Synergistic Cache Layout For Reuse and Compression

International audienceRecent advances in research on compressed caches make them an attractive design point for effective hardware implementation for last-level caches. For instance, the yet another compressed cache (YACC) layout leverages both spatial and compression factor localities to pack compressed contiguous memory blocks from a 4-block super-block in a single cache block location. YACC requires less than 2% extra storage over a conventional uncompressed cache. Performance of LLC is also highly dependent on its cache block replacement management. This includes allocation and bypass decision on a miss as well as replacement target selection which is guided by priority insertion policy on allocation and priority promotion policy on a hit. YACC uses the same cache layout as a conventional set-associative uncompressed cache Therefore the LLC cache management policies that were introduced during the past decade can be transposed to YACC. However, YACC features super-block tags instead of block tags. For uncompressed block, these super-block tags can be used to monitor the reuse behavior of blocks from the same super-block. We introduce the First In Then First Use Bypass (FITFUB) allocation policy for YACC. With FITFUB, a missing uncompressed block that belongs to a super-block that is already partially valid in the cache is not stored in the cache on its first use, but only on its first reuse if any. FITFUB can be associated with any priority insertion/promotion policy. YACC+FITFUB with compression turned off, achieves an average 6.5%/8% additional performance over a conventional LLC, for single-core/multi-core workloads, respectively. When compression is enabled, the performance benefits associated with compression and FITFUB are almost additive reaching 12.7%/17%. This leads us to call this design the Synergistic cache layout for Reuse and Compression (SRC). SRC reaches the performance benefit that would be obtained with a 4X larger cache, but with less than 2% extra storage