charting sum statistic is meaningless for cumulative meters

[Eoghan Glynn]

The Value dropdown list on the Stats tab of the admin/Resource Usage panel allows the average, minimum, maximum or sum of all meters to be charted.

However it doesn't really make sense to apply the sum aggregate to a cumulative meter (or even a gauge meter for that matter).

As this panel allows the user to graph the sum of samples for any meter aggregated over a tenant, depending on the duration chosen, they would generally end up with a meaningless sawtooth pattern in the graph for cumulative meters.

The exact shape seen depends on how many samples happen to fall into each period timeslice. Horizon arbitrarily divides the duration by 400 to arrive at a period length, whereas ceilometer allows the metric gathering cadence to be configured via the /etc/ceilometer/pipeline.yaml.

Given the typical time horizons that users tend to be interested in (last day, last week etc.), the slices are very unlikely in general to match the pipeline interval, unless the user happened to configure this to a fairly improbable value (216s, 1512s etc.).

So we end up seeing a sawtooth pattern that doesn't reflect peaks and troughs in actual usage, rather it simply follows from the number of samples that happen to fall into each arbitrarily sized period.

I'll attach two screenshots illustrating this pattern for cumulative and gauge meters. In the first case it appears that a monotonically increasing value for CPU time is veering up & down (superimposed over an upward trend).

Even more misleading is the second case, which seems to show regular churn in the numbers of instances in use by individual tenants (whereas in fact the number of instances spun up has been broadly static, and the apparent churn is just an illusion caused by a mismatch between the requested period and the actual metric cadence).

Proposed solution: make the Value dropdown context-sensitive, such that the aggregate functions provided for the current meter always make sense for the meter type. In particular, exclude the Sum aggregate for all but delta meters.

[Ladislav Smola (lsmola)]

Agree, I have already started patch, that is adding some default settings to each meter. We should put it there.

[Ladislav Smola (lsmola)]

Should be add a enhancement of this https://review.openstack.org/#/c/47579/

[Gary W. Smith (gary-w-smith)]

Ladislav Smola: was this addressed in https://review.openstack.org/#/c/47579/ ?

[Ying Zuo (yingzuo)]

Ceilometer dashboard has been moved out of Horizon.