A disadvantage of directories is that it requires some extra memory.
Levy
Directory based protocol avoids broadcast but potentially increases the latency, if the interconnection itself is broadcast based (e.g. ethernet)
l8b
Directories can avoid unnecessary communication because you only have to communicate with the controllers you know care about this memory line. But you need more memory to store this information; so directories might be most helpful when a very small number of controllers are dealing with each line.
A disadvantage of directories is that it requires some extra memory.
Directory based protocol avoids broadcast but potentially increases the latency, if the interconnection itself is broadcast based (e.g. ethernet)
Directories can avoid unnecessary communication because you only have to communicate with the controllers you know care about this memory line. But you need more memory to store this information; so directories might be most helpful when a very small number of controllers are dealing with each line.