Telemetry Alert for GC_SLOW_TASK on 2019-10-22

[telemetr...@mozilla.com]

We have detected a change in the Telemetry probe GC_SLOW_TASK in Firefox Nightly builds from 2019-10-22.

Alert details: http://alerts.telemetry.mozilla.org/index.html#/detectors/1/metrics/2368/alerts/?from=2019-10-22&to=2019-10-22
Changes new to Nightly builds on 2019-10-22: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=563f437f24b9e12495b0f2c364538a2b8d8a0ca7&tochange=47d7c18620c845c3a42610fea5468ccc50889bae
The value of GC_SLOW_TASK over time: https://telemetry.mozilla.org/new-pipeline/evo.html#!measure=GC_SLOW_TASK

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_SLOW_TASK on 2019-10-22 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_SLOW_TASK'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 indicates that has been an increase in slow SWEEP_MARK phases. 
This is probably due to bug 1564136 which landed during this period. 
I've filed a bug to investigate:

https://bugzilla.mozilla.org/show_bug.cgi?id=1591889

Jon