Why CPU metrics mislead autoscalers running single-threaded services

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A single-threaded Node.js API was experiencing high latency and request backlogs, yet its autoscaler remained idle because average CPU utilization across a four-vCPU task showed only around 30 percent. The root cause was architectural: a single-threaded process can saturate only one CPU core, making peak load appear as roughly 25–30 percent when averaged across all cores. This meant the autoscaling policy was effectively measuring load divided by four, rather than actual service strain. Engineers identified two durable fixes — resizing tasks to one vCPU so full saturation registers as 100 percent, and switching scaling signals to latency or request-count metrics that directly reflect workload pressure. The broader lesson applies to any single-threaded runtime: average CPU utilization answers the wrong question for services that cannot distribute work across cores.

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