The Dirty-Block Index

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 find that this approach is in efficient and inhibits several cache optimizations. In this work, we propose a new way of organizing the dirty bit information that enables simpler and more efficient implementations of several optimizations. In our proposed approach, we remove the dirty bits from the tag store and organize it differently 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 benefits of DBI by using it to simultaneously and efficiently implement three optimizations proposed 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.