### USL, universal scalability law, is good to know

The USL is worth understanding. USL is short for universal scalability law and was created by Neil Gunther for use in capacity planning. I don't do much capacity planning but the USL is also valuable for explaining performance. Performance problems for the old InnoDB rw-lock (circa 2007) would have been easy to describe via the USL because "wake all" on unlock is an N 2 overhead -- see the 𝛽 parameter in the USL. A longer version of the USL is here . The USL predicts how throughput will change (increase or decrease) with concurrency. One version of the formula where X(i) is throughput with i concurrent clients is below. X(1) * N X(N) = ------------------- 1 + α(N-1) + ꞵN(N-1) The denominator determines how throughput changes with concurrency. When it is one then there is linear scaleup. The best way to get linear scaleup is to cheat and choose an easy base case but otherwise the denominator is greater than 1 and the USL ex