Excellent explanatory mailing list post by Martin Thompson to the mechanical-sympathy group, discussing Little’s Law vs the USL:
Little’s law can be used to describe a system in steady state from a queuing perspective, i.e. arrival and leaving rates are balanced. In this case it is a crude way of modelling a system with a contention percentage of 100% under Amdahl’s law, in that throughput is one over latency. However this is an inaccurate way to model a system with locks. Amdahl’s law does not account for coherence costs. For example, if you wrote a microbenchmark with a single thread to measure the lock cost then it is much lower than in a multi-threaded environment where cache coherence, other OS costs such as scheduling, and lock implementations need to be considered. Universal Scalability Law (USL) accounts for both the contention and the coherence costs. http://www.perfdynamics.com/Manifesto/USLscalability.html When modelling locks it is necessary to consider how contention and coherence costs vary given how they can be implemented. Consider in Java how we have biased locking, thin locks, fat locks, inflation, and revoking biases which can cause safe points that bring all threads in the JVM to a stop with a significant coherence component.
jwz may indeed have invented this feature way back in Netscape Mail. FWIW I think he’s right — Netscape Mail was the first usage of HTML email I recall