Abstract: Hybrid main memories are composed of DRAM which can provide much larger storage capacity than traditional main memories used as a cache to scalable non-volatile memories,such as phase-change memory．However,for hybrid main memories with high performance and scalability,a key challenge is to effectively manage the metadata (e.g.,tags) for data cached in DRAM in a fine- granularity．Based on this observation:storing metadata on off-chip cache line in the same row as their data corresponding to the metadata exploits DRAM row buffer locality,this paper reduced the overhead of fine-grained DRAM cache by using a small buffer to cache chip cache line which has recently been accessed．We also developed an adaptive policy to choose the best granularity when migrating data into DRAM．On a hybrid me-mory with a 512MB DRAM cache,our proposal using an 8KB on-chip buffer can increase the performance within 6％ and save 18％ better energy efficiency than a conventional 8MB SRAM metadata store,even when the energy overhead due to large SRAM metadata storage is not considered.

Key words: Cache,Tag memory,Non-volatile memories,Hybrid main memories