Talos has detected a Firefox performance regression from push fbb40278805f. As author of one of the patches included in that push, we need your help to address this regression. This is a list of all known regressions and improvements related to the push: https://treeherder.mozilla.org/perf.html#/alerts?id=599 On the page above you can see an alert for each affected platform as well as a link to a graph showing the history of scores for this test. There is also a link to a treeherder page showing the Talos jobs in a pushlog format. To learn more about the regressing test(s), please see: https://wiki.mozilla.org/Buildbot/Talos/Tests#tp5 Reproducing and debugging the regression: If you would like to re-run this Talos test on a potential fix, use try with the following syntax: try: -b o -p win32 -u none -t tp5o-e10s[Windows XP] --rebuild 5 # add "mozharness: --spsProfile" to generate profile data (we suggest --rebuild 5 to be more confident in the results) To run the test locally and do a more in-depth investigation, first set up a local Talos environment: https://wiki.mozilla.lorg/Buildbot/Talos/Running#Running_locally_-_Source_Code Then run the following command from the directory where you set up Talos: talos --develop -e [path]/firefox -a tp5o --e10s Making a decision: As the patch author we need your feedback to help us handle this regression. *** Please let us know your plans by Tuesday, or the offending patch(es) will be backed out! *** Our wiki page outlines the common responses and expectations: https://wiki.mozilla.org/Buildbot/Talos/RegressionBugsHandling

:emk, can you take a look at this and help us figure out why this might be problematic and what we can do to reduce this regression?

I have no idea how bug 1257877 can cause a performance regression. The change is only code removal.

Either the measurement or the attribution has to be bogus. There's no way for bug 1257877 to have caused an effect on responsiveness.

some pushes to try: https://treeherder.mozilla.org/#/jobs?repo=try&author=jmaher@mozilla.com&fromchange=1d5a583bc0ff&tochange=eb5c73f977b9 lets see how this ends up

pushing to try with the suspect revision and the one prior to build pgo and run tp5 on windows xp, I see the same difference: https://treeherder.mozilla.org/perf.html#/compare?originalProject=try&originalRevision=eb5c73f977b9&newProject=try&newRevision=1d5a583bc0ff&framework=1&showOnlyImportant=0 it appears that this code removal is causing pgo to do something different and show a regression. :avih, this is a heads up for the e10s tracking as this is a e10s only regression and will make the e10s dashboard show less of a win for tp5 responsiveness. Can you own determining what this means as I will not be working this next week?

:avih, did you have any thoughts or progress on this while I was out all of last week on pto?

Sorry for the delay. Yeah, it's not huge, and what happens with PGO is mostly beyond our control. Also, XP... We'll live with that.