Group-caching for NoC based multicore cache coherent systems

Most CMPs use on-chip networks to connect cores and tend to integrate more simple cores on a single die. Low-radix networks, such as 2D-MESH, are widely used in tiled CMPs since they can be mapped to on-chip networks efficiently. However, low-radix networks introduce high network latency caused by long diameter. In this paper, we propose the use of group-caching design in NoC based multicore cache coherent systems. In our design, on-chip L2 banks are organized to form multiple groups. Each cache group behaves like a shared L2 cache for the cores inside cache group while the cache coherence between cache groups is maintained by coherence messages. Besides, group-caching also adopts the new cache replacement policy to improve the inefficient use of the aggregate L2 cache capacity. Compared to banked and shared L2 design, as most L2 accesses are served by local cache group, the hop count is significantly reduced. Experiment results based on full-system simulation show that for 2D-MESH, group-caching can increase the performance by 2%~8% compared to banked and shared L2 design, with network energy consumption reduced by 11%~13%. Experiment results also show that the communication overhead inside cache group plays an important role in the performance of group caching.