Explain Non-CPU Latency With Prometheus + proxymock

A reproducible SRE lab demonstrates how to diagnose p95 latency regressions caused by waiting rather than CPU work, using a checkout API whose pricing client is intentionally limited to two pooled connections while handling eight concurrent users. Prometheus measures request-duration latency, connection-acquisition wait time through Go httptrace, and CPU consumption, showing that a roughly 190 ms p95 request latency is largely explained by a 140 ms p95 connection wait while CPU remains near 0.05 cores. Proxymock records the real inbound request and outbound pricing dependency exchange, then replays an identical 1,200-request workload for baseline and candidate versions, while functional replay and response diffs verify that performance changes do not alter stable response behavior. Raising the client connection budget from 2 to 16 eliminates queueing, reducing replay p95 latency from 173 ms to 46 ms and Prometheus connection-wait p95 to about 1 ms, while increasing throughput without introducing failures or response differences. The lab emphasizes using histograms and saturation metrics first to distinguish wait time from compute time, reserving CPU profiling for work-bound incidents, and cautions that production dependencies must be validated before increasing concurrency limits.

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