Telemetry Alert for GC_RESET_REASON on 2019-10-29

[telemetr...@mozilla.com]

We have detected a change in the Telemetry probe GC_RESET_REASON in Firefox Nightly builds from 2019-10-29.

Alert details: http://alerts.telemetry.mozilla.org/index.html#/detectors/1/metrics/2041/alerts/?from=2019-10-29&to=2019-10-29
Changes new to Nightly builds on 2019-10-29: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=c65ef27b6fc78ec5140068913209bad3b55f1139&tochange=25bf8e097e604cca2842af6754c2c1698db5618a
The value of GC_RESET_REASON over time: https://telemetry.mozilla.org/new-pipeline/evo.html#!measure=GC_RESET_REASON

What to do about this:

1. File a bug to track your investigation. You can just copy this email into the bug Description to get you started.
2. Reply-All to this email to let the list know that you are investigating. Include the bug number so we can help out.
3. Triage the alert. You can find instructions here: https://docs.telemetry.mozilla.org/tools/alerts.html#triaging-a-telemetry-alert-email

If you have any problems, please ask for help on the #telemetry IRC channel or on Slack in #fx-metrics. We'll give you a hand.

What this is:
We have a system called cerberus[1] that compares Telemetry collected on different Nightly builds and looks for sudden changed in value distributions using the Bhattacharyya Distance[2]. It found such a change in GC_RESET_REASON on 2019-10-29 so it asked its buddy medusa[3] to send this email to the dev-telemetry-alerts mailing list and to all email addresses listed in the alert_emails field of GC_RESET_REASON's definition.

You can do this!

Your Friendly, Neighbourhood Firefox Telemetry Team
[1]: https://github.com/mozilla/cerberus
[2]: https://en.wikipedia.org/wiki/Bhattacharyya_distance
[3]: https://github.com/mozilla/medusa

[Jon Coppeard]

This alert and the one about GC_NON_INCREMENTAL_REASON were caused by
landing bug 1591711.

https://bugzilla.mozilla.org/show_bug.cgi?id=1591711

This returns the number of reset GCs to its original level since bug
1570905 landed.  I don't fully understand why the distribution changed
in the GC_NON_INCREMENTAL_REASON telemetry but the total number of non
incremental GCs hasn't changed significantly (see GC_NON_INCREMENTAL
telemetry) so I don't think it's anything to worry about.

Jon

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