We have detected a platform microbenchmarks regression from push: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=611582e42451dd26fcaa38f251e2203a6e8b0bf9&tochange=ac60233276ed2798c2f2f42772362f729dec3468 As author of one of the patches included in that push, we need your help to address this regression. Regressions: 164% Stylo Servo_DeclarationBlock_GetPropertyById_Bench linux32 opt 243,843.29 -> 642,725.30 161% Stylo Servo_DeclarationBlock_GetPropertyById_Bench windows7-32 opt 224,732.54 -> 586,337.10 144% Stylo Servo_DeclarationBlock_GetPropertyById_Bench windows10-64 opt 184,395.42 -> 449,097.60 130% Stylo Servo_DeclarationBlock_GetPropertyById_Bench osx-10-10 opt 199,551.58 -> 457,979.75 You can find links to graphs and comparison views for each of the above tests at: https://treeherder.mozilla.org/perf.html#/alerts?id=9669 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 jobs in a pushlog format. To learn more about the regressing test(s), please see: https://developer.mozilla.org/en-US/docs/Mozilla/Performance/Automated_Performance_Testing_and_Sheriffing/Platform_Microbenchmarks

Michael, I see you are the owner of the servo merge from https://github.com/servo/servo/pull/18642 Can we resolve these regressions? Or should we accept/backout them?

I missed converting these locations when I made the previous patch, which caused an extra allocation to occur whenever using fmt::Write. This should fix that issue. MozReview-Commit-ID: 38WZKDU1cLg

Pushed by manishearth@gmail.com: https://hg.mozilla.org/integration/autoland/rev/1e3c2c6f12d1 Stop allocating in fmt::Write for nsA[C]String, r=froydnj

I confirm the fix resolved the regressions: == Change summary for alert #9704 (as of September 27 2017 19:19 UTC) == Improvements: 66% Stylo Servo_DeclarationBlock_GetPropertyById_Bench linux32 opt 647,356.50 -> 219,750.67 62% Stylo Servo_DeclarationBlock_GetPropertyById_Bench windows7-32 opt 585,077.29 -> 225,259.44 60% Stylo Servo_DeclarationBlock_GetPropertyById_Bench linux64 opt 470,252.83 -> 188,702.18 59% Stylo Servo_DeclarationBlock_GetPropertyById_Bench windows10-64 opt 445,805.33 -> 184,229.44 55% Stylo Servo_DeclarationBlock_GetPropertyById_Bench osx-10-10 opt 456,356.33 -> 203,380.33 For up to date results, see: https://treeherder.mozilla.org/perf.html#/alerts?id=9704