(In reply to Chris H-C :chutten from comment #4)
Could this change be responsible for a sudden decrease in the number of samples (and a decrease in the values of the samples still being reported) for the Scalar
contentprocess.os_priority_raised? It showed a Telemetry alert: http://alerts.telemetry.mozilla.org/index.html#/detectors/1/metrics/2892/alerts/?from=2019-08-16&to=2019-08-16
Yes, I believe this change is responsible for the shift in the histograms.
Is this an improvement? A regression?
This is neither - by disabling the privileged about content process for about:home, we've reduced the number of content processes that are being opened by default.
We're also preventing preloaded about:newtab's from loading in that privileged about content process, which I think is the bigger factor here: before this patch landed, if a user had no about:newtab / about:home tabs in the foreground, we'd kick off an idle task that would preload the next about:newtab in the privileged about content process, and because the preloaded about:newtab is hidden in the background, that privileged about content process would have its priority lowered, and then raised as soon as the next about:newtab was opened.
With the privileged about content process disabled, the default behaviour is to preload the next about:newtab by attaching to a pre-existing content process. There's a higher likelihood that the pre-existing content process is already in the foreground and won't need to have its process priority shifted when the user opens that about:newtab.
Is this intentional? Is this expected?
I had forgotten these probes existed, but upon reflection, I think what happened here makes sense.
Is this probe still measuring something useful?
Yes, I believe so.