feat(vegas): prefetch_gate on by default, after an A/B/C soak on hdpi

Two runs per arm, about 81,000 frames each, order A B C C B A:

  A  step 1 as is            0.90% late, 20.1 per 10k two+ refreshes late
  B  switch_interval_ms 1    0.78% late, 15.8 per 10k
  C  prefetch_gate           0.60% late,  2.5 per 10k

No freezes in any arm, and the next group was ready at every strip
extension, so parking the prefetch thread (3-6s per 8-minute run) cost
nothing visible. The gate is now on unless vegas_scroll.prefetch_gate is
false; on a stock binding it cannot work and says so at INFO once a run
rather than warning on every install. switch_interval_ms stays off.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/CONFIG_REFERENCE.md

@@ -128,6 +128,8 @@ Read by `src/vegas_mode/config.py` (`VegasScrollConfig.from_config`). See
 | `min_cut_gap` | int, `6` |
 | `continuous_scroll` | bool, `true` |
 | `offscreen_prefetch` | bool, `true` — render every plugin's ticker content on the background thread, each on its own canvas. `false` restores handing canvas-bound plugins to the render thread, one pause at a time. Temporary; see [OFFSCREEN_RENDERING.md](OFFSCREEN_RENDERING.md) |
+| `prefetch_gate` | bool, `true` — let that background thread run Python only while the render thread is waiting for the panel, so the render thread never waits for the GIL when a refresh comes round. Only takes effect with the rebuilt rgbmatrix binding (`scripts/build_rgbmatrix_nogil.sh`). See [OFFSCREEN_RENDERING.md](OFFSCREEN_RENDERING.md) |
+| `switch_interval_ms` | float, `0` — experimental: shorten Python's GIL switch interval to this many ms while Vegas runs. `0` leaves the default (5 ms) alone |
 | `smooth_scroll` | bool, `true` — move a whole number of pixels per panel refresh, locked to vsync. `scroll_speed` is snapped to the nearest speed the panel can show that way (at 95Hz: 95, 47.5, 31.7 px/s…), measured against the panel's real refresh rate once scrolling starts |
 | `sub_pixel_blend` | bool, `false` — the older smoothing: advance by elapsed time and blend neighbouring pixel columns. Looks anti-aliased in the web preview but shimmers on the panel and is not locked to the refresh. Overrides `smooth_scroll` when on |
 | `extend_threshold_screens` | float, `2.0` |

docs/OFFSCREEN_RENDERING.md

@@ -21,6 +21,28 @@ moved to the prefetch thread still needs the GIL, and the render thread waits
 for it (risk 5 below). The late rate did not improve overall. The 1–2 s
 freezes appear in both builds and have a separate, not yet identified cause.
 
+The GIL fix, measured on hdpi (90 px/s, `pwm_bits` 8, preview open, 8-minute
+runs after a 2-minute warm-up, order A B C C B A, 2026-09-24). Each arm pools
+two runs, about 81,000 frames:
+
+| arm | late | by 1 | 2 | 3–5 | 6+ | 2+ late per 10k frames | freezes |
+|---|---|---|---|---|---|---|---|
+| A: step 1 as is | 0.90% | 575 | 64 | 91 | 9 | 20.1 | 0 |
+| B: `switch_interval_ms` 1 | 0.78% | 510 | 105 | 23 | 2 | 15.8 | 0 |
+| C: `prefetch_gate` | **0.60%** | 471 | 11 | 7 | 2 | **2.5** | 0 |
+
+The gate removes the frames the render thread spent waiting for the GIL, and
+it costs the prefetch nothing that shows: it parked the thread for 3–6 s per
+run, and the next group was ready at every strip extension in every arm.
+`prefetch_gate` is therefore on by default; `switch_interval_ms` stays an
+off-by-default experiment. What is left is almost all one refresh late, which
+is the per-frame budget (a 6.75 ms p50 blit in a refresh the panel holds at
+83–85 Hz while rendering), not contention.
+
+The runs restart the service, so the hourly sports refresh never fell inside
+one. That refresh is its own case: about twenty ESPN chunk-fetch threads at
+once, which the gate does not cover (it gates only the prefetch thread).
+
 ## The problem
 
 Vegas mode builds its ticker from every plugin's content. Most of that work
@@ -285,9 +307,8 @@ updates are disabled while sync is active.
    it, and a waiting thread only gets it back after the switch interval
    (default 5 ms). Expect some single-refresh late frames while a prefetch
    runs. Measure with the soak. A render process separate from plugin work
-   is the structural answer (the "native presenter" step). Two opt-in
-   experiments try to get most of the way first, both off by default until
-   the soak says otherwise:
+   is the structural answer (the "native presenter" step). Two experiments
+   get most of the way first (results under Status, above):
    - `vegas_scroll.switch_interval_ms` lowers the switch interval for a Vegas
      run (1 ms is the obvious try), so the render thread waits at most that
      long behind bytecode. It does nothing for a C call that keeps the GIL.
@@ -297,7 +318,8 @@ updates are disabled while sync is active.
      parks it the rest of the time. That covers C calls too, since the gate is
      checked before each one starts. It never parks the thread while it holds
      a lock the render thread takes, and never for more than 50 ms. It needs
-     the rebuilt binding, which releases the GIL during the swap.
+     the rebuilt binding, which releases the GIL during the swap. On by
+     default.
 
 ## What this does not fix
 

src/vegas_mode/config.py

@@ -87,16 +87,17 @@ class VegasModeConfig:
     # 5ms. Plugin rendering on the prefetch thread and plugin updates hold the
     # GIL in Pillow and Python code, and a frame waiting its turn for 5ms at a
     # time misses its refresh. 0 leaves the interpreter default alone.
-    # Experimental: measured with scripts/frame_soak.py before it gets a default.
+    # Experimental. On hdpi it did less than prefetch_gate (0.90% -> 0.78% late
+    # against 0.60%; see docs/OFFSCREEN_RENDERING.md), so it stays off.
     switch_interval_ms: float = 0.0
 
     # Let the prefetch thread run Python only while the render thread is
     # blocked waiting for vsync, and park it the rest of the time, so the
     # render thread never waits for the GIL when its refresh comes round. Needs
-    # a binding that releases the GIL in SwapOnVSync; ignored otherwise.
-    # Experimental: see src/common/render_gate.py and measure with
-    # scripts/frame_soak.py before it gets a default.
-    prefetch_gate: bool = False
+    # a binding that releases the GIL in SwapOnVSync; off otherwise. On hdpi
+    # it cut frames two or more refreshes late eightfold, and late frames
+    # overall from 0.90% to 0.60%. See src/common/render_gate.py.
+    prefetch_gate: bool = True
 
     # Keep one continuous strip, extending it with the next group of plugins as
     # the scroll approaches the end, instead of composing a fresh strip and
@@ -229,7 +230,7 @@ class VegasModeConfig:
             continuous_scroll=vegas_config.get('continuous_scroll', True),
             offscreen_prefetch=bool(vegas_config.get('offscreen_prefetch', True)),
             switch_interval_ms=float(vegas_config.get('switch_interval_ms', 0.0) or 0.0),
-            prefetch_gate=bool(vegas_config.get('prefetch_gate', False)),
+            prefetch_gate=bool(vegas_config.get('prefetch_gate', True)),
             extend_threshold_screens=float(
                 vegas_config.get('extend_threshold_screens', 2.0)),
             auto_trim=vegas_config.get('auto_trim', True),

src/vegas_mode/coordinator.py

@@ -340,12 +340,14 @@ class VegasModeCoordinator:
         if getattr(self.display_manager, 'render_gate', None) is not None:
             return
         releases = render_gate.swap_releases_gil()
+        if releases is None:
+            logger.debug("Vegas: no prefetch gate -- no hardware binding loaded")
+            return
         if not releases:
-            logger.warning(
-                "Vegas: prefetch_gate ignored -- %s",
-                "this rgbmatrix binding keeps the GIL in SwapOnVSync "
-                "(scripts/build_rgbmatrix_nogil.sh)" if releases is False
-                else "no hardware binding loaded")
+            # On by default, so this is every stock install: say so once per
+            # run, not as a warning.
+            logger.info("Vegas: no prefetch gate -- this rgbmatrix binding keeps "
+                        "the GIL in SwapOnVSync (scripts/build_rgbmatrix_nogil.sh)")
             return
         gate = render_gate.RenderGate()
         # Locks the render thread takes too: never park the prefetch holding one.

test/test_render_gate.py

@@ -311,10 +311,16 @@ class TestVegasWiring:
         c.display_manager = type("DM", (), {"render_gate": None})()
         return c
 
-    def test_off_by_default(self, monkeypatch):
+    def test_on_by_default(self, monkeypatch):
         monkeypatch.setattr(render_gate, "swap_releases_gil", lambda: True)
         c = self._coordinator()
         c._install_render_gate()
+        assert isinstance(c.display_manager.render_gate, RenderGate)
+
+    def test_can_be_turned_off(self, monkeypatch):
+        monkeypatch.setattr(render_gate, "swap_releases_gil", lambda: True)
+        c = self._coordinator(prefetch_gate=False)
+        c._install_render_gate()
         assert c.display_manager.render_gate is None
 
     def test_installed_for_the_run_and_removed_after(self, monkeypatch):
@@ -338,12 +344,12 @@ class TestVegasWiring:
 
     def test_read_from_config(self):
         from src.vegas_mode.config import VegasModeConfig
-        on = VegasModeConfig.from_config(
-            {"display": {"vegas_scroll": {"prefetch_gate": True}}})
-        assert on.prefetch_gate is True
-        assert on.to_dict()["prefetch_gate"] is True
+        off = VegasModeConfig.from_config(
+            {"display": {"vegas_scroll": {"prefetch_gate": False}}})
+        assert off.prefetch_gate is False
+        assert off.to_dict()["prefetch_gate"] is False
         assert VegasModeConfig.from_config(
-            {"display": {"vegas_scroll": {}}}).prefetch_gate is False
+            {"display": {"vegas_scroll": {}}}).prefetch_gate is True
 
     def test_the_prefetch_runs_inside_the_gate(self):
         from src.vegas_mode.render_pipeline import RenderPipeline