However, due to the interconnection network and scalability requirements, centralized protocols have drawbacks in these architectures.
Coherence is maintained by software which has several disadvantages.
Compiler mechanisms are very limited. They have to be very conservative, e.g. treat a block on another processor as dirty even though it may not be. Software coherence must be run each time a word is needed. Latencies to remote memory are relatively high. Snooping The lack of scalability of snooping coherence schemes is a problem in DSMs.