Perfetto’s vsync-app, vsync-sf, and app doFrame tracks can look like the same event. This article follows a frame request from SurfaceFlinger scheduling to the app, then relates it to RenderThread, buffers, and FrameTimeline.

The examples mainly concern 120 Hz operation. Phase settings, hardware-callback enablement, and pending frame requests affect the tracks; separate those meanings before interpreting latency.

Version note (reviewed October 2026): Java, JNI, and scheduling code are checked against android-16.0.0_r1; app/appSf roles also reference Android 13 source. Historical screenshots are not universal Android 16/17 configurations. Excerpts omit surrounding state where noted and come from different classes, not one compilable program.

During a scrolling slowdown, the main thread may be doing layout or may already have handed drawing work to RenderThread. This article follows their hand-off, buffer submission, and SurfaceFlinger activity in Perfetto screenshots to locate waits.

The focus is the hardware-accelerated View/HWUI path. Other engines and software drawing can use different threads. FrameTimeline and presentation-side evidence are needed to determine what reached the screen.