acdream/docs/plans/2026-05-10-perf-tiers-2-3-roadmap.md

# Performance Tiers 2 + 3 — Future Roadmap

**Created:** 2026-05-10 during Phase A.5 polish.
**Status:** Future planning — not for current execution.
**Context:** A.5 shipped two-tier streaming with the entity dispatcher landing at ~3.5ms median (post-Bug-A and Bug-B fixes). Tier 1 (entity-classification cache) lands as A.5 polish and brings the dispatcher inside the 2.0ms spec budget. Tiers 2 + 3 are the "next big perf wins" beyond Tier 1.

---

## Background — why this exists

Discussion captured 2026-05-10: user observed 200-240 FPS at radius=12 on a Radeon 9070 XT @ 1440p and asked why an "old game like AC" doesn't deliver Unreal-level (1000+ FPS) on this hardware.

The honest answer: the bottleneck is *architectural*, not hardware. The CPU is single-threaded and rebuilds the entire draw plan from scratch every frame. Modern engines pre-bake static-world batches at content-cook time and rebuild only what changes.

AC's design — server-spawned per-entity world streamed at runtime — doesn't naturally batch the way Unreal's pre-cooked content does. Closing the gap requires backporting modern techniques while preserving AC's data model. Tiers 2 and 3 are that backporting work.

---

## Tier 2 — Static/dynamic split with persistent groups

**Estimated effort:** ~10-15 days (2-week phase).
**Estimated win:** entity dispatcher ~3.5ms → **~0.5-1ms median** at radius=12.
**Total frame time:** ~4-5ms → **~2-3ms = 400-600 FPS at standstill.**

### The core idea

Today, `WbDrawDispatcher._groups` (the dictionary of "(mesh + texture + blend) → list of instances to draw") is cleared and rebuilt from scratch every frame.

For trees, rocks, buildings, and other static entities (~95% of the world), the answer is identical every frame forever. Tier 2 makes the static-group instance buffers **persistent GPU-resident data**, just like Unreal's pre-baked world. The CPU only orchestrates "which groups are visible" per frame.

### Architectural shift

```csharp
class StaticInstancedGroup
{
    public GroupKey Key;
    public Matrix4x4[] Matrices;          // grown as entities spawn
    public BitArray ActiveSlots;          // for free-list reuse
    public bool NeedsGpuUpload;           // dirty flag for delta upload
    public Dictionary<uint, int> EntityToSlot;   // for despawn lookup
    public uint InstanceBufferOffset;     // start of group's slice in global SSBO
}
```

**On entity spawn (atlas-tier static):** allocate a slot in each relevant group, write the matrix, mark dirty.

**On entity despawn:** free the slot, mark dirty.

**Per frame:**
- Static groups: LB-cull each group (cheap). For visible groups, flag for draw. **No matrix copy. No list rebuild.**
- Dynamic entities (~50 NPCs/players): today's per-frame walk-and-classify. Keeps the existing slow path for things that legitimately change every frame.
- Upload only the dirty groups' matrix slices (delta upload, not full reupload).
- Issue 2 multi-draw-indirect calls.

### Sub-decisions

**Frustum cull granularity at the group level:** at group level you can't reject individual instances; you draw the whole group or none of it. Two strategies:

- **Per-LB subgroups:** split each group into per-landblock subgroups. LB-frustum-culls reject subgroups whose LB is invisible. ~2K groups × ~5 LBs per group on average = ~10K subgroups. Each subgroup AABB cull is ~0.3 µs → ~3 ms per frame. Roughly a wash with today's per-entity cull.
- **Per-instance GPU cull (Tier 3):** compute pre-pass on the GPU writes which instances are visible to a draw-indirect buffer. ~0.05ms CPU. The right long-term answer.

For Tier 2 alone, per-LB subgroups are the recommended approach — keep CPU culling, just at coarser granularity than per-entity.

**Dynamic entities crossing LB boundaries:** when an NPC walks across a landblock boundary, it stays in the same group key but its "spatial bucket" changes. Solution: dynamic entities are tracked in a single global "dynamic group" outside the per-LB structure; they don't need spatial bucketing because there are only ~50 of them.

**Palette override invalidation:** server event swaps an NPC's clothing color → group key changes. Treat as despawn-from-old + spawn-into-new. NPCs are dynamic so this just rebuckets them.

**Animation overrides on static entities:** static entities don't animate. Trees don't bend (foliage wave is a vertex shader effect, not a group-key change). Buildings don't move. So the static path never invalidates.

**EnvCell visibility:** dungeon entities are gated by per-cell visibility state. Need to track which group instances are tied to which cell, and during visibility cull, gate per-cell. Keep using existing `ParentCellId` field on WorldEntity.

**Streaming load/unload integration:** when an LB unloads, all its static entity matrices need to be removed from their groups. Free-list management. Matches existing `LandblockSpawnAdapter` lifecycle.

### Effort breakdown

| Task | Days |
|---|---|
| Design + invariants document | 2 |
| Spawn-time slot allocator + free-list | 3 |
| Per-frame visibility + dirty-flag delta upload | 2 |
| Dynamic entity path (NPCs, projectiles) | 2 |
| Invalidation (palette/ObjDesc events) | 2 |
| EnvCell visibility integration | 1 |
| Streaming load/unload integration | 1 |
| Conformance testing | 2-3 |
| **Total** | **~10-15 days** |

### Risks

- **Slot management bugs** = double-frees or leaks (entities draw at random positions — visible).
- **Invalidation bugs** = stale matrices (entity teleports back to spawn point when palette changes).
- **Dynamic entity tracking** adds complexity around the static/dynamic boundary.

### Mitigations

- **Conformance test:** render a fixed scene through both pipelines, compare draw output. Adds CI infrastructure.
- **Per-frame validation in debug:** walk all groups, assert no orphan slots.
- **Hash invariant test:** static entities should produce stable group keys frame-over-frame. Add a debug assertion that fires once per frame in Debug builds.

---

## Tier 3 — GPU-side culling (compute pre-pass)

**Estimated effort:** ~1 month (longer phase).
**Estimated win:** entity dispatcher ~0.5-1ms (post-Tier-2) → **~0.05ms median.**
**Total frame time:** ~2-3ms → **~1.5-2ms = 600-1000+ FPS at standstill.**

### The core idea

Today (and after Tier 2), the CPU does per-LB or per-subgroup frustum culling and tells the GPU which groups to draw.

Tier 3 moves per-instance frustum cull to the GPU via a compute shader pre-pass. The CPU just uploads "here are all 1M instance matrices" once; the GPU compute shader writes which ones are visible to a draw-indirect buffer; the rasterizer draws only those.

This is the level Unreal is at. With this, per-frame CPU work for the entity dispatcher becomes essentially "tell the GPU what to do" + a tiny scratch upload.

### Why Tier 3 needs Tier 2 first

Without Tier 2's persistent group structure, GPU culling has nothing stable to operate on. The compute shader needs an addressable "here are the static instances" buffer to read from; that buffer only exists after Tier 2.

### Sub-decisions to be made

**Compute shader API:** OpenGL 4.3+ compute shaders are sufficient. We're already at GL 4.3+ for bindless. No additional capability requirement.

**Indirect draw command generation:** the compute shader writes a `DrawElementsIndirectCommand[]` buffer per pass. Render thread issues `glMultiDrawElementsIndirect` reading from that buffer. No CPU readback.

**LOD selection:** opportunity to add per-instance LOD selection in the compute shader (distance-based mesh detail). Not needed for A.5's scope; could be a Tier 4 follow-up.

**Per-light shadow map culling:** if shadows ship, GPU culling extends naturally to per-light frustum cull. Significant win for shadow rendering.

### Effort breakdown

| Task | Days |
|---|---|
| Compute shader design + GLSL implementation | 4 |
| Buffer layout coordination with Tier 2 | 2 |
| Silk.NET compute dispatch integration | 3 |
| Indirect command compaction logic | 4 |
| LOD selection (optional, ~stretch) | 4 |
| Validation: per-instance cull matches CPU cull within epsilon | 3 |
| Conformance + regression testing | 5 |
| **Total** | **~21-25 days, ~1 month** |

### Risks

- **GPU stalls** if the compute shader takes longer than expected (esp. on lower-end GPUs).
- **Sync overhead** between compute pre-pass and rasterizer pass.
- **Debugging difficulty** — GPU compute bugs are harder to diagnose than CPU bugs.

### Mitigations

- **Profile-driven design:** measure compute shader runtime on target hardware before committing.
- **Fallback path:** keep CPU cull as a runtime-toggleable option (env var) so we can A/B compare.
- **GPU debugging tools:** RenderDoc captures + frame-by-frame compute shader inspection.

---

## When to schedule these

**Tier 2:**
- Best fit: dedicated 2-week phase after a SHIP cycle. Treat it like a Phase B/C/N (i.e., name it Phase A.6 or N.7).
- Trigger: user wants to push radius beyond 12 (e.g., to 15 or 20 for true continent-scale horizon).
- Trigger: user wants to add 100+ active NPCs in a city without dropping below 240Hz.

**Tier 3:**
- Best fit: after Tier 2 has been live and stable for at least one cycle.
- Trigger: shadow map work begins (GPU cull + shadow cull share the same compute pre-pass infrastructure).
- Trigger: user wants 500+ FPS sustained for very-high-refresh scenarios (360Hz monitors, future hardware).

**Both:**
- Don't bundle with other phases. These are dedicated perf phases with their own brainstorm + spec + plan + SHIP cycles.

---

## What's "free" or smaller (out of Tier 1/2/3 scope but worth noting)

- **Plumb `JobKind` properly through `BuildLandblockForStreaming`** (~30 min). Today's Bug A patch wastes worker-thread CPU on hydration that gets thrown away for far-tier. Cleaner code, slight CPU savings on worker.
- **Eliminate `ToEntries` adapter allocation in `Draw`** (~15 min). Tiny win (~25 KB / frame). Could fold into Tier 1.
- **Persistent-mapped indirect buffer** (~2 days). Today's `glBufferData` per frame becomes a pre-mapped persistent buffer. Marginal win on RDNA 4; meaningful on lower-end GPUs.
- **Multi-thread mesh-build worker pool** (~1 day). 2.7s first-traversal horizon-fill drops to 0.7s with 4 workers. UX win on first walk-into-region.

These are good candidates for a "perf polish" mini-phase or to backfill into Tier 2.

---

## The architectural ceiling

Even with all three tiers, **a faithful AC client written in C# with bindless OpenGL tops out around 800-1500 FPS at radius=12 on RDNA 4 hardware**. Beyond that requires:

- Native C++ rendering core (eliminate .NET GC + JIT overhead)
- DX12/Vulkan API (eliminate driver state validation)
- Offline content cooking (eliminate runtime mesh/texture decode)

Each of those is a several-month undertaking and represents "becoming a different engine." The realistic target for acdream is 240-500 FPS at the user's monitor refresh, comfortably ahead of the visible-stutter threshold. Tier 1 + Tier 2 alone should deliver that for radius=12-15.

For "Unreal-level FPS at full quality," that's a different project.