# NVFP4 MegaMoE Debug Log

## Current State (May 16, 2026 — 01:15 UTC)

**Status:** GEMM verified correct, SF remap verified correct, B layout verified correct, L2 slot_token cleaned up. vLLM still produces garbage. The checkpoint `input_scale` red herring is documented below. The bug remains unidentified.

**What's verified correct (DO NOT re-investigate):**
1. SF remap: roundtrip verifier = 0 errors, forward mapping with `layout_sf(make_coord(mn, k_elem, 0))`
2. GEMM math: uniform FP4 + uniform SF → exact output (72.0 = 1.5² × K)
3. B matrix layout: byte transpose correct, column-dependent weight test passes
4. L2 GEMM input: slot-major, no gather needed (cleaned up dead `slot_token` param)

**What's still broken:**
- "The capital of France is" → garbage tokens (varies, e.g. `-W'MSG173`, `( z tractor`)
- All magnitudes reasonable, no NaN anywhere

## Red Herring: Checkpoint input_scale (May 16, 00:10 UTC)

Mike's code review suggested using the checkpoint's `input_scale` as the activation global scale instead of the dynamic `amax/(6*448)`. This was **wrong** and has been reverted (commit `79b9bec`).

**What happened:**
- Applied `stage_activation(hidden_states, input_global_scale=w13_input_scale)` 
- `w13_input_scale = 2.86e-4` (from checkpoint, same for all experts in a layer)
- Dynamic `amax/(6*448)` = 3.70e-3 (13x larger)
- Using 2.86e-4 for normalization: `x / 2.86e-4` produces values in the thousands for typical hidden states (amax ~5)
- ALL block scales saturated to 448.0 (max float8_e4m3 value)
- Output: still garbage, but now with quantized-to-death activations

**Why it's wrong:**
The checkpoint `input_scale` is NOT the `input_global_scale = amax/(6*448)` normalization constant. They are different quantities:
- `input_global_scale` normalizes data to [0,1] before FP4 quantization
- `input_scale` is a calibration constant from the Quark quantization tool

The calibration amax for `input_scale = 2.86e-4` would be `2.86e-4 * 6 * 448 = 0.77`. Runtime hidden states have amax ~5-10. The `input_scale` was computed on a different data distribution (probably calibration data, not actual inference data).

**The correct use of `input_scale` is still unknown.** The Quark path computes `alpha = input_scale * weight_scale_2`, but this may assume BF16 activations (not FP4-quantized). Our CUTLASS kernel requires FP4 input, so we must quantize with the dynamic scale.

**Preserved for future use:** `_w13_input_scale` and `_w2_input_scale` are now saved in `finalize_weights` (not dropped) in case we need them for alpha computation later.

**Checkpoint input_scale values (layer 0, all experts identical):**
- `gate_proj.input_scale` = `up_proj.input_scale` = 2.862840e-04
- `down_proj.input_scale` = 3.069196e-02
- `weight_scale_2` = 4.650298e-05 (all projections)
- Scales vary by layer: layer 0 = 2.86e-4, layer 60 = 1.07e-2

## SF Remap — Final Correct Implementation (commit `6626b75`)

```cpp
int mn = tid / K_sf;
int k_sf = tid % K_sf;
int k_elem = k_sf * 16;
int dst_idx = layout_sf(cute::make_coord(mn, k_elem, 0));
dst[dst_idx] = src[mn * src_stride_mn + k_sf * src_stride_ksf];
```
Source strides: SFA=(K_sf, 1), SFB=(1, N)
Allocation: `cute::size(cute::filter_zeros(layout))`

## Previous Bugs Fixed

### BF16 reference comparison (May 15, 23:37 UTC)
The 0.2 cosine against the Python BF16 dequantization reference was a RED HERRING. The reference is wrong, not the GEMM. 8+ iterations of SF remap changes all produced the same 0.2 cosine because it was never about the remap. **A wrong reference is worse than no reference.**

### `cute::size` vs `cute::cosize` (commit `c384198`)
Iteration bound used `size` (logical) instead of `cosize` (physical). Fixed but insufficient alone.

### M/K coordinate extraction in `idx2crd` (commits `deb6b32` → `30b6c89`)
Original had M/K swapped. Mike's correction: `mn = f0 + 32*f1 + 128*f2`, `k_sf = f4 + 4*f5`.

### `if/else if` fallthrough (commit `6626b75`)
Dead `dst_idx=0` when no branch matched. Fix: branchless `layout_sf(make_coord(...))`.

### `col_major_src` ambiguity (commit `7285331`)
Boolean flag → explicit `src_stride_mn, src_stride_ksf`.

### Allocation size (commit `6626b75`)
`cosize` → `size(filter_zeros(layout))`.

### L2 slot_token cleanup (commit `bb5a1ba`)
`nvfp4_moe_l2` accepted `slot_token` but never passed it to GEMM. Removed dead parameter.

## Mike's Code Review Answers

1. **E2M1 packing:** Confirmed correct — element 2j in low nibble, 2j+1 in high nibble. Suggested hardware oracle with `__nv_cvt_bfloat16raw2_to_fp4x2`.
2. **A RowMajor:** Confirmed correct — no micro-tiling for A.
3. **B ColumnMajor:** Byte transpose confirmed correct by test. Mike flagged theoretical concern about nibble-level transpose but our test passed.
4. **Alpha/global scale:** Mike suggested `alpha = input_scale * weight_scale_2` (from checkpoint). We tried it — wrong for activation normalization. The correct use of `input_scale` in our pipeline is still TBD.
5. **Gate/up correction:** Mathematically valid. `up_half *= up_weight_gs / gate_weight_gs` is equivalent to per-column alpha.

## Architecture Notes

### NVFP4 MoE Pipeline
```
stage_activation(hidden_states) → x_fp4, x_sf, input_global_scale
L1 GEMM: (x_fp4, x_sf) @ (l1_w, l1_sf) with alpha=igs*l1_global_sf → gate_up
SiLU(gate) * up → activated
stage_activation(activated) → l1_fp4, l1_sf, l1_igs
L2 GEMM: (l1_fp4, l1_sf) @ (l2_w, l2_sf) with alpha=l1_igs*l2_global_sf → output
scatter with routing weights → y
```

### Per-element multiply order
`res += A_fp4 * SFA_fp8 * B_fp4 * SFB_fp8`

## Next Steps

1. **Compare against BF16 model** — run the same prompt on a known-good implementation to see if the attention layers are working and only MoE is broken
2. **Check the vLLM model integration** — how does the MoE output get mixed with the residual? Is `hc_post` correct?
3. **Understand the Quark input_scale contract** — maybe we need to NOT quantize activations to FP4 and instead use BF16 input
4. **Add per-layer token output logging** — see which layer the tokens go off the rails
5. **Check o_a_proj BF16 handling** — it's kept in BF16 in the checkpoint, is it being processed correctly?