The Choreographer vsync callback reaches the UI thread only 2.5-6 ms
AFTER the vsync edge (measured on the phone). With a 2.5-5 ms Skia
render plus present, that left no phase margin inside the 8.3 ms slot:
51% of frames landed one slot late (judder) and ~4% two slots late
(hitches), on every build tested so far.
Attempted fixes, all measured and retired (see commit history):
- render-ahead hold (present previous frame at next render start):
the held present lands after the NEXT wake's 3.2-6 ms Choreographer
latency -> 52% one-slot late.
- present-at-end + present GATE (sleep on the UI thread to quantize
the present to the vsync grid): the gate sleep (2.5-4 ms) added to
the slot budget (wake 5.3 + core 3 + gate 3 + present 1 = 12.3 ms >
8.3 ms) -> cadence collapsed to 60 Hz.
Working fix (this commit):
- vsync.rs: an 'early waker' thread learns the vsync grid from the
callback's frame_time_nanos (vkprobe::note_vsync_edge) and wakes the
main looper ~0.8 ms BEFORE each edge while animating. Render start
moves from edge+2.5..6 to edge-0.8..+0.5, so the present lands at a
natural ~edge+3..5 ms phase with no UI-thread sleep.
- lib.rs: a render freshness guard (min gap = period - 1.5 ms) makes
the FIRST wake of each slot (early or the late Choreographer
fallback) render and rejects the second (double render = 60 Hz
collapse). Threshold sits between late-after-early (5.8 ms, reject)
and early-after-late (7.5 ms, accept).
- lib.rs: locked renders no longer consume pending_redraw (it carried
the motion invalidations; consuming it starved the follow-up
unlocked iteration -> 60 Hz collapse).
- stg.rs: IT lines now log the render decision (locked/saw_wake/
pr_before/do_render); vkprobe logs gate sleep (GATE_US, now 0).
Measured (phone, 8580-file grid, slow 4 s drags):
in-burst cadence: 8.29-8.44 ms median, p90 8.6-10.2 ms
(was 8.36 median / 10.2 p90 with 3.7-20% two-slot gaps)
in-burst >16.7 ms gaps: 0-2 per burst (was 3-13)
present-to-present: 27% exact one slot, 33% one-slot-late steady
state (imvisible), 3% f-jumps (Skia core 2.5-5 ms variance).
Residual judder lever: Skia core cost variance (next step).
Test hygiene reminders (HANDOFF 8.10): verify ResumedActivity, keep
the task clean (sz=1), record the perf-log byte offset before each
run (the file accumulates across process launches).
Three root causes, all fixed and verified on the Pixel 9 Pro:
1. Present phase (backend + renderer): VSYNC_LOCKED_RENDER defers the
render to the vsync wake; RENDER_AHEAD presents the previous frame at
the start of render() (phase = wake + ~1.1 ms) and holds the new
frame one slot. Crash fix: a held wgpu frame must be PRESENTED, never
merely dropped, before the next acquire (AlreadyAcquired panic at
launch otherwise); discard_pending_frame presents too.
2. Full-res prefetch churn (src/lib.rs): the 8-slot full cache cycled
through a ~20-cell viewport forever (137 decodes/20 s) and each
eviction free()'d a 50 MB Vec on the UI thread (20-52 ms CPU spikes,
~7/s = the residual SLOWEV NonMain jank). Fix: 60 s tombstone
cooldown on evicted paths (idle prefetch skips, urgent tap bypasses)
+ off-thread reaper for the freed blobs (insert: 21 ms -> 4 us).
3. (earlier) Skia final flush + Mailbox present mode (vendor, M3).
Instrumentation (all behind --features perf): OTHR/INVK/INVK_TAG/FLCK
lines for cross-thread closure timing and push-site tagging.
Warm measured swipe, 8580-file grid: cadence median 8.33 ms (exact
120 Hz), p99 15.5 ms, 5/552 gaps >16.7 ms (0.9%, max 28.3 ms, zero
>33 ms); PRES phase median ~994 us, zero late frames. Docs updated
(docs/BUGS/scroll-jank-120hz.md, full scorecard + line reference).
Also: vendored i-slint-core + wgpu-hal (vkshim interposition patch),
vkshim profiling tooling, backend made a non-optional dep, debuggable
flag for simpleperf (REMOVE before shipping).
The 19-26 ms mid-scroll stalls (Mali-only, fixed content offsets) were
the Skia renderer's final synchronous GrDirectContext::flush() blocking
the UI thread on Mali driver back-pressure — proven by per-stage
timestamps in the vendored renderer (stall frames: flush 19-24 ms,
walk 1-3 ms, 1 binding re-eval, image imports 0.1 ms each).
- vendor/i-slint-renderer-skia (pinned 1.18.1, [patch.crates-io]):
remove that one flush. Redundant on the wgpu backend: the surface
already calls flush_and_submit before queue.present, same Vulkan
queue, in-order. Phone: 0 stalls across 4 runs (max 3.6-7.4 ms vs
the 8.3 ms budget), display verified clean.
- docs/BUGS/slint-skia-final-flush-jank.md: full evidence, prior art
(slint-ui/slint#12030 closest), open question for a future
upstream filing (not filed yet).
- Profiling also retired the O(n)-walk theory: Slint 1.18 prunes the
item walk to ~133-152 items; the windowed Flickable built on that
theory was already reverted (ListView is smooth with the fix).
- Strip all jank-hunt instrumentation: FR/SCR/TICK/APLYD/STG logging,
stg markers, .gallery_* file knobs, jsamp thread sampler,
DbgScroll programmatic scroll, nblog perf-file mirroring.
Also fixes the 'launches into unstoppable fast scroll' bug: a
leftover .gallery_cmd was re-read on every launch (file never
deleted) and overrode user swipes — the hook is gone.
- Keep: nblog (logd backpressure is real), zero-dimension decode
guards, keep-alive (default-off).
- HANDOFF: replace stale windowed-Flickable section with the actual
root cause/fix; 15 unit tests pass.
Verified on phone (aarch64, 8580 photos) and emulator (x86_64).
