Parallel Computer Architecture

8.3 Assessing Directory Protocols and Trade-Offs 571

Summary of Directory Organization Alternatives

To summarize, there are many different ways to organize how directories store the

cache state of memory blocks. Simple bit vector representations work well for

machines that have a moderate number of nodes visible to the directory protocol.

For larger machines, many alternatives are available to reduce the memory over-

head. The organization chosen does, however, affect the complexity of the coher-

ence protocol and the performance of the directory scheme against various sharing

patterns. Hierarchical directories have not been popular on real machines ...