Universal Scalability Law
C(N) = N / (1 + α(N−1) + βN(N−1)). Throughput vs concurrency.
Origin
Neil Gunther formulated USL in the 1990s, building on Amdahl with an extra term for coordination cost. Predicts the negative-return region — beyond a critical N, more workers makes things slower.
Where it shows up in production
- Database lock contention At some concurrency threshold, throughput inverts and falls. USL fits the curve.
- Scalability modeling Fit α and β to throughput-vs-load data and predict where adding capacity will stop helping.
On Semicolony
Sources & further reading
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