Nabla Path Tracer runtime compare 2026-03-28

Nabla Path Tracer runtime compare from Nsight Graphics

This directory contains one paired Nsight Graphics GPU Trace probe for Nabla Path Tracer.

Protocol:

• 1 run per variant
• same capture point: frame 1000
• same effective render path:
  • geometry: sphere
  • effective method: solid angle
• runtime numbers below come directly from Nsight Graphics exports:
  • FRAME.xls
  • GPUTRACE_FRAME.xls
• measurement machine:
  • measurement_machine.md

Variant matrix

Case	Checkout source	Nabla	DXC	SPIRV-Headers	SPIRV-Tools	Mode
master_source_off	master_runcheck local worktree	e11b118dd2e80393b5b7eb309c6abb25f51a818c	d76c7890b19ce0b344ee0ce116dbc1c92220ccea	057230db28c7f7d1d571c9e61732da44815f2891	91ac969ed599bfd0697a5b88cfae550318a04392	local Release, SOURCE, runtime builtins OFF
devshfixes_upstream	unroll_dxc_df_upstream_check local worktree	c13c33662c3733b54d9014988a5ac602ab0c3245	74d6fbbad7388813c65ae269b20f15b4e971df9c	10b37414a3c9269b9bd8861cc759bd7fdf09760d	2c75d08e3b31a673726ce6be80ab528250247064	local Release, SOURCE, runtime builtins OFF
unroll_artifact	CI install artifact from run 23599197849	262a8b72f295ec95d3cf83170f1768a43972c9ab	07f06e9d48807ef8e7cabc41ae6acdeb26c68c09	c141151dd53cbd5b1ced0665ad95ae3e91e8f916	2a730e127a32ac8b0713f5e1490d7b9be9d1cc9a	CI Release install artifact
unroll_v2	unroll_o1_local local worktree after the latest -O1experimental changes	6ee8dbc04df55db97c9440d078eef160522a6af1	891d1d7bd6fb20757a3af07f5a7a33ef59f7c15e	c141151dd53cbd5b1ced0665ad95ae3e91e8f916	0ecbcc95a108f1a3313ea184260b10d21e158a47	local RelWithDebInfo, SOURCE, runtime -O1experimental

Legend

flowchart LR
  A["master_source_off\nNabla e11b118d\n2026-03-26"] --> B["devshfixes_upstream\nNabla c13c3366\n2026-03-28"] --> C["unroll_artifact\nNabla 262a8b72\n2026-03-26"] --> D["unroll_v2\nlocal O1experimental refresh\n2026-03-28"]

This report compares four checkpoints:

• master_source_off: current master-side baseline
• devshfixes_upstream: the same line after refreshing devshFixes with newer DXC upstream state
• unroll_artifact: that refreshed line plus the unroll PR work packaged in the published CI artifact
• unroll_v2: an up-to-date local measurement with the new -O1experimental flag after the latest unroll-line changes

Runtime Probe

Variant	GPU frame ms	Dispatch count	Compute active	SM throughput	PCIe write GB/s
master_source_off	21.4304	2	83.2501%	35.5388%	2.62710
devshfixes_upstream	19.6157	2	82.9923%	38.2916%	2.64694
unroll_artifact	21.5935	2	83.9945%	34.3346%	2.62212
unroll_v2	19.2360	2	86.9712%	38.7311%	2.64514

Runtime deltas

Comparison	Delta ms	Delta %
devshfixes_upstream vs master_source_off	-1.8147	-8.47%
unroll_artifact vs master_source_off	+0.1631	+0.76%
unroll_artifact vs devshfixes_upstream	+1.9778	+10.08%
unroll_v2 vs master_source_off	-2.1944	-10.24%
unroll_v2 vs devshfixes_upstream	-0.3797	-1.94%
unroll_v2 vs unroll_artifact	-2.3575	-10.92%

Cold startup Vulkan API probe

Cold startup vkCreateComputePipelines was measured on the same published runnable bundles with cleared pipeline/shader cache and Vulkan API tracing enabled for the process.

Variant	vkCreateComputePipelines calls	vkCreateComputePipelines start->next sum ms
master_source_off	13	3737.11
devshfixes_upstream	21	3332.55
unroll_artifact	21	1418.86
unroll_v2	2	354.707

TODO: need to recheck vkCreateComputePipelines, those are wrong metrics

Main conclusion

The measured latest upstream refresh baseline is faster than master_source_off in this probe. At the same time unroll_artifact is effectively at parity with master_source_off here at only +0.76%, while the remaining gap appears only against devshfixes_upstream.

The up-to-date unroll_v2 follow-up, measured with the new -O1experimental flag, goes further: in this probe it is now faster than master_source_off by 10.24% on steady-state GPU frame time (19.2360 ms vs 21.4304 ms).

Taken together, the measured runtime cost points at the unroll side of the experiment, not at the generic DXC/SPIRV-Tools upstream refresh. That tradeoff is also aligned with the intent of the experiment: reduce shader build time aggressively while accepting a small runtime cost.

In practice this is also a strong argument for the new explicit -O1experimental path. For the Nabla Path Tracer builds behind this comparison the shader-build wall time is about 10x worse without -O1experimental, while the newest unroll_v2 follow-up is already faster than the current master baseline on this measured path. On this workload -O1experimental delivers the intended development tradeoff directly: a major build-time win together with favorable measured runtime.

unroll_v2 is the current local follow-up checkpoint after those latest changes. It keeps the same high-level workload shape (dispatch_count = 2) and shows where the updated -O1experimental line lands relative to the published unroll_artifact and the current master baseline.

Deeper Nsight signals from the same exports

Frame-level exports also show:

• dispatch_count = 2 and gr__ctas_launched_queue_sync.sum = 14401 in all three variants
• unroll_artifact has lower SM throughput than devshfixes_upstream
• unroll_artifact also shows higher total executed instructions and much higher L1/LSU/shared pressure than devshfixes_upstream
• unroll_v2 raises SM throughput back to 38.7311% while keeping dispatch_count = 2

This points at a compute-side codegen / execution-mix difference with higher L1/LSU/shared pressure on the unroll side.

Directory map

Runtime stats

• master_source_off/stats.json
• devshfixes_upstream/stats.json
• unroll_artifact/stats.json
• unroll_v2/stats.json

Machine spec

• measurement_machine.md

Executable locations

• master_source_off: runnable/master_source_off_minimal/31_hlslpathtracer.exe
• devshfixes_upstream: runnable/devshfixes_upstream_minimal/31_hlslpathtracer.exe
• unroll_artifact: runnable/unroll_artifact_minimal/31_hlslpathtracer.exe
• unroll_v2: runnable/unroll_v2_minimal/31_hlslpathtracer_rwdi.exe

Capture files

• master_source_off/run01/master_source_off_frame1000_run01.ngfx-capture
• devshfixes_upstream/run01/devshfixes_upstream_frame1000_run01.ngfx-capture
• unroll_artifact/run01/unroll_artifact_frame1000_run01.ngfx-capture
• unroll_v2/run01/unroll_v2_frame1000_run01.ngfx-capture

Raw Nsight exports

• master_source_off/run01/gpu-trace/BASE/FRAME.xls
• master_source_off/run01/gpu-trace/BASE/GPUTRACE_FRAME.xls
• devshfixes_upstream/run01/gpu-trace/BASE/GPUTRACE_FRAME.xls
• unroll_artifact/run01/gpu-trace/BASE/GPUTRACE_FRAME.xls
• unroll_v2/run01/gpu-trace/BASE/FRAME.xls
• unroll_v2/run01/gpu-trace/BASE/GPUTRACE_FRAME.xls

Cold startup Vulkan API trace

• startup_vkcreate_api_probe/api_dump_cold_compare/summary.csv
• startup_vkcreate_api_probe/api_dump_cold_compare/master_source_off/vkcreate_stats.json
• startup_vkcreate_api_probe/api_dump_cold_compare/devshfixes_upstream/vkcreate_stats.json
• startup_vkcreate_api_probe/api_dump_cold_compare/unroll_artifact/vkcreate_stats.json
• startup_vkcreate_api_probe/api_dump_cold_compare/unroll_v2/vkcreate_stats.json

Startup logs

• startup_devshfixes_upstream/stats.json
• startup_unroll_artifact/stats.json
• startup_unroll_v2/stats.json

Additional unroll_v2 diagnostics

• unroll_v2/diagnostics/build_runtime_rwdi.log
• unroll_v2/diagnostics/rebuild_spirv_rwdi.log
• unroll_v2/diagnostics/legality_rwdi_all_spv.log
• startup_unroll_v2/stdout.txt