Page "Cache coherence" Paragraph 17

The two most common types of coherence that are typically studied are Snooping and Directory-based, each having its own benefits and drawbacks.

Snooping protocols tend to be faster, if enough bandwidth is available, since all transactions are a request / response seen by all processors.

The drawback is that snooping isn't scalable.

Every request must be broadcast to all nodes in a system, meaning that as the system gets larger, the size of the ( logical or physical ) bus and the bandwidth it provides must grow.

Directories, on the other hand, tend to have longer latencies ( with a 3 hop request / forward / respond ) but use much less bandwidth since messages are point to point and not broadcast.

For this reason, many of the larger systems (> 64 processors ) use this type of cache coherence.