Efficient memory hierarchy design is critical due to the increasing gap between the speed of the processors and the memory. One of the sources of inefficiency in current caches is the non-uniform distribution of the memory accesses on the cache sets. Its consequence is that while some cache sets may have working sets that are far from fitting in them, other sets may be underutilized because their working set has fewer lines than the set. In this paper we present a technique that aims to balance the pressure on the cache sets by detecting when it may be beneficial to associate sets, displacing lines from stressed sets to underutilized ones. This new technique, called Set Balancing Cache or SBC, achieved an average reduction of 13% in the miss rate of ten benchmarks from the SPEC CPU2006 suite, resulting in an average IPC improvement of 5%.

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SUMMARYTwo of the main sources of inefficiency in current caches are the non‐uniform distribution of the memory accesses across the cache sets, which causes misses due to the mapping restrictions of non fully associative caches and the access patterns with little locality that degrade the performance of caches under the traditional least recently used. replacement policy. This paper proposes a technique to tackle in a coordinated way both kinds of problems in the context of chip multiprocessors, whose last level caches can be shared by threads with different patterns of locality. Our proposal, called thread‐aware mapping and replacement miss reduction (TAMR2) policy, tracks the behavior of each thread in each set in order to decide the appropriate combination of policies to deal with these problems. Despite its small overhead, TAMR2 achieved in our experiments average power consumption and memory latency reductions of 10% and 12%, respectively, resulting in an average throughput improvement of 5.6%, relative to a traditional cache design using four cores. TAMR2 also outperformed many recent related approaches in the field. Copyright © 2013 John Wiley & Sons, Ltd.

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First-level caches are usually split for both instructions and data instead of unifying them in a single cache. Although that approach eases the pipeline design and provides a simple way to independently treat data and instructions, its global hit rate is usually smaller than that of a unified cache. Furthermore, unified lower-level caches usually behave and process memory requests disregarding whether they are data or instruction requests. In this article, we propose a new technique aimed to balance the amount of space devoted to instructions and data for optimizing set-associative caches: the Virtually Split Cache or VSC. Our technique combines the sharing of resources from unified approaches with the bandwidth and parallelism that split configurations provide, thus reducing power consumption while not degrading performance. Our design dynamically adjusts cache resources devoted to instructions and data depending on their particular demand. Two VSC designs are proposed in order to track the instructions and data requirements. The Shadow Tag VSC (ST-VSC) is based on shadow tags that store the last evicted line related to data and instructions in order to determine how well the cache would work with one more way per set devoted to each kind. The Global Selector VSC (GS-VSC) uses a saturation counter that is updated every time a cache miss occurs either under an instruction or data request applying a duel-like mechanism. Experiments with a variable and a fixed latency VSC show that ST-VSC and GS-VSC reduce on average the cache hierarchy power consumption by 29% and 24%, respectively, with respect to a standard baseline. As for performance, while the fixed latency designs virtually match the split baseline in a single-core system, a variable latency ST-VSC and GS-VSC increase the average IPC by 2.5% and 2%, respectively. In multicore systems, even the slower fixed latency ST-VSC and GS-VSC designs improve the baseline IPC by 3.1% and 2.5%, respectively, in a four-core system thanks to the reduction in the bandwidth demanded from the lower cache levels. This is in contrast with many techniques that trade performance degradation for power consumption reduction. VSC particularly benefits embedded processors with a single level of cache, where up to an average 9.2% IPC improvement is achieved. Interestingly, we also find that partitioning the LLC for instructions and data can improve performance around 2%. ACM Reference Format:Rolán, D., Fraguela, B. B., and Doallo, R. 2013. Virtually split cache: An efficient mechanism to distribute instructions and data. ACM Trans.