Anthony Eid 297c4a4d78 Bench app context phase 2 (#58202) ...

This PR builds out GPUI's benchmark harness so render benchmarks measure
realistic frame costs using GPUI-owned, runtime-gated instrumentation.
It adds measurements for frame draw time, dirty-to-draw latency,
invalidation coalescing, and frame-budget overruns, and runs benchmark
workloads with production-like concurrency.

## How it works

**Frame timings flow through the GPUI profiler.** `Window::draw` emits a
`FrameTiming { window_id, dirty_at, invalidations, draw_start, draw_end
}` event into a global ring buffer in `gpui::profiler`, mirroring the
existing task-timing channel. Collection is runtime-gated by
`profiler::set_frame_trace_enabled` (one relaxed atomic load when
disabled — no `Instant::now` calls in production). `BenchReport` is a
pure listener: it drains events through a cursor-based
`FrameTimingCollector` and builds histograms in the bench layer.
`Window` carries no bench-only cfg fields, and the same event channel
can later feed the miniprofiler UI or an in-app frame-time HUD.

**`BenchDispatcher`** is a multithreaded `PlatformDispatcher` for
benchmarks: background tasks run on a worker pool (same priority queue
as `LinuxDispatcher`, with task-profiler hooks), timers fire in real
time on a dedicated thread, and foreground tasks queue until the bench
thread drains them with a blocking `run_until_idle()`. Unlike
`TestDispatcher`, work executes in parallel in real time, so wall-clock
measurements reflect production concurrency. In-flight accounting is
panic-safe via drop guards.

**`gpui::bench_platform()`** returns a per-process `TestPlatform` backed
by the `BenchDispatcher`, cached in a thread-local so worker threads
persist across Criterion calibration passes. This replaces the earlier
approach of constructing a real platform per invocation, which had
process-global singleton issues, never ran foreground tasks (no run loop
pumped the main queue), and couldn't open windows on headless CI.

**Text shaping** uses `NoopTextSystem`: deterministic across
machines/font installations and CI-portable. Measured cost of this
trade: ~10% of editor draw time vs `MacTextSystem` (Noop still emits one
glyph per character at fixed advances, so downstream layout/paint
structure is preserved).

**GPU coverage (macOS only for now).** `PlatformHeadlessRenderer` gained
`render_scene`, which encodes and submits the scene to Metal against a
cached offscreen target without blocking on completion or reading pixels
back — matching production `present()` CPU cost (`render_scene_to_image`
would overstate it: it waits for the GPU and copies pixels back).
`TestWindow::draw` forwards scenes to the renderer, the real
`MetalAtlas` means glyph/SVG rasterization happens during paint, and
`bench_renderer` presents after each measured update. Platforms without
a headless renderer degrade to discarding the scene.

## Example

```rust
#[gpui::bench]
fn editor_render(cx: &mut BenchAppContext) {
    init_context(cx);

    let buffer = cx.update(|cx| { /* build a MultiBuffer */ });

    let mut window = cx.add_empty_window();
    let editor = window.update(|window, cx| {
        let editor = window.replace_root(cx, |window, cx| {
            let mut editor = Editor::new(EditorMode::full(), buffer, None, window, cx);
            editor.set_style(editor::EditorStyle::default(), window, cx);
            editor
        });
        window.focus(&editor.focus_handle(cx), cx);
        editor
    });

    let mut move_down = true;
    cx.bench_renderer(editor, move |editor, window, cx| {
        if move_down {
            editor.move_down(&MoveDown, window, cx);
        } else {
            editor.move_up(&MoveUp, window, cx);
        }
        move_down = !move_down;
    });
}
```

## Example output (release, M-series)

```
editor_render           time:   [329.75 µs 330.17 µs 330.69 µs]

GPUI bench report (all observed iterations): editor_render
  note: includes Criterion warmup/calibration
  window dirty-to-draw:
    samples: 31533
    mean: 0.321ms
    p50: 0.322ms
    p90: 0.336ms
    p95: 0.342ms
    p99: 0.360ms
    max: 0.504ms
    frame budget overruns total: 0
    frame budget overruns max: 0
  window draw:
    samples: 31533
    mean: 0.295ms
    p50: 0.295ms
    p90: 0.307ms
    p95: 0.313ms
    p99: 0.330ms
    max: 0.455ms
    frame budget overruns total: 0
    frame budget overruns max: 0
  invalidations per frame: mean 5.00, max 5
```

(`invalidations per frame: mean 5.00` is real signal: each `move_down`
notifies the window five times before the draw.)

## Known limitations

- **Draw-per-flush**: the harness draws synchronously when effects flush
rather than coalescing invalidations to a vsync tick, so `dirty-to-draw`
excludes queueing delay, and `frame budget overruns` is a draw-time
budget proxy rather than actual missed presents. A frame-paced mode is
natural follow-up work.
- **GPU submission is measured on macOS only**; other platforms have no
headless renderer yet.
- The GPUI report includes Criterion warmup/calibration samples (noted
in the output); Criterion's `time` is the regression-gating number.
- `run_until_idle` waits for queued, running, and already-due work, but
not for timers that haven't reached their due time — the dispatcher runs
in real time and can't skip ahead like `TestDispatcher`'s virtual clock.

## Future work

- A vsync-like frame-pacing mode (suppress draw-on-flush; tick-driven
draw + present) so dirty-to-draw captures queueing delay
- Record present duration in `FrameTiming` so the report can split draw
vs present
- Benches that scroll through novel content (cold layout caches) and an
agent-panel render bench
- Headless renderers for Windows/Linux
- Move benches into a dedicated crate

Self-Review Checklist:
- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the UI/UX checklist
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A

2026-06-10 09:26:50 +00:00

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benches

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src

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Cargo.toml

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LICENSE-GPL

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