Paolo Bennati

Exploiting locality to improve leakage reduction in embedded drowsy I-caches at same area/speed

In this paper, a technique to reduce the leakage power consumption in embedded drowsy instruction caches (I caches) is proposed. The technique is called “Improved Drowsy” (ID), and adopts a more efficient strategy than standard Drowsy Caches (DCs) to turn off unused cache lines, based on locality. The implementation of ID caches requires minor changes, and the area/speed overhead associated with the additional circuitry is insignificant. The proposed technique is assessed through circuit and cycle accurate simulations on an L1 instruction cache embedded in an ARM XScale processor based system in a 65 nm CMOS technology. Results show that this technique is able to reduce the leakage power by 69% on average. Leakage of DC is shown to be significantly lowered with the proposed ID approach, being DC leakage greater than that of ID by up to 53%, and 10 15% typically.

Filtering drowsy instruction cache to achieve better efficiency

Leakage power in cache memories represents a sizable fraction of total power consumption, and many techniques have been proposed to reduce it. As a matter of fact, during a fixed period of time, only a small subset of cache lines is used. Previous techniques put unused lines, for example, to drowsy in order to save power. Our idea is to adaptively select the most used cache lines. In the case of instruction cache, we found that this can automatically achieved by coupling a tiny cache acting as a filter cache (ILO cache) with a drowsy-cache. Our experiments, with complete MiBench suite for ARM based processor, show a 25% improvement in leakage saving versus drowsy.

Reducing Leakage through Filter Cache

We evaluate the leakage reduction for both instruction and data cache in presence of drowsy or decay techniques. We discovered that a filter cache,traditionally used for reducing active power, can help reduce also leakage. The key idea is to reduce the lifetime of the lines that are in high-power state inside a leakage-saving cache. Power consumption has become one of the main concerns for designers, together with the performance. Caches account for the largest fraction of on-chip transistors in most modern processors. Therefore, they are a primary candidate for attacking the problem of the leakage. In average with the proposed solution, for instruction cache 24% improvement in leakage savings and 1.5% in IPC (Instruction Per Cycle) can be achieved with respect to drowsy cache. For data caches, 5% and 5.4% improvement can be achieved respectively. Experiments have been performed also with decay cache showing fewer benefits.