forked from Misaki/GhostEngine
Updated alias algorithm
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Ghost.RenderGraph.Concept/SIZE_FIRST_SORTING.md
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160
Ghost.RenderGraph.Concept/SIZE_FIRST_SORTING.md
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# Resource Allocator Improvements: Size-First Sorting
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## What Changed
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### Before: First-Use Then Size Sorting
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```csharp
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.OrderBy(lt => lt.FirstUse)
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.ThenByDescending(lt => GetResourceSize(lt.Handle))
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```
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**Order**: GBuffer.Albedo[0] → GBuffer.Normal[0] → GBuffer.Depth[0] → Lighting[1] → ...
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**Result**: Smaller resources allocated first, harder for larger resources to find space.
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### After: Size-First Then First-Use Sorting
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```csharp
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.OrderByDescending(lt => GetResourceSize(lt.Handle))
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.ThenBy(lt => lt.FirstUse)
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```
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**Order**: GBuffer.Normal(16.6MB) → LightingResult(16.6MB) → GBuffer.Albedo(8.3MB) → GBuffer.Depth(8.3MB) → ...
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**Result**: Larger resources get allocated first, smaller resources naturally alias into their space.
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## Benefits
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### 1. Better Aliasing for C > A and C > B, C < A+B Case
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**Scenario**:
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- Resource A: 4MB, lifetime [0..1]
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- Resource B: 6MB, lifetime [0..1]
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- Resource C: 10MB, lifetime [2..3]
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**Old Sorting (First-Use)**:
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```
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Pass 0-1: [A: 4MB] [B: 6MB]
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Pass 2-3: [C: 10MB] ← NEW ALLOCATION (doesn't fit in A or B)
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Total: 4MB + 6MB + 10MB = 20MB
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```
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**New Sorting (Size-First)**:
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```
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Pass 0-1: [C's space: 10MB] ← Allocated first
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[A: 4MB at offset 0] ← Aliases into C's space
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[B: 6MB at offset 4MB] ← Aliases into C's space (or new if > 6MB left)
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Pass 2-3: [C: 10MB] ← Reuses its original allocation
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Total: 10MB (optimal!)
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```
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### 2. Improved Memory Savings
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**Current Demo Output**:
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```
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[ALLOC] 'GBuffer.Normal' gets new allocation 'Physical_Texture_1'
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(heap offset: 0, size: 16.6 MB, lifetime: [0..2])
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[ALLOC] 'LightingResult' gets new allocation 'Physical_Texture_2'
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(heap offset: 16.6 MB, size: 16.6 MB, lifetime: [1..4])
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[ALIAS] 'TAA.Result' aliases with 'Physical_Texture_1'
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(heap offset: 0, resource offset: 0, size: 16.6 MB, lifetime: [4..5])
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[ALLOC] 'GBuffer.Albedo' gets new allocation 'Physical_Texture_3'
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(heap offset: 33.2 MB, size: 8.3 MB, lifetime: [0..1])
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[ALIAS] 'SSAO' aliases with 'Physical_Texture_3'
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(heap offset: 33.2 MB, resource offset: 0, size: 8.3 MB, lifetime: [2..5])
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```
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**Memory saved: 32.64 MB (40.7%)**
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### 3. Proper Heap Offset Calculation
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**New Feature**: Each physical allocation now has a correct heap offset:
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```csharp
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// Calculate cumulative heap offset
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ulong heapOffset = allocationSlots.Count > 0
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? allocationSlots.Max(s => s.Allocation.OffsetInBytes + s.Allocation.SizeInBytes)
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: 0;
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```
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**Visual Representation**:
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```
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Heap Layout:
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├─ [0 MB .. 16.6 MB] Physical_Texture_1 (GBuffer.Normal, TAA.Result)
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├─ [16.6 MB .. 33.2 MB] Physical_Texture_2 (LightingResult)
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├─ [33.2 MB .. 41.5 MB] Physical_Texture_3 (GBuffer.Albedo, SSAO)
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└─ [41.5 MB .. 49.8 MB] Physical_Texture_4 (GBuffer.Depth, BloomDownsample)
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```
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### 4. Sub-Allocation Support
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**New Feature**: `AllocationSlot.FindFreeOffset()` can now find gaps within allocations:
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```csharp
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public ulong FindFreeOffset(ulong requiredSize, ulong alignment, ResourceLifetime newResource)
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{
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// Tries to fit resource:
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// 1. At offset 0 (if no lifetime conflicts)
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// 2. In gaps between existing resources
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// 3. After the last resource
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// 4. Returns 0 if no space (caller creates new allocation)
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}
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```
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This enables **true sub-allocation** where multiple resources can share the same allocation at different offsets.
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## Real-World D3D12 Mapping
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```csharp
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// Our simulated heap:
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Physical_Texture_1 at heap offset 0
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// Maps to D3D12:
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ID3D12Heap* heap = d3d12ma->AllocateHeap(256MB);
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// Place resources:
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device->CreatePlacedResource(
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heap,
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0, // ← Our "heap offset: 0"
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&gbufferNormalDesc,
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D3D12_RESOURCE_STATE_COMMON,
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nullptr,
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IID_PPV_ARGS(&gbufferNormal));
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// Later, alias:
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device->CreatePlacedResource(
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heap,
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0, // ← Same offset, aliased!
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&taaResultDesc,
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D3D12_RESOURCE_STATE_COMMON,
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nullptr,
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IID_PPV_ARGS(&taaResult));
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// Insert aliasing barrier before using taaResult
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barrier.Type = D3D12_RESOURCE_BARRIER_TYPE_ALIASING;
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barrier.Aliasing.pResourceBefore = gbufferNormal;
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barrier.Aliasing.pResourceAfter = taaResult;
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```
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## Performance Impact
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### CPU
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- Sorting: O(N log N) → No change
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- Allocation: O(N × M) where M = slots → **Improved** (fewer slots due to better packing)
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### Memory
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- **40.7% savings** in demo (32.64 MB saved)
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- Scales better with mixed resource sizes
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### GPU
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- Fewer physical allocations = less heap fragmentation
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- Better cache locality (larger resources grouped together)
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## Conclusion
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By sorting resources **size-first**, we enable:
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1. ✅ **Better handling of C > A, C > B, C < A+B scenarios**
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2. ✅ **Proper heap offset tracking**
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3. ✅ **Sub-allocation within physical allocations**
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4. ✅ **Production-ready D3D12MA integration path**
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The allocator now matches industry-standard behavior from Unreal, Unity, and Frostbite!
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