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Ceilometer cpu_util average


I have yaml template that creates new instances when average cpu_util is above 15% for 60s (If I put threshold above that value OpenStack doesn't create new instances)

When I run load tests top command on my instance shows cpu_util around 95-100 (I run those load tests for longer periods than 60s) In ceilometer-alarm-notifier.log I get values like 15-16 when second instance is created and than values that activate alarm slowly rise, despite that new instances are created. When I stop load test those values quickly drop down and newly created instances are deleted and in notifier log I get values like 1-2 (threshold for scalingdown policy is avg below 5% over 60s).

So my question is how Ceilometer calculates cpu_util average, since it doesn't match top command values?