Using dead blocks as a virtual victim cache
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Caches mitigate the long memory latency that limits the performance of modern processors. However, caches can be quite inefficient. On average, a cache block in a 2MB L2 cache is dead 59% of the time, i.e., it will not be referenced again before it is evicted. Increasing cache efficiency can improve performance by reducing miss rate, or alternately, improve power and energy by allowing a smaller cache with the same miss rate. This paper proposes using predicted dead blocks to hold blocks evicted from other sets. When these evicted blocks are referenced again, the access can be satisfied from the other set, avoiding a costly access to main memory. The pool of predicted dead blocks can be thought of as a virtual victim cache. For a set of memory-intensive single-threaded workloads, a virtual victim cache in a 16-way set associative 2MB L2 cache reduces misses by 26%, yields an geometric mean speedup of 12.1% and improves cache efficiency by 27% on average, where cache efficiency is defined as the average time during which cache blocks contain live information. This virtual victim cache yields a lower average miss rate than a fully-associative LRU cache of the same capacity. For a set of multi-core workloads, the virtual victim cache improves throughput performance by 4% over LRU while improving cache efficiency by 62%. Alternately, a 1.7MB virtual victim cache achieves about the same performance as a larger 2MB L2 cache, reducing the number of SRAM cells required by 16%, thus maintaining performance while reducing power and area. © 2010 ACM.
author list (cited authors)
Khan, S. M., Jiménez, D. A., Burger, D., & Falsafi, B.