Enhancing LTP-Driven Cache Management Using Reuse Distance Information

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Traditional caches employ the LRU management policy to drive replacement decisions. However, previous studies have shown LRU can perform significantly worse than the theoretical optimum, OPT [1]. To better match OPT, it is necessary to aggressively anticipate the future memory references performed in the cache. Recently, several researchers have tried to approximate OPT management by predicting last touch references [2, 3, 4, 5]. Existing last touch predictors (LTPs) either correlate last touch references with execution signatures, like instruction traces [3, 4] or last touch history [5], or they predict cache block life times based on reference [2] or cycle [6] counts. On a predicted last touch, the referenced cache block is marked for early eviction. This permits cache blocks lower in the LRU stackbut with shorter reuse distances to remain in cache longer, resulting in additional cache hits. This paper investigates two novel techniques to improve LTP-driven cache management. First, we pro- pose exploiting reuse distance information to increase LTP accuracy. Specifically, we correlate a memory references last touch outcome with its global reuse distance history. Our results show that for an 8-way 1 MB L2 cache, a 74 KB RD-LTP can reduce the cache miss rate by 11.5% and 14.5% compared to LvP and AIP [2], two state-of-the-art last touch predictors. These performance gains are achieved because RD-LTPs exhibit a much higher prediction rate compared to existing LTPs, and RD-LTPs often avoid evicting LNO last touches [5], increasing the proportion of OPT last touches they evict. Second, we also propose predicting actual reuse distance values using reuse distance predictors (RDPs). An RDP is very similar to an RD-LTP except its predictor table stores exact reuse distance values instead of last touch outcomes. Because RDPs predict reuse distances, we can distinguish between LNO and OPT last touches more accurately. Our results show an 80 KB RDP can improve the miss rate compared to an RD-LTP by an additional 3.7%.