# Linux71 — SurfaceWorld generation timing

## Like-for-like result

All runs used the same host and CPU affinity. Unreal and headless resolved plan hash `bcc0d0d33ea39435` and the exact same 2,850 tile keys at LOD0–8. The live asset trace still prints clip 6/max 6; the captured tile keys are identical to headless radius 3/max 8.

| Run | Tile workers | Hashing | Sum of Core/worker time |
|---|---:|---:|---:|
| L70 Unreal median | 8 slots | on | Core 42.69s; worker 42.91s |
| L70 headless | 12 | off | worker 20.06s |
| L71 Unreal median | 8 slots | off | Core 14.42s; worker 14.62s |
| L71 headless | 8 | off | worker 14.26s |

At matched W8 the Core sum differs by 1.2% from headless end-to-end worker time; Unreal worker time differs by 2.5%. The same-worker L71 per-LOD comparison is within about ±11%. Unreal’s initial-set-ready milestone is 4.71–4.83s; the 53s commandlet lifetime includes the trace’s `UntilEndPlay` sampling after all tiles finished. Headless whole-view readiness was 2.16s.

## Cause and tail

The L70 timing trace enabled `bCaptureVoxelForgeCoreHashesForValidation`, which set `TileGenerationOptions.hashDensity`. That diagnostic disables the native heightfield-band shortcut and whole-tile early skips. A direct W1 headless A/B on the same keys went from 9.00s/hash-off to 28.46s/hash-on: field calls rose from 5.46M to 33.82M and 654 whole-tile skips fell to zero. Turning hashing off in the Unreal trace reduced its Core sum 66%, from 42.69s to 14.42s. The no-hash profile shows LOD0 marching-cubes density-grid work as the largest near-field cost; LOD8 also spends time in MC, ground refinement and caps.

The L71 Core p50/p95/max is 1.78/17.56/130.52ms; worker p50/p95 is 1.80/17.97ms. LOD0 p50 is 0.50ms and p95 34.8–39.1ms; LOD1 is 0.60–0.64ms and 10.3–10.7ms. Most LOD0/1 tiles are empty (441/512 and 372/448); the nonempty passage tiles have p50/p95 of 18.8/105.1ms at LOD0 and 8.6/65.0ms at LOD1. The slow keys cluster around the spawn mouth and first passage bend. This is real per-tile generation variance. Queue p95 is separately 129.45ms, so it increases arrival latency but is not counted in `core_generate_s`.

## Changes and verification

The stream-trace commandlet now leaves hash capture off; dedicated hash/parity tests still enable it. This changes no world data or hashes. No shared Core or build-setting change was warranted by the matched result. No Windows build-flag change is indicated by this Linux run. The only `Source/` change is `Source/VoxelForgeEditor/Private/VoxelForgeExploreCommandlet.cpp`; Linux VFHostEditor Development UBT succeeded with `-MaxParallelActions=4` (log: `Saved/Linux/l71/ubt-source-change.log`). No Unreal build ran during timing.

Historical fixtures and controls used directly or indirectly by verify tiers now live under `Standalone/Tools/verify/baselines/`; `verify.py` and its helper scripts point there. Gate thresholds and outcomes are unchanged. The Clang fast tier passed in 267.2s (log: `Saved/Linux/l71/verify-fast-tier.log`).

`perf` and stack-sampling tools were unavailable, so stage counters and per-tile traces supplied the profile. Initial UE launches used an output path outside the host plugin’s Saved tree and stopped before measurement; corrected launches all passed. The first UBT attempt could not open its default user log; retrying with an explicit `-Log` succeeded. The first fast invocation lacked its output binary; after the Clang build, the rerun passed. No commit, push or stash was made.