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Fix Vulkan capture perf collapse: read back off the present thread

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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>
This commit is contained in:
BlackMark
2026-06-23 08:19:47 +02:00
parent 60957775b7
commit 304857dcf0
13 changed files with 1564 additions and 799 deletions
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15
tests/CMakeLists.txt
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@@ -232,6 +232,20 @@ target_link_libraries(opengl_hook_test PRIVATE
add_test(NAME opengl_hook_test COMMAND opengl_hook_test)
# Reproducer + regression guard for the Vulkan capture performance collapse (the ~370 ms/frame
# present-thread stall that dropped a 144 FPS game to ~3 FPS). Reuses the shipping VkCapture; builds
# a known image on a real device and asserts the fixed present-thread cost is a small fraction of a
# synchronous read-back (and the captured image is byte-correct). `--sync` re-demonstrates the
# regression (the assertion fails on the synchronous design). Skips without a Vulkan ICD.
add_executable(vk_capture_perf_test
vk_capture_perf_test.cpp
${CMAKE_SOURCE_DIR}/hook/src/vk_capture.cpp)
target_include_directories(vk_capture_perf_test PRIVATE
${CMAKE_SOURCE_DIR}/hook/src
${CMAKE_SOURCE_DIR}/third_party/Vulkan-Headers/include)
target_link_libraries(vk_capture_perf_test PRIVATE coop_common d3d11 dxgi)
add_test(NAME vk_capture_perf_test COMMAND vk_capture_perf_test)
# Headless UI-fit check (M1): drives the real Controllers + Audio panels at reference
# resolutions with Debug details on and the richest content, and asserts no panel overflows
# its assigned size. Pure ImGui layout (no GPU / window). The Audio panel renders in its
@@ -278,4 +292,5 @@ coop_output_subdir(tests
opengl_hook_test
mock_game_test
audio_verify_test
vk_capture_perf_test
ui_fit_test)