Divide-and-Conquer Way Access for Low Power Mobile Caches

Abstract

As multi-core design concept is becoming dominant, power consumption of the shared level-2 caches is one of the critical issues along with its performance. This is more significant for mobile processors which are used in battery-powered devices. Designing a cache memory, increasing the cache size or adding more set-associativity is one of the simplest ways to improve the performance for both mobile processor and even general-purpose processors. In mobile processors, however, simple increase of the cache size can significantly affect the chip area and power consumption. Modern integration technologies allow integrating the bigger caches into tiny single-chip multi-core mobile processors, but relevant architectural issues cannot be ignored. These issues will be worse in mobile processors which are integrated in a small form-factor without any cooling fan. In this paper, we propose a novel cache mechanism to reduce power consumption in level-2 cache by using the divide-and-conquer way-accessing technique. This idea is derived from the observation that cache way-hit distribution shows the unbalanced pattern in which primary-way(s) have more way-hits. Based on this way utilizations and activities, we came up with a biased way-access mechanism to get benefits from lower associativity which requires relatively lower power consumption. Considering unbalanced way-hit distribution of the cache, all other ways but primary way(s) can be accessed one step later when the primary way-access has across the miss. By doing that, if way-hits are achieved from the primary-way, power can be saved by turning only primary-way on. However, the penalties from the primary-way misses will not be ignored in access time. Through the careful tradeoff analysis between power-saving and penalty in access time, appropriate cache configurations can be chosen in early design stage.