The Dirty-Block Index Vivek Seshadri,Abhishek Bhowmick,Onur Mutlu,

On-chip caches maintain multiple pieces of metadata about each cached block—e.g., dirty bit, coherence information, ECC. Traditionally, such metadata for each block is stored in the corresponding tag entry in the tag store. While this approach is simple to implement and scalable, it necessitates a full tag store lookup for any metadata query—resulting in high latency and energy consumption. We Vnd that this approach is ineX-cient and inhibits several cache optimizations. In this work, we propose a new way of organizing the dirty bit information that enables simpler and more eXcient im-plementations of several optimizations. In our proposed ap-proach, we remove the dirty bits from the tag store and orga-nize it diUerently in a separate structure, which we call the Dirty-Block Index (DBI). The organization of DBI is simple: it consists of multiple entries, each corresponding to some row in DRAM. A bit vector in each entry tracks whether or not each block in the corresponding DRAM row is dirty. We demonstrate the beneVts of DBI by using it to simul-taneously and eXciently implement three optimizations pro-posed by prior work: 1) Aggressive DRAM-aware writeback, 2) Bypassing cache lookups, and 3) Heterogeneous ECC for clean/dirty blocks. DBI, with all three optimizations enabled, improves performance by 31 % compared to the baseline (by 6 % compared to the best previous mechanism) while reducing overall cache area cost by 8 % compared to prior approaches. 1