部分数据会同时被多个内核访问,而仅被单个内核访问的私有数据则不需要在目录中维护其一致性.本文通过在共享缓存中增加CoreID字段来区分共享和私有数据,并使一致性目录只维护共享数据的一致性,降低目录的存储开销和功耗等,同时提高系统性能.