nvfp4-megamoe-kernel/README.md

# nvfp4-megamoe-kernel

Native NVFP4 block-scaled MoE kernel for DeepSeek-V4-Pro on NVIDIA Blackwell (SM100).

Replaces the broken `fp8_nvfp4_mega_moe` kernel from DeepGEMM with a working CUTLASS-based implementation that emits real `SM100_MMA_MXF4_SS` tensor core instructions.

---

## Architecture

DeepSeek-V4-Pro is a 384-expert MoE model with expert parallelism across 8 ranks (B200 GPUs). Each rank handles 48 experts. For each token, the router picks the top-6 experts.

### The MoE Forward Pass

```
Input hidden states (BF16)
        │
        ▼
┌─────────────────┐
│  Shared Experts │  ← vLLM native FlashInfer CUTLASS NVFP4 path
│  (gate + up →  │     (not our kernel)
│   SiLU * up →   │
│   down)         │
└─────────────────┘
        │
        ▼
  Staging Kernel (vLLM built-in)
  BF16 → packed E2M1 (int8) + UE4M3 block-16 scales (uint32)
  Writes to SymmBuffer.x / SymmBuffer.x_sf
        │
        ▼
  Router (vLLM built-in)
  Writes topk_ids / topk_weights to SymmBuffer
        │
        ▼
┌─────────────────────────────────────────────────┐
│          nvfp4_mega_moe_full                    │  ← nvfp4_mega_moe.py
│                                                 │
│  1. Read staged activation from buffer          │
│  2. Build slot mapping (token, topk) → local    │
│     expert, routing weight                      │
│  3. L1 GEMM: gate_up_proj (slot-based)         │  ← CUTLASS NVFP4 block-scaled
│     E2M1 × E2M1 + UE4M3 scales                 │    SM100_MMA_MXF4_SS PTX
│     → BF16 per-slot output (6144-wide)         │
│  4. SiLU(gate) * up PER SLOT                   │
│     (nonlinearity before combining paths)       │
│  5. stage_activation: BF16 → FP4               │  ← proper E2M1 quantization
│  6. L2 GEMM: down_proj (slot-based)            │  ← CUTLASS NVFP4 block-scaled
│     E2M1 × E2M1 + UE4M3 scales                 │    SM100_MMA_MXF4_SS PTX
│     → BF16 per-slot output (7168-wide)         │
│  7. Final scatter:                              │
│     y.index_add_(slot_token,                    │
│       slot_weight * l2_slots)                   │
│     Routing weight applied ONCE at scatter      │
└─────────────────────────────────────────────────┘
        │
        ▼
  Cross-rank all-reduce (vLLM built-in)
```

### Slot-Based Dispatch

The kernel uses a **slot representation** instead of collapsing expert outputs early. A slot is one `(token, topk_expert)` pair. For a batch of T tokens with top-6 routing, there are up to 6T slots (fewer if some experts are out of the local rank's range).

**Why slots?** Two bugs in the previous approach:

1. **SiLU after summing is mathematically wrong.** `silu(Σ wᵢ·gateᵢ) * (Σ wᵢ·upᵢ) ≠ Σ wᵢ·silu(gateᵢ)·upᵢ`. The nonlinearity must happen per-expert-path before combining.

2. **Routing weights applied twice.** The old grouped GEMM applied `topk_weights` in its scatter loop, and was called for both L1 and L2 — squaring the weights.

The slot approach fixes both: SiLU+Mul happens per-slot, and routing weights are applied exactly once at the final `index_add_` scatter.

### SFB (Weight Scale Factors) — Remapped Per-Call, NOT Cached

Weight scale factors (SFB) are remapped from row-major to CUTLASS interleaved layout on every GEMM call. This is a lightweight scatter kernel (~µs) and is NOT the bottleneck compared to the GEMM itself.

⚠️ **DO NOT ADD A PREPACK CACHE FOR SFB.** Previous attempts caused critical issues:

| Problem | Impact |
|---------|--------|
| **OOM** | ~1.75 GiB per prepacked tensor × 61 MoE layers × 2 (L1+L2) = ~214 GiB — exceeds B200 capacity |
| **Peak memory 2×** | `torch.stack` held all expert tensors + final stack simultaneously before LRU eviction |
| **CUDA graph trap** | LRU eviction frees tensors that CUDA graphs still reference → use-after-free → silent corruption or crash |
| **M-dependent layout** | `prepack_sfb(M=128)` assumed SFB layout size is M-independent (never verified). If wrong, entire prepack is invalid |
| **Cross-layer cache collision** | Tag-based cache (`"l1"`/`"l2"`) returned layer N-1's data for layer N. Fixed with data_ptr key, but the cache itself was the root problem |

The per-call remap costs microseconds. The cache cost was hours of debugging. Don't repeat this mistake.

---

### vLLM Startup Sequence (how our code plugs in)

```
1. vLLM engine init
   └─ ModelOptNvFp4Config selected (NVFP4 quantization scheme)
   └─ FlashInferCutlassNvFp4LinearKernel for linear layers

2. Model construction
   └─ DeepseekV4ForCausalLM → DeepseekV4MoE → DeepseekV4DecoderLayer
       Each layer has: attention + MoE block
       MoE block has: shared experts + 384 routed experts

3. Weight loading
   └─ 95 safetensor shards loaded
   └─ weight, weight_scale, weight_scale_2 loaded per linear

4. process_weights_after_loading  ← THIS IS WHERE WE HOOK IN
   └─ ModelOptNvFp4LinearMethod swizzles/pads weights for CUTLASS
   └─ finalize_mega_moe_weights()
       └─ weight_transform.py: transform_nvfp4_weights_for_mega_moe()
           • Folds weight_scale_2 (global scale) into weight_scale (block scale)
           • UE4M3 block-16 scales: 4 values packed per uint32
           • Returns ((l1_w, l1_sf), (l2_w, l2_sf)) per rank

5. SymmBuffer allocation
   └─ symm_buffer.py: get_symm_buffer_for_nvfp4_mega_moe()
       • Pre-allocates GPU buffers for:
         - x: int8 packed E2M1 activations
         - x_sf: uint32 packed UE4M3 activation scales
         - topk_idx: int32 expert indices
         - topk_weights: float32 routing weights
         - buffer: BF16 all-reduce buffer

6. Profile run (warmup)
   └─ First forward pass to allocate KV cache, etc.
   └─ This is where the CUTLASS GEMM first executes
   └─ SFB weight scales remapped per-expert inside CUTLASS (no cache)

7. Ready to serve
```

---

## File Map

```
nvfp4_megamoe_kernel/
├── __init__.py              # Public API exports
├── nvfp4_mega_moe.py       # Main kernel: nvfp4_mega_moe_full, L1/L2, stage_activation
├── weight_transform.py     # Weight prep: fold global scale, pack UE4M3
├── symm_buffer.py          # GPU buffer allocation for MoE dispatch
│
└── cutlass_nvfp4_gemm/     # CUTLASS CUDA extension (the actual hardware kernel)
    ├── cutlass_nvfp4_gemm.cu   # CUDA: CUTLASS GEMM + SF remap + prepack SFB + prepacked-SFB GEMM path
    ├── pytorch_binding.cpp     # PyTorch C++ binding (forward, forward_prepacked_sfb, prepack_sfb)
    ├── kernel.py               # Python: cutlass_grouped_nvfp4_gemm (slot-based, per-expert loop)
    ├── sf_layout.py            # CUTLASS SF layout reference docs
    ├── setup.py                # Build config (nvcc, CUTLASS include paths)
    ├── build.sh                # Build script
    ├── test_gemm.py            # Standalone test
    └── README.md
```

### What each file does (in call order)

| File | When it runs | What it does |
|------|-------------|--------------|
| `weight_transform.py` | Once at startup (weight loading) | Takes raw NVFP4 checkpoint weights, folds global scales into block scales. Returns scales as `float8_e4m3fn` (not packed uint32). Output: `((l1_w, l1_sf), (l2_w, l2_sf))` |
| `symm_buffer.py` | Once at startup (buffer alloc) | Pre-allocates GPU tensors for activations, scales, routing data, and all-reduce. These persist across forward passes. |
| `nvfp4_mega_moe.py` | Every forward pass | Orchestrates the MoE: reads from symm buffer → build slot mapping → L1 GEMM → SiLU+Mul per-slot → re-quantize → L2 GEMM → final index_add_ scatter with routing weights. Contains `stage_activation` (BF16→FP4) and `unpack_ue4m3_u32`. NO prepack cache — SFB remapped per-call inside CUTLASS. |
| `cutlass_nvfp4_gemm/kernel.py` | Every forward pass (called by nvfp4_mega_moe) | Slot-based per-expert loop: gather slots for each expert, call CUTLASS GEMM (SFB remapped inside C extension), write results to slot buffer. No routing weights — caller handles scatter. |
| `cutlass_nvfp4_gemm/cutlass_nvfp4_gemm.cu` | Every forward pass (CUDA kernel) | The actual CUTLASS kernel: native NVFP4 block-scaled GEMM + GPU-side SFA and SFB remap. |
| `cutlass_nvfp4_gemm/sf_layout.py` | Reference only | Documents the CUTLASS SfAtom layout. Not used at runtime (remap is in CUDA). |

---

## Data Formats

### Weights
- **Packed E2M1** (`int8`): 2 FP4 values per byte. Shape: `(E_per_rank, N, K//2)`, K-major layout.
- **UE4M3 block scales** (`float8_e4m3fn`): 1 scale per 16 FP4 values (group_size=16). Shape: `(E_per_rank, N, K//16)`. Returned as `float8_e4m3fn` from `weight_transform.py` — NOT packed uint32. The CUTLASS GEMM consumes float8 directly.

### Activations (after staging kernel)
- **Packed E2M1** (`int8`): Shape: `(num_tokens, K//2)`.
- **UE4M3 scales** (`uint32`): 4 UE4M3 values packed per uint32. Shape: `(num_tokens, K//64)`. Unpacked to `float8_e4m3fn` via `unpack_ue4m3_u32` before reaching the CUTLASS GEMM.

### GEMM dimensions (DeepSeek-V4-Pro)
- **L1 (gate_up_proj):** M×6144×7168 (per expert)
- **L2 (down_proj):** M×7168×3072 (per expert)
- 48 experts per rank (384 total / 8 ranks), top-6 routing

---

## CUTLASS Scale Factor Remap

CUTLASS's `Sm1xxBlockScaledConfig` expects scale factors in a specific interleaved layout, not simple row-major. The SfAtom is:

```
Atom Shape:  Shape<Shape<32, 4>, Shape<16, 4>>
Atom Stride: Stride<Stride<16, 4>, Stride<0, 1>>
Tiling:      Step<_2, _1>  (M tiled with step 2, K with step 1)
```

Our source data is row-major `(M, K_sf)` where `K_sf = K / 16`. The remap kernel (`remap_sf_to_cutlass_kernel` in `cutlass_nvfp4_gemm.cu`) converts from row-major to CUTLASS's interleaved layout.

### How the remap works

The kernel iterates over CUTLASS destination indices, uses `cute::idx2crd` to get the hierarchical coordinate, then `cute::flatten` to get a flat tuple of 8 sub-indices. From those, we extract logical `(m, k_sf)` and read from the row-major source.

### Flattened coordinate decomposition (flat_rank=8)

From the SfAtom layout with Step<_2, _1> tiling, `flatten(idx2crd(idx, ...))` produces 8 values:

```
f0 = inner_m  (0..31)  — varies fastest within M atom
f1 = sub_m    (0..3)   — second M sub-coordinate
f2 = tile_m   (0..)    — M tile index
f3 = step_m stride     — degenerate (always = sfa_size, not a coordinate)
f4 = sub_k    (0..3)   — K sub-coordinate within atom
f5 = tile_k   (0..)    — K tile index
f6 = 0                  — unused
f7 = 0                  — unused
```

#### Empirical coordinate dump (MN=8192, K_sf=448, T = sfa_size = 58720256)

| idx | f0 | f1 | f2 | f3 | f4 | f5 | f6 | f7 |
| ----- | --- | --- | --- | --- | --- | --- | --- | --- |
| 0 | 0 | 0 | 0 | T | 0 | 0 | 0 | 0 |
| 1 | 0 | 0 | 0 | T | 1 | 0 | 0 | 0 |
| 4 | 0 | 1 | 0 | T | 0 | 0 | 0 | 0 |
| 16 | 1 | 0 | 0 | T | 0 | 0 |0 | 0 |
| 511 | 31 | 3 | 0 | T | 3 | 0 | 0 | 0 |
| 512 | 0 | 0 | 0 | T | 0 | 1 | 0 | 0 |
| 1024 | 0 | 0 | 0 | T | 0 | 2 | 0 | 0 |
| 2048 | 0 | 0 | 0 | T | 0 | 4 | 0 | 0 |
| 4096 | 0 | 0 | 0 | T | 0 | 8 | 0 | 0 |
| 8192 | 0 | 0 | 0 | T | 0 | 16 | 0 | 0 |
| 65536 | 0 | 0 | 1 | T | 0 | 16 | 0 | 0 |
| 131072 | 0 | 0 | 2 | T | 0 | 32 | 0 | 0 |

#### Extraction formula

CuTe uses "first sub varies fastest" for `Shape<32, 4>`:

```cpp
m    = f0 + f1 * 32 + f2 * 128;
k_sf = f4 + f5 * 4;
```

This was verified with 6 independent probes:

| Probe | Source | Expected | Result |
|-------|--------|----------|--------|
| SFA[1, 0] = 2.0 | row 1 changes | ✅ only row 1 | Confirms f0 term |
| SFA[32, 0] = 2.0 | row 32 changes | ✅ only row 32 | Confirms f1*32, rules out f0*4+f1 |
| SFA[128, 0] = 2.0 | row 128 changes | ✅ only row 128 | Confirms f2*128 |
| SFA[0, 1] = 2.0 | row 0 changes (k=1) | ✅ only row 0 | Confirms f4 term |
| SFA[0, 4] = 2.0 | row 0 changes (k=4) | ✅ only row 0 | Confirms f5*4 term |
| SFA[0, 100] = 2.0 | row 0 changes (k=100) | ✅ only row 0 | Confirms tile-overflow range |

#### Why the previous remap was broken

The previous code used `cute::get<0>(flat)` and `cute::get<1>(flat)` to extract (m, k). Since flatten produces `(inner_m, sub_m, tile_m, ...)` in order, `get<0>` and `get<1>` are both **M sub-indices** — they carry no K information. This caused only `k_group=0` to work; all other K-groups were silently mapped to the wrong source offset.

Additionally, the dest buffer must be zero-initialized before remap because CUTLASS pads to tile boundaries (128 × 64), making the dest buffer larger than `M * K_sf`. Unmapped padding slots reading garbage caused sporadic wrong results.

---

## Bugs Found & Fixed

### 1. unpack_ue4m3_u32: value cast vs bit reinterpret
**File:** `nvfp4_mega_moe.py`  
**Bug:** `(x_u32 & 0xFF).to(torch.int32).to(torch.float8_e4m3fn)` converts integer 63 → float8(63.0).  
**Fix:** `(x_u32 & 0xFF).to(torch.uint8).view(torch.float8_e4m3fn)` reinterprets bit pattern 0x3F → float8(~0.984).  
**Also:** `uint32` lacks CUDA bitwise ops — cast to `int32` first.  
**Impact:** Corrupted every activation scale fed to the L1 GEMM. Weight scales were fine (already float8 from weight_transform). "Structured garbage" recipe.

### 2. stage_activation: three independent bugs
**File:** `nvfp4_moe.py`  
**Bug A:** `clamp(0, 15)` zeroed every negative value. E2M1 is sign-magnitude 4-bit (bit3=sign, bits2:0=mag).  
**Bug B:** Stored `block_max` but divided by `block_max/6.0` → stored scale was 6× too large.  
**Bug C:** Uniform 0.5 step doesn't match E2M1 values {0, ±0.5, ±1, ±1.5, ±2, ±3, ±4, ±6} — non-uniform above ±2.  
**Fix:** Rewrote with proper nearest-neighbor E2M1 quantization.  
**Impact:** Half the L1→L2 activation was zeroed, 6× scale mismatch, quantization noise on top.

### 3. _fold_global_scale: logical_widths branch
**File:** `weight_transform.py`  
**Bug:** `logical_widths=[3072, 3072]` caused the function to apply expert 0's scale to gate half and expert 1's scale to up half of ALL experts. All other experts' global scales were discarded.  
**Fix:** Removed the `logical_widths` branch entirely. The `else` branch correctly broadcasts each expert's own `(E, 1)` global scale across `(E, N, K//16)`.

### 4. L1 weight interleave removed (transpose still needed)
**File:** `weight_transform.py`  
**Bug:** `_interleave_l1_weights` assumed gate/up were pre-interleaved in groups of 16 and that the kernel used 2CTA UMMA layout. vLLM uses plain concat `[gate; up]` along the output dim, and our CUTLASS kernel uses `ClusterShape<1, 1, 1>`.  
**Fix:** Removed the interleave function. Weights still need a transpose from checkpoint layout `(N, K_half)` row-major to CUTLASS layout `(K_half, N)` column-major — this is standard row→column conversion, not interleaving. Both L1 and L2 weights and scales are transposed.

### 5. SF remap: idx2crd+flatten coordinate extraction
**File:** `cutlass_nvfp4_gemm.cu`  
**Bug:** `cute::flatten(coord)` produces 8 sub-indices (flat_rank=8). `get<0>` and `get<1>` are both M sub-indices (inner_m, sub_m), carrying zero K information. Only k_group=0 worked; all other K-groups were silently wrong.  
**Fix:** Correct extraction: `m = f0 + f1*32 + f2*128`, `k_sf = f4 + f5*4`. Zero-init dest buffer before remap.  
**Diagnostic trail:** Constant-scale test (all SF=1.0) → cosine 1.0 proved FP4 path was correct. Real scales → cosine 0.83 proved SF remap was broken. Single-element probes (SFA[0,0] vs SFA[0,3]) proved only k_group=0 worked. Printf dump of flat coordinates at specific indices revealed flat_rank=8 and the correct extraction formula.

### 6. SiLU after summing expert paths (math error)
**File:** `nvfp4_mega_moe.py`  
**Bug:** The old grouped GEMM collapsed expert outputs into a weighted sum, then applied SiLU+Mul on the sum. `silu(Σ wᵢ·gateᵢ) * (Σ wᵢ·upᵢ) ≠ Σ wᵢ·silu(gateᵢ)·upᵢ`. The nonlinearity must happen per-expert-path.  
**Fix:** Slot-based dispatch — L1 GEMM returns per-slot output, SiLU+Mul applied per-slot, L2 GEMM per-slot, routing weights applied once at final `index_add_` scatter.

### 7. Routing weights applied twice
**File:** `cutlass_nvfp4_gemm/kernel.py`  
**Bug:** `cutlass_grouped_nvfp4_gemm` applied `topk_weights` in its scatter loop. Called for both L1 and L2, each expert's contribution was scaled by `topk_weight²`.  
**Fix:** GEMM returns per-slot results with no routing weights. Single `y.index_add_(0, slot_token, slot_weight * l2_slots)` at the end.

### Diagnostic: constant-scale test (smoking gun for SF bugs)

When all scale factors are set to UE4M3(1.0):
- **Cosine = 1.0000, MSE = 0.19** (expected FP4 quantization noise)

With real (variable) scale factors and the broken remap:
- **Cosine = 0.83** → scales are misaligned, not fundamentally broken

After the fix with correct coordinate extraction:
- **Cosine = 1.0000, MSE = 0.0** → perfect match with dequantized reference

---

## Build & Deploy (B200)

```bash
# On B200 host — CUTLASS must be cloned and mounted
cd /root/nvidia-meeting/deepseek-v4-quant/

# Rebuild container (CUTLASS is host-mounted at /root/cutlass)
KERNEL_CACHE_BUSTER=$(date +%s) docker compose build --no-cache
docker compose up -d
```

The CUTLASS extension builds inside the container during `pip install` of the nvfp4-megamoe-kernel package. It needs:
- CUDA 13.0 toolkit (in the vllm/vllm-openai:nightly image)
- CUTLASS headers at `/root/cutlass/include/`
- CCCL headers at `/usr/local/cuda-13.0/targets/x86_64-linux/include/cccl/`
- Device with SM100 compute capability (B200)

---

## Known Issues / TODO

1. ~~**MoE dispatch is slow**~~ — Fixed. Slot-based `index_add_` replaces the Python double loop over tokens×topk. Routing weights applied once at final scatter.

2. **stage_activation is Python** — Re-quantization from L1 BF16 output to FP4 for L2 input runs in PyTorch. Should use the Triton staging kernel for speed and consistency with vLLM's built-in staging.

3. ~~**SF remap allocates every call**~~ — Fixed. SFB weight scales are prepacked into CUTLASS layout once (lazy, cached per layer). Only SFA (activation scales) remapped dynamically.

4. **Per-expert GEMM dispatch is serial Python loop** — The `cutlass_grouped_nvfp4_gemm` iterates over 48 experts in a Python `for` loop. Each iteration launches one CUTLASS GEMM. Could benefit from a true grouped GEMM kernel or CUDA-side expert dispatch.

---

## Environment Variables

| Variable | Default | Description |
|----------|---------|-------------|
| `MEGA_MOE_STATIC` | 0 | Set to 1 to skip MoE kernel entirely (return zeros) |
| `MEGA_MOE_DEBUG` | 0 | Set to 1 for verbose logging |
| `SKIP_ATTENTION` | 0 | Skip attention layers (debug) |

---

## Repos

- **Kernel:** `sweetapi.com/biondizzle/nvfp4-megamoe-kernel` (branch: master)
- **Deployment:** `sweetapi.com/biondizzle/deepseek-v4-quant` (branch: modelopt-nvfp4)
- **Local:** `~/dev/nvfp4-megamoe-kernel/`, `~/dev/deepseek-v4-quant/`