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1. IL-0209 A too-high HPA CPU target kept GitLab.com web pods from scaling up as traffic moved from VMs to Kubernetes, breaching the web latency SLO for 15 minutes

   GitLab2021

   Symptoms

   The web service breached its latency SLO at 07:30 UTC and its Apdex dropped: some requests to the GitLab.com web front end slowed down for about 15 minutes. This happened while web traffic was being shifted from virtual machines to the web pods on Kubernetes, and the pods did not scale up as the traffic arrived.

   High latencyAutoscaler misbehaviour

   Trigger

   Shifting web traffic from VMs to Kubernetes pods during the GitLab.com web migration.

   Root cause

   Configuration change

   Component

   Horizontal pod autoscaler

   Contributing factor

   Unverified assumption

   Detection

   The web latency SLO breach at 07:30 UTC; an engineer declared the incident in Slack at 07:40.

   Remediation

   • Lowered the target CPU utilisation of the web pods' autoscaler to a more conservative value (merge request gitlab-com/gl-infra/k8s-workloads/gitlab-com!1149)

   Action items

   • Adjust the target utilisation for the web pods (gitlab-com/gl-infra/k8s-workloads/gitlab-com!1149)

   Lessons

   When traffic is moved onto pods that rely on CPU-based autoscaling, a CPU target set too high can keep the pods from scaling as the traffic arrives; GitLab fixed it by setting a more conservative target.

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