Ahmdal's law describes the scaling behavior of systems with contention ("serialized workload portion"). This was generalized to the Universal Scalability Law by Neil Gunther which additionally takes into account "cross talk" for coordination between nodes. Systems with cross talk always run into configurations where adding more nodes will decrease the throughput of a system.

This is best explained in Baron Schwarz's book: https://raw.githubusercontent.com/VividCortex/ebooks/master/...

My notes on this topic are here: https://nbviewer.jupyter.org/github/HeinrichHartmann/Statist...