Dynamic Indexing: Leakage-Aging Co-Optimization for Caches

Traditional implementations of low-power states based on voltage scaling or power gating have been shown to have a beneficial effect on the aging phenomena caused by negative bias temperature instability (NBTI), which can be explained in terms of the intuitive correlation between the idleness and the reduced workload of a system. Such a joint benefit has been exploited only partially because of the different nature of energy and aging as cost functions: as a performance figure, aging is affected by the worst idleness pattern. Therefore, large potential energy savings usually result in limited aging reductions. In this paper, we address this problem in the context of power-managed caches, which represent a critical target for NBTI-reduced aging: given their symmetric structure, SRAM structures are, in particular, sensitive to NBTI effects because they cannot take advantage of the value-dependent recovery typical of NBTI. We propose a strategy called dynamic indexing, in which the cache indexing function is changed over time in order to uniformly distribute the idleness over all the various power managed units (e.g., lines). This distribution allows fully using the leakage optimization potential and extending the lifetime of a cache. We explore various alternatives, in particular different granularities of the power managed units as well as different reindexing functions. Experimental analysis shows that it is possible to simultaneously reduce leakage power and aging in caches, with minimal power consumption overhead.