Decoupled sectored caches: conciliating low tag implementation cost

Abstract

Sectored caches have been used for many years in order to reconcile low tag array size and small or medium block size. In a sectored cache, a single address tag is associated with a sector consisting on several cache lines, while validity, dirty and coherency tags are associated with each of the inner cache lines.Maintaining a low tag array size is a major issue in many cache designs (e.g. L2 caches). Using a sectored cache is a design trade-off between a low size of the tag array which is possible with large line size and a low memory traffic which requires a small line size.This technique has been used in many cache designs including small on-chip microprocessor caches and large external second level caches. Unfortunately, as on some applications, the miss ratio on a sectored cache is significantly higher than the miss ratio on a non-sectored cache (factors higher than two are commonly observed), a significant part of the potential performance may be wasted in miss penalties.Usually in a cache, a cache line location is statically linked to one and only one address tag word location. In the decoupled sectored cache we introduce in this paper, this monolithic association is broken; the address tag location associated with a cache line location is dynamically chosen at fetch time among several possible locations.The tag volume on a decoupled sectored cache is in the same range as the tag volume in a traditional sectored cache; but the hit ratio on a decoupled sectored cache is very close to the hit ratio on a non-sectored cache. A decoupled sectored cache will allow the same level of performance as a non-sectored cache, but at a significantly lower hardware cost.