Decoupled compressed cache: Exploiting spatial locality for energy-optimized compressed caching

In multicore processor systems, last-level caches (LLCs) play a crucial role in reducing system energy by (i) filtering out expensive accesses to main memory and (ii) reducing the time spent executing in high-power states. Cache compression can increase effective cache capacity and reduce misses, improve performance, and potentially reduce system energy. However, previous compressed cache designs have demonstrated only limited benefits due to internal fragmentation and limited tags. In this paper, we propose the Decoupled Compressed Cache (DCC), which exploits spatial locality to improve both the performance and energy-efficiency of cache compression. DCC uses decoupled super-blocks and non-contiguous sub-block allocation to decrease tag overhead without increasing internal fragmentation. Non-contiguous sub-blocks also eliminate the need for energy-expensive re-compaction when a block's size changes. Compared to earlier compressed caches, DCC increases normalized effective capacity to a maximum of 4 and an average of 2.2 for a wide range of workloads. A further optimized Co-DCC (Co-Compacted DCC) design improves the average normalized effective capacity to 2.6 by co-compacting the compressed blocks in a super-block. Our simulations show that DCC nearly doubles the benefits of previous compressed caches with similar area overhead. We also demonstrate a practical DCC design based on a recent commercial LLC design.