LA-LLC: Inter-Core Locality-Aware Last-Level Cache to Exploit Many-to-Many Traffic in GPGPUs

The reply network is a severe performance bottleneck in General Purpose Graphic Processing Units (GPGPUs), as the communication path from memory controllers (MC) to cores is often congested. In this paper, we find that instead of relying on the congested communication path between MCs and cores, the unused core-to-core communication path can be leveraged to transfer data blocks between cores. We propose the inter-core Locality-Aware Last-Level Cache (LA-LLC), which requires only few bits per cache block and enables a core to fetch shared data from another core's private cache instead of the LLC. Leveraging inter-core communication, LA-LLC transforms few-to-many traffic to many-to-many traffic, thereby mitigating the reply network bottleneck. For a set of applications exhibiting varying degrees of inter-core locality, LA-LLC reduces memory access latency and increases performance by 21.1 percent on average and up to 68 percent, with negligible hardware cost.