Data Coherence Problem in a Multicache System

A coherence problem may occur in a multicache system as soon as data inconsistency exists in the private caches and the main memory. Without an effective solution to the coherence problem, the effectiveness of a multicache system will be inherently limited. The problem is closely examined in this paper and previous solutions, both centralized approaches and distributed approaches, are analyzed based on the notion of semicritical sections. A state model is then presented which clarifies various coherence mechanisms as well as introduces a new state to enable the multicache system to more efficiently handle the processor writes. Software guidance, for performance and not for integrity, is advocated in a new proposal which in a practical multicache environment explores the benefit of the new state with little cost.