Improved Policies for Drowsy Caches in Embedded Processors

In the design of embedded systems, especially battery- powered systems, it is important to reduce energy consumption. Cache are now used not only in general-purpose processors but also in embedded processors. As feature sizes shrink, the leakage energy has contributed to a significant portion of total energy consumption. To reduce the leakage energy of cache, the Drowsy cache was proposed, in which the cache lines are periodically moved to the low- leakage mode without loss of its content. However, when a cache line in the low-leakage mode is accessed, one or more clock cycles are required to transition the cache line back to the normal mode before its content can be accessed. As a result, these penalty cycles may significantly degrade the cache performance, especially in embedded processors without out-of-order execution. In this paper, we propose four mode transition policies which aim at high energy reduction with the minimum performance degradation. We also compare our policies with existing policies in the context of embedded processors. Experimental results demonstrate the effectiveness of the proposed policies.