Fr0zka
7313201e91
Three orchestration findings on the parked PERF item, none measured yet: 1. The perf A/B needs no new code -- two Insights traces on a deterministic world are a clean before/after. But tile skipping changes how many times GenerateMesh runs, so totals conflate "cheaper per tile" with "fewer tiles". TilesMeshed from task 001 is the denominator. 001 is therefore a PREREQUISITE for the perf work, not a parallel item as the handoff had it. 2. The suspects don't share an archetype: SurfaceWorld is the column memo, TunnelNetwork is 19 virtual calls per voxel (16 + 3 structural post). One lumped number can't be acted on -- measure one strate at a time. 3. GetColumn is a direct-mapped hashed table where GSurfColCache is a direct-indexed box, and the mesher pre-samples Z-OUTERMOST (verified, VoxelMarchingCubesMesher.cpp ~226) so every column is revisited ~34x per tile. The table's sizing comment reasons about fullness; a direct-mapped table evicts on collision. At load factor 0.28 that is still ~25% of columns recomputing the full height stack every Z plane -- derived ~9x. Task 002 measures that and fixes nothing, so the instrument and the fix cannot land in the same build and make each other unreadable. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
113 lines
6.0 KiB
Markdown
113 lines
6.0 KiB
Markdown
# Codex task 002 — is the op-stack column memo thrashing? Count, don't guess.
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**Owner:** Codex (Model Luna, xHigh) · **Orchestrator:** Claude · **Branch:** `experimental`
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**Status:** specified, not started
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**Depends on:** `CODEX-TASK-001` — this adds counters to the **same** `stat VoxelForge` group.
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Do 001 first; this task assumes `VoxelStats.h` already exists.
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---
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## Why this exists
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The operator-stack density path is measurably slower than the `switch` it replaces, and the cause
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has never been attributed. There are three standing suspects. **This task measures the first one,
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and does not fix anything.** That order is deliberate: `AUDIT §C10` cost six builds and five refuted
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hypotheses by reasoning first, and the last session re-learned it.
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### The hypothesis, derived from the code
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`FSurfaceColumnSource::GetColumn` (`VoxelDensityOpStack.cpp` ~615) memoises a computed column in a
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**direct-mapped, 4096-entry `thread_local` table**, indexed by a hash of the two XY floats:
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```cpp
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struct FSlot { uint64 Key; float X, Y; FColumn C; }; // 40 bytes
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thread_local FSlot Slots[4096] = {}; // 160 KB per worker
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const uint32 Idx = ((HX * 0x9E3779B9u) ^ (HY * 0x85EBCA6Bu)) >> 20; // [0,4095]
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```
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The original path, `GSurfColCache` in `GetDensityAt`, is instead a **direct-indexed box**:
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`CI = (IY - Box.BaseY) * Dim + (IX - Box.BaseX)`, with a `Computed[CI]` flag. **No hash, therefore
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no collisions, therefore every column is computed exactly once.**
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Now the sampling order, which is the premise that makes this bite. `FVoxelMarchingCubesMesher`
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pre-samples with **Z as the OUTERMOST loop** (`VoxelMarchingCubesMesher.cpp` ~226):
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```cpp
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for (int32 gz = GzLo; gz <= GzHi; gz++)
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for (int32 gy = -1; gy <= GridDim; gy++)
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for (int32 gx = -1; gx <= GridDim; gx++)
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Generator->GetDensityAt(...);
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```
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So the mesher sweeps a **whole XY plane at every Z level**. Every column in the tile is revisited
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once per Z plane — roughly 34 times.
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The table's own comment sized it for this: *"A chunk is CHUNK_SIZE² columns (1024), so the first
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draft's 256 entries could not even hold one chunk and thrashed inside a single tile. 4096 covers
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four chunks."* **That reasoning has a gap.** A direct-mapped table does not need to be full to
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evict — it needs two live keys to collide. At ~1156 columns per plane in 4096 slots (load factor
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0.28), the expected number of columns sharing a slot with another is **~285, about 25 %**. Those
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columns evict each other, miss again on the next Z plane, and recompute the **entire height stack** —
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structural source, cliff (four structural resamples), terrace, layer-line, beach, and the ceiling
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stack — every single plane.
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Order of magnitude if that is right: ~1156 column computations on the original path versus
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~1156 + 285 × 34 ≈ **10 000** on the op path. Roughly **9×** the column work, on the plugin's most
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expensive archetype.
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**That is a derivation, not a measurement, and it is exactly the kind of confident chain this
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project has watched reverse six times.** Hence: count first.
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## What to build — two counters, no behaviour change
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Add to the existing `stat VoxelForge` group from task 001:
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| counter | incremented when |
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|---|---|
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| `ColumnMemoHit` | `GetColumn` found a live entry (the `if` body did **not** run) |
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| `ColumnMemoMiss` | `GetColumn` recomputed (the `if` body ran) |
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That is the entire change. Two `INC_DWORD_STAT` calls inside `FSurfaceColumnSource::GetColumn`,
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around the existing `if (S.Key != ColumnKey || S.X != WorldX || S.Y != WorldY)`.
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## ⚠️ Invariants
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1. **Change nothing else in `GetColumn`.** Not the table size, not the hash, not the key comparison.
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The point of this task is to produce a number that decides whether the fix is worth writing; a
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change bundled in makes the number unattributable. **Do not "obviously improve" the table while
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you are in there** — if the fix ships in the same build as the instrument, we learn nothing, and
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this project has a written lesson about exactly that.
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2. **`GetColumn` is `const` and runs on mesher workers.** `INC_DWORD_STAT`, never a `static int32++`
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— same rule as task 001, same reason.
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3. **The key comparison is load-bearing and must stay complete.** `S.Key != ColumnKey || S.X != X ||
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S.Y != Y` — the full key is compared on every touch precisely so a hash collision can only cost a
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recompute and never return **someone else's column**. Do not shorten it to feed a counter.
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4. **Zero cost when `STATS == 0`.** No value computed outside the macros.
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5. `FCaveCliffMod` and the overhang read this same memo through
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`Column->GetColumn(...)` (~line 864). They are legitimate traffic and must be counted, not
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excluded — they are part of why a miss is expensive.
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## Acceptance
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Fly a route through a **SurfaceWorld** strate that has `bUseOperatorStack` ticked, `stat VoxelForge`
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on screen, and read the ratio.
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- **`ColumnMemoMiss` ≈ one per distinct column per tile** (hit rate climbing toward ~97 % as Z
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planes accumulate) ⇒ **the hypothesis is WRONG**, the table is fine, and the perf cost is
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elsewhere. Record that and move to suspect 2 (per-voxel virtual dispatch). *A negative result here
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is a real result and must be written down, not quietly dropped.*
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- **`ColumnMemoMiss` in the neighbourhood of 20–30 % of all lookups, and staying there** ⇒
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**confirmed**: the table is evicting on collision every Z plane. The fix is then obvious and cheap
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(direct-indexed box keyed like `GSurfColCache`, or a set-associative table), and gets its own
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task with the before/after this run supplies.
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Report **both numbers**, not the ratio alone — a ratio cannot distinguish "few lookups" from "many".
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## Notes for the reviewer (Claude)
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- Confirm the miss counter sits inside the `if`, and the hit counter in an `else` — not computed
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from a subtraction, which would silently agree with itself.
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- Confirm `GetColumn`'s early-out path (if any is added later) cannot skip both counters.
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- Confirm nothing else in `VoxelDensityOpStack.cpp` changed. `git diff --stat` should show one file
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and a handful of lines.
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