Question

Your team runs a fleet of 40 Compute Engine instances behind a managed instance group serving a REST API. You create a Cloud Monitoring alerting policy on the metric compute.googleapis.com/instance/cpu/utilization to page on-call when CPU is sustained above 85%. During a real incident where several individual instances spiked to 95% while the group average stayed near 60%, the alert never fired. You inspected the policy and found it aligns the series and then applies a cross-series reducer of 'mean' before evaluating the threshold. What change will make the alert correctly detect per-instance saturation?

Accepted Answer

Remove the cross-series reducer (or set it to 'none') and group the time series by instance so the threshold is evaluated per instance rather than against a single aggregated series.

Answer

Increase the alignment period from 60s to 300s so the aligner smooths out brief per-instance spikes before evaluation.

Answer

Change the cross-series reducer from 'mean' to 'sum' so the combined utilization of all instances exceeds the 85% threshold.

Answer

Lower the threshold from 85% to 55% so the group average of 60% during the incident would trigger the alert.