Alerting on Saturation-related Issues

Measure saturation #

Saturation measures the fullness of our services and the system. We should be aware if replicas of our services are processing too many requests and being forced to queue some of them. We should also monitor whether usage of our CPUs, memory, disks and other resources reaches critical limits.

Measure CPU usage #

For now, we’ll focus on CPU usage. We’ll start by opening the Prometheus's graph screen.

open "http://$PROM_ADDR/graph"

Let’s see if we can get the rate of used CPU by node (instance). We can use node_cpu_seconds_total metric for that. However, it is split into different modes, and we’ll have to exclude a few of them to get the “real” CPU usage. Those will be idle, iowait, and any type of guest cycles.

Please type the expression that follows, and press the Execute button.

sum(rate(
  node_cpu_seconds_total{
    mode!="idle", 
    mode!="iowait", 
    mode!~"^(?:guest.*)$"
  }[5m]
)) 
by (instance)

Switch to the Graph view.

The output represents the actual usage of CPU in the system. In my case (screenshot below), excluding a temporary spike, all nodes are using less than a hundred CPU milliseconds. The system is far from being under stress.

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