Telemetry Alerts

Many Telemetry probes were created to show performance trends over time. Sudden changes happening in Nightly could be the sign of an unintentional performance regression, so we introduced a system to automatically detect and alert developers about such changes.

Thus we created Telemetry Alerts. It comes in two pieces: Cerberus the Detector and Medusa the Front-end.


Every day Cerberus grabs the latest aggregated information about all non-keyed Telemetry probes from and compares the distribution of values from the Nightly builds of the past two days to the distribution of values from the Nightly builds of the past seven days.

It does this by calculating the Bhattacharyya distance between the two distributions and guessing whether or not they are significant and narrow.

It places all detected changes in a file for ingestion by Medusa.


Medusa is in charge of emailing people when distributions change and for displaying the website which contains pertinent information about each detected regression.

Medusa also checks for expiring histograms and sends emails notifying of their expiry.

What it can do

Telemetry Alerts is very good at identifying sudden changes in the shapes of normalized distributions of Telemetry probes. If you can see the distribution of GC_MS shift from one day to the next, then likely so can Cerberus.

What can't it do

Telemetry Alerts is not able to see sudden shifts in volume. It is also very easily fooled if a change happens over a long period of time or doesn't fundamentally alter the shape of the probe's histogram.

So if you have a probe like SCALARS_BROWSER.ENGAGEMENT.MAX_CONCURRENT_TAB_COUNT, Cerberus won't notice if:

  • The number of pings reporting this value decreased in half, but otherwise reported the same spread of numbers
  • The value increases very slowly over time (which I'd expect it to do given how good Session Restore is these days)
  • We suddenly received twice as many pings from 200-tab subsessions (the dominance of 1-tab pings would likely ensure the overall shape of the distribution changed insufficiently much for Cerberus to pick up on it)

Telemetry Alert Emails

One of the main ways humans interact with Telemetry Alerts is through the emails sent by Medusa.

At present the email contains a link to the alert's page on and a link to a pushlog on detailing the changes newly-present in the Nightly build that exhibited the change.

Triaging a Telemetry Alert Email

Congratulations! You have just received a Telemetry Alert!

Now what?

Assumption: Alerts happen because of changes in probes. Changes in probes happen because of changes in related code. If we can identify the code change, we can find the bug that introduced the code change. If we can find the bug, we can ni? the person who made the change.

Goal: Identify the human responsible for the Alert so they can identify if it is good/bad/intentional/exceptional/temporary/permanent/still relevant/having its alerts properly looked after.


1. Is this alert just one of a group of similar changes by topic? By build?

  • If there's a group by topic (SPDY, URLCLASSIFIER, ...) check to see if the changes are similar in direction/magnitude. They usually are.
  • If there's a group by build but not topic, maybe a large merge kicked things over. Unfortunate, as that'll make finding the source more difficult.

2. Open the and links in tabs

  • On alerts.tmo, does it look like an improvement or regression? (This is just a first idea and might change. There are often extenuating circumstances that make something that looks bad into an improvement, and vice versa.)
  • On, does the topic of the changed probe exist in the pushlog? In other words, does any part of the probe's name show up in the summaries of any of the commits?

3. From alerts.tmo, open the link by clicking on the plot's title. Open another tab to the Evolution View.

  • Is the change temporary? (might've been noticed elsewhere and backed out)
  • Is the change up or down?
  • Has it happened before?
  • Was it accompanied by a decrease in submission volume? (the second graph at the bottom of the Evolution View)
  • On the Distribution View, did the Sample Count increase? Decrease? (this signifies that the change could be because of the addition or subtraction of a population of values. For instance, we could suddenly stop sending 0 values which would shift the graph to the right. This could be a good thing (we're not handling useless things anymore) a bad thing (something broke and we're no longer measuring the same thing we used to measure) or indifferent)

4. If you still don't have a cause

  • Use DXR or searchfox to find where the probe is accumulated.
  • Click "Log" in that view.
  • Are there any changesets in the resultant list that ended up in the build we received the Alert for?

5. If you _still_ don't know what's going on

  • find a domain expert on IRC and bother them to help you out. Domain knowledge is awesome.

From pursuing these steps or substeps you should now have two things: a bug that likely caused the alert, and an idea of what the alert is about.

Now comment on the bug. Feel free to use this script:

This bug may have contributed to a sudden change in the Telemetry probe <PROBE_NAME>[1] which seems to have occurred in Nightly <builddate>[2][3].

There was a <describe the change: increase/decrease, population addition/subtraction, regression/improvement, change in submission/sample volume...>.
This might mean <wild speculation. It'll encourage the ni? to refute it :) >

Is this an improvement? A regression?

Is this intentional? Is this expected?

Is this probe still measuring something useful?

[1]: <the alerts.tmo link>
[2]: <the link for the pushlog>
[3]: <the link showing the Evolution View>

Then ni? the person who pushed the change. Reply-all to the dev-telemetry-alerts mail with a link to the bug and some short notes on what you found.

From here the user on ni? should get back to you in fairly short order and either help you find the real bug that caused it, or help explain what the Alert was all about. More often than not it is an expected change from a probe that is still operating correctly and there is no action to take...

...except making sure you never have to respond to an Alert for this probe again, that is. File a bug in that bug's component to update the Alert'ing probe to have a valid, monitored "alert_emails" field so that the next time it misbehaves they can be the ones to explain themselves without you having to spend all this time tracking them down.

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