The GL renderer may skip SDL_GL_SwapWindow when the displayed rect is
unchanged, so a double-buffered 30 Hz game does not swap an identical
front buffer on the intervening vblank. The frontend calls present once
per guest vblank, so eliding a swap also elides whatever that swap was
blocking on.
That is only safe while the wall-clock pacer owns cadence, because the
pacer runs per present regardless of what the renderer does. When driver
vsync owns cadence instead (~60 Hz panel, present_vsync_owns_cadence()),
the pacer is deliberately not run -- the two are XOR to avoid
double-blocking -- and the swap block becomes the only throttle. An
elided frame then blocks on nothing, and a game presenting every other
vblank advances two guest vblanks per block: exactly 2x real time.
Measured on BreathOfFire3Recomp (SLPS-00990, 60 Hz panel, OpenGL):
guest vblank counter sampled against wall clock ran 120.4 fps / 2.01x
before, 59.8 fps / 0.997x after. Swaps per second are unchanged at 60
either way -- the fix removes a free-running guest frame, not a swap --
and per-frame GPU cost is unchanged (present_gpu_ms_avg 12.2 -> 12.6,
scene_gpu_ms_avg 4.5 -> 4.1).
Gate both skip sites (VRAM and wide-FBO paths) on the cadence owner,
alongside the existing OSD and interpolation-cadence guards. Only the GL
renderer has this optimisation; the software and Vulkan paths always
present and were never affected.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Root cause (freeze dump 1781046069, addr2line-verified): the wall-clock
pacing loop in sdl_vblank_present read the performance counter twice per
iteration — once in the while-condition, once to compute remaining time.
If the frame deadline was crossed between the two reads (a preemption
window), the unsigned subtraction underflowed, *1000 overflowed, and
SDL_Delay received ~2^64/10^4 ms mod 2^32 ≈ 24.7 days at the standard
10 MHz QPC. The app parks in one SDL_Delay with every counter frozen,
wedged between 'i_stat |= VBLANK' (interrupts.c:286) and exception
delivery — which is why dumps showed i_stat=1 pinned. VBLANK was being
ACK'd every frame right up to the wedge in every dump examined; the
'VBLANK-ACK reentry storm' framing was a symptom misread.
- frame_pacing.c/h: shared race-free pacer (single counter read per
decision, compare-before-subtract, sleep clamped to one period).
Both call sites switched: main.cpp sdl_vblank_present and
beetle_main.cpp. Unit test pins the underflow case.
- Vsync self-heal: the 'recovered storm' dump (1781045865) was 8/8
main-thread samples inside wglSwapBuffers — NV driver blocking ~1.5s
per present. After 3 presents >250ms, drop driver vsync (we pace to
59.94 Hz ourselves). Counters surfaced in the freeze dump.
- Watchdog classifier: 'reentry_storm' threshold (5K/sec) was below the
healthy baseline (~2K reentry blocks/frame × 30-60 fps), mislabeling
normal play and host stalls — that label misdirected two prior
investigations. Now normalized per frame (>20K/frame), plus a new
slow_frames kind (<5 fps) with multi-sample stack capture.
Verified: test_frame_pacing ALL PASS; live Tomba paced at 59.962 fps
over 20s (single-process measurement).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Matthew Stanley