# 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)