Design and Verification of Adaptive Cache Coherence Protocols ...

Chapter 4
The Base Cache Coherence
Protocol
The Base protocol is the most straightforward implementation of the CRF model. An attractive
characteristic of Base is its simplicity: no state needs to be maintained at the memory side. In
Base, a commit operation on a dirty cell forces the data to be written back to the memory, and
a reconcile operation on a clean cell forces the data to be purged from the cache. This is ideal
for programs in which only necessary commit and reconcile operations are performed.
In this chapter, we rst present a novel protocol design methodology called Imperative-&-
Directive that will be used throughout this thesis. Section 4.2 describes the system con guration
of the Base protocol and gives the message passing rules for non-FIFO and FIFO networks.
We de ne the imperative rules of Base in Section 4.3, and present the complete Base protocol
in Section 4.4. Section 4.5 proves the soundness of Base by showing that the imperative Base
rules can be simulated in CRF Section 4.6 proves the liveness of Base by showing that each
processor can always make progress (that is, a memory instruction can always be completed
eventually).
4.1 The Imperative-&-Directive Design Methodology
In spite of the development of various cache coherence protocols, it is di cult to discern any
methodology that has guided the design of existing protocols. A major source of complexity
in protocol design is that the designer often deals with many di erent issues simultaneously.
Are coherence states being maintained correctly? Is it possible that a cache miss may never be
serviced? What is the consequence if messages get reordered in the network? How to achieve
better adaptivity for programs with di erent access patterns? Answering such questions can be
di cult for sophisticated protocols with various optimizations. The net result is that protocol
design is viewed as an enigma, and even the designer is not totally con dent of his or her
understanding of the protocol behavior.
We propose a two-stage design methodology called Imperative-&-Directive that separates
soundness and liveness concerns in the design process (see Figure 4.1). Soundness ensures that
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