BlackMark 304857dcf0 Fix Vulkan capture perf collapse: read back off the present thread ...

Against Sphere Spectacle (144 FPS, runs without Steam) the implicit-layer
capture dropped the game to ~3 FPS. Measured cause (per-stage trace in the
layer): the read-back ran on the game's PRESENT THREAD and spent ~370 ms per
1080p frame -- not the GPU copy (~2 ms) but the CPU swizzle, because the staging
buffer was a plain HOST_VISIBLE|HOST_COHERENT type (write-combined / uncached on
a discrete GPU), where a scattered CPU read runs at PCIe latency. 3 captures/s =
the 3 FPS the user saw.

Test-first: tests/vk_capture_perf_test reproduces the stall as a deterministic
unit test (372 ms/present, ratio 1.0 -> FAIL via `--sync`), then proves the fix
(0.02 ms/present, byte-correct BGRA->RGBA, ratio ~0 -> PASS).

Fix: extract the near-identical read-back from vk_hook.cpp and coop_vk_layer.cpp
into one shared coop::hook::VkCapture that:
  * has the present thread only record + submit the copy (sub-ms) and return;
  * runs a dedicated reaper thread for the fence wait + swizzle + D3D upload, off
    the critical path, with a ring of in-flight slots (game never waits);
  * allocates HOST_CACHED staging (fast CPU read), invalidating when non-coherent;
  * throttles capture to ~150 Hz (a guest stream is <= the host refresh; no point
    mirroring an uncapped 400+ FPS game and burning reaper CPU).

Real-game A/B: present rate now matches the no-capture baseline (605->470 vs
593->405 over the same ramp) with the mirror at ~130 fps -- no measurable impact.

Also adds present-thread overhead guards to the other GPU backends' hook tests
(present_overhead.hpp): DX11 0.05 ms, DX12 0.34 ms, OpenGL 0.09 ms overhead, all
asserted < one 60 Hz frame, so any future synchronous-stall regression fails.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

2026-06-23 08:19:47 +02:00

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