perf(linux): app is capped at 60 fps because the GTK embedder has no vsync #980

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1. ZhuchkaTriplesix

   added

   performanceTheme parser epic label: performanceTheme parser epic label: performance

   motionAnimation system, transitions, motion tokensAnimation system, transitions, motion tokens

   on Sep 25, 2026
2. ZhuchkaTriplesix commented on Sep 25, 2026

   

   ZhuchkaTriplesix

   on Sep 25, 2026

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   Local test: engine 3.47.1 + vsync part of flutter/flutter#192342

   Built libflutter_linux_gtk.so (profile) from the 3.47.1 tag with only the production vsync changes of flutter/flutter#192342 (fl_engine.cc, fl_display_monitor.cc/.h). The Wayland subsurface part of that PR does not apply: 3.47.1 has no subsurface renderer yet. The patched library was swapped into a profile bundle of benchmark/app_perf_bench.dart; everything else identical. Hyprland, 120 Hz, window 2056×2616.

   Note: the patch is a display-rate timer (GTK exposes no vsync for the window), not a hardware vsync.

   Scenario	stock engine	patched engine
   Grid horizontal scroll	59 fps	119 fps
   Hover over grid	60 fps	120 fps
   Grid vertical scroll	60 fps	110 fps
   Tree scroll (1500 tables)	60 fps	112 fps
   Tree expand / collapse	60 fps	114 fps
   Sidebar toggle animation	60 fps	60 fps

   Sidebar stays at 60 fps

   Dart work per frame is ~4.5 ms (Animator::BeginFrame) + ~1.1 ms semantics, well under 8.3 ms, yet every other tick is missed. Experiment: wrapping the app in ExcludeSemantics (bench only) raises the sidebar to ~91 fps (median interval 8.33 ms, ~30 % of frames still late) with the same Dart cost. The sidebar animation changes the geometry of the whole semantics tree every frame; on Linux those updates are turned into ATK objects on the GTK main thread, which is also where the engine's vsync callback runs, so ticks are delayed. This ties into #981 (fewer semantics nodes) and #984 (sidebar relayout).

   Conclusion

   • The engine change works: Linux reaches the display rate for scroll, hover and tree interactions.
   • To ship it before upstream lands we would need our own engine build per Flutter upgrade (this build: gclient sync without Android / JDK / Fuchsia deps, gn --runtime-mode release, ninja libflutter_linux_gtk.so, ~4000 steps).
   • Semantics volume is the next limiter on Linux even with vsync.
3. ZhuchkaTriplesix commented on Sep 27, 2026

   

   ZhuchkaTriplesix

   on Sep 27, 2026

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   CI wiring: patched engine repo + splice script (PR #995)

   Turned the local test above into infra:

   • QueryaHub/flutter-engine-linux-120hz — new repo holding the vsync patch (patches/vsync-fl_engine-fl_display_monitor.patch, just the fl_engine.cc/fl_display_monitor.cc/.h part of Use vsync in Linux embedder flutter/flutter#192342), a scripts/build.sh/publish_release.sh pair to rebuild it for a new Flutter version, and the first published build: release-and-profile libflutter_linux_gtk.so for engine content hash 11d79658c444477b06513d32b52c8c4ccb7276b0 (Flutter 3.47.1's engine).
     • Note: .github/workflows/build.yml in that repo (a workflow_dispatch job to rebuild in CI) is committed to the branch but not yet pushed — my token lacked the workflow OAuth scope needed to push files under .github/workflows/. Either grant that scope (gh auth refresh -s workflow) or add the file by hand from the branch; it's a straightforward, documented-but-unvalidated translation of the manual recipe used for this issue's benchmark.
   • querya-desktop#995: scripts/linux/apply_patched_engine.sh splices the matching release asset into build/linux/x64/release/bundle/lib/libflutter_linux_gtk.so after flutter build linux --release, keyed by the running Flutter SDK's engine content hash. No match → warns and ships the stock engine, never fails the build.

   Blocker before this actually changes what ships: our CI pins Flutter 3.41.6; the published patched build matches 3.47.1's engine (built against my local SDK while validating the fix). Need a call on:

   1. Bump CI's Flutter pin to 3.47.1 (then perf(linux): splice in vsync-patched engine on release builds (#980) #995's splice starts working immediately), or
   2. Build+publish a patched engine for 3.41.6's specific content hash instead (keeps the current Flutter pin).

   Leaving this issue open until one of those lands and a Linux release build actually ships at the display refresh rate (the original acceptance criteria).
4. ZhuchkaTriplesix

   added a commit that references this issue on Sep 27, 2026

   Merge pull request #995 from QueryaHub/issue/980-linux-vsync-patched-…

   ...

   d3352da
5. ZhuchkaTriplesix commented on Oct 8, 2026

   

   ZhuchkaTriplesix

   on Oct 8, 2026

   MemberAuthor

   More actions

   Done in #995
6. ZhuchkaTriplesix

   closed this as completedon Oct 8, 2026