Greg Pomerantz
c72bfa481d
Measurements only, no behavior change: - vsync.rs: EW lines (every early-waker landing, EW_ON flag, off by default) - lib.rs/stg.rs: IT lines gain dec (locked/saw_wake/pr_before/do_render) and pcpu (CPU us inside poll_events — 2% = asleep in epoll, rules out CPU spin/deschedule as the wake latency) Findings (docs/BUGS/hypotheses-120hz.md, sltest baseline): - Choreographer callback fires at edge+0.52ms (p90 0.65) — the 'late Choreographer' model from earlier sessions was a mislabeled STG field (poll block time, not callback latency). The pipeline actually works at median: present at edge+~4ms, inside the 5.5ms deadline. - sltest zero-jank definition measured: 98.8% of frames land in the next display slot; residual = 0.5-0.7% drops + 0.7-0.9% doubles + 1.4ms phase spread (p10-p90) = the visible micro-judder. - Residual cost is VARIANCE, not median: per-frame render 3.5ms median (SKD 1.07 + CLR 0.8 + PRE 1.0 + SKF/ORD 0.65) with spikes to 10.7ms (SKD max 5.2, CLR max 3.2, PRE max 2.5). Spikes push presents past the deadline (drops) and shake the phase (judder). Next: classify spikes (CPU work vs GPU/driver block) via render-window CPU splits, then kill the top source.
43 lines
2.2 KiB
Markdown
43 lines
2.2 KiB
Markdown
# 120 Hz zero-jank campaign — hypothesis log
|
|
|
|
Target: sltest (minimal 100-row grid, no textures) on the phone's Mali G715.
|
|
Definition of ZERO JANK (measured from PRES mon= + VSYNC ftn= lines):
|
|
1. no display-slot skip: for consecutive presents, slot_index(n+1) == slot_index(n)+1 (100%)
|
|
2. no same-slot double: two presents never land in the same display slot
|
|
3. phase stable: present phase within its slot varies < 0.5 ms
|
|
Test: 2x `input swipe 640 1900 640 900 4000` (slow drags) + fling, ~10 s.
|
|
A "burst" = consecutive presents with gaps < 20 ms; evaluate inside bursts only.
|
|
|
|
## Hypotheses (order = priority)
|
|
|
|
- H6 [sltest confound]: sltest FileLogger is blocking (flush per line) on the
|
|
render path (PRES/STG/IT at ~240 lines/s).
|
|
FIX: non-blocking drain-thread logger. TEST: rebuild, re-measure baseline.
|
|
- H1: early waker misses edges (thread descheduled / nanosleep jitter) →
|
|
render starts late (edge+5 late wake wins) → present phase bimodal.
|
|
TEST: render-start phase histogram vs edges; early-wake hit rate.
|
|
- H2: Skia render core variance → present phase jitter.
|
|
TEST: VK SKD distribution in sltest (trivial scene, expect <1.5 ms).
|
|
- H3: wgpu/Mali submit+present latency variance (CLR/SKF/ORD/PRE).
|
|
TEST: VK PRE / CLR / SKF distributions in sltest.
|
|
- H4: event-loop overhead (timers, change tracker, spin iterations) eats the
|
|
slot budget.
|
|
TEST: IT poll/timers/ev/fe distributions; iters/s during scroll.
|
|
- H5: CPU scheduling of the UI thread (deschedule mid-slot, core migration,
|
|
frequency).
|
|
TEST: wall vs CPU split for the render window (vkprobe thread_cpu_ns).
|
|
- H7 [method confound]: adb `input swipe` motion is bursty (~20-60 Hz) →
|
|
grid movement itself is non-uniform regardless of render cadence.
|
|
TEST: pointer-location overlay + user finger, later.
|
|
|
|
## Rules
|
|
- One hypothesis per build; keep the change minimal; measure before/after
|
|
with the identical test; record numbers here.
|
|
- A hypothesis is RULED OUT only when the measurement it predicts is absent
|
|
(not "the fix didn't help").
|
|
- Phone hygiene before each run: clean task (sz=1), ResumedActivity,
|
|
dismiss 16KB dialog (sltest is debuggable), record log byte offsets.
|
|
|
|
## Results
|
|
(append below)
|