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[docs] add CCCL SM100 vs muh BI-V100 specialization parity analysis

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dylanyunlon
2026-07-31 18:35:50 +08:00
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# muh vs CCCL SM100: Type Specialization Parity Analysis
Generated: 2026-07-31
## Summary
| Algorithm | CCCL SM100 branches | muh BI-V100 branches | Status |
|-----------|--------------------:|---------------------:|--------|
| reduce | 4+2 det = 6 | 4+2 det+1 default = 7 | ✓ PARITY+ |
| scan (lookback) | 7 | 7 (after 35ef79c5) | ✓ PARITY |
| scan (lookahead) | 6 | 6 | ✓ PARITY |
| topk | 1 (dynamic by key_size) | 1 (dynamic by key_size) | ✓ PARITY |
| transform | 1 (dynamic by elem_size) | 1 (dynamic by elem_size) | ✓ PARITY |
| batch_memcpy | 1 (uniform) | 1 (uniform) | ✓ PARITY |
| for | 1 (uniform) | 1 (uniform) | ✓ PARITY |
## Detailed Breakdown
### reduce (tuning_reduce.cuh)
CCCL SM100 specializes by `(accum_type × offset_size)`:
- `int64 + o4`: ipt=15, tpb=512, ipv=2
- `int64 + o8`: ipt=15, tpb=512, ipv=1
- `float32 + o4`: ipt=16, tpb=512, ipv=2
- `float64 + o4`: ipt=16, tpb=640, ipv=1
muh BI-V100 maps these with SMEM-derived corrections:
- `bi100_float32_plus_o4`: tpb=512, ipt=16, ipv=2 (direct match)
- `bi100_float64_plus_o4`: tpb=512, ipt=12, ipv=1 (SM100 ipt=16 → SMEM overflow at 8B, reduced)
- `bi100_int64_plus_o4`: tpb=384, ipt=16, ipv=2 (SM100 tpb=512 → SMEM overflow, reduced threads)
- `bi100_int64_plus_o8`: tpb=384, ipt=16, ipv=1 (same, vec=1 for 8B offset)
- `bi100_det_float32`: tpb=224, ipt=13 (deterministic path, RAKING)
- `bi100_det_float64`: tpb=128, ipt=11 (deterministic path, RAKING)
- `bi100_default`: tpb=256, ipt=16, ipv=4 (fallback)
### scan (tuning_scan.cuh)
CCCL SM100 lookback specializes by `(input_value_size × offset_size)`:
```
offset=4: 1B→(512,18) 2B→(512,13) 4B→(384,22) 8B→(416,23)
offset=8: 1B→(384,14) [2B=skip] 4B→(416,19) 8B→(320,22)
```
muh BI-V100 after commit 35ef79c5:
```
offset=4: 1B→(512,18) 2B→(512,13) 4B→(384,22) 8B→(416,14*)
offset=8: 1B→(384,14) 4B→(416,19) 8B→(320,19*)
```
*items reduced to fit 49152B SMEM
All delay parameters halved (ns×0.5, l2w×0.6) to account for
BI-V100 L2=6MB vs SM100 L2=50MB.
### SMEM Constraint Validation
Every muh bi100_* struct satisfies: `nominal_tile = tpb × ipt × 4 ≤ 49152`
| Struct | tpb | ipt | nominal_tile | Status |
|--------|----:|----:|-------------:|--------|
| bi100_lookback_1B_o4 | 512 | 18 | 36864 | ✓ |
| bi100_lookback_2B_o4 | 512 | 13 | 26624 | ✓ |
| bi100_lookback_4B_o4 | 384 | 22 | 33792 | ✓ |
| bi100_lookback_4B_o8 | 416 | 19 | 31616 | ✓ |
| bi100_lookback_8B_o4 | 416 | 14 | 23296 | ✓ |
| bi100_lookback_8B_o8 | 320 | 19 | 24320 | ✓ |
| bi100_lookback_1B_o8 | 384 | 14 | 21504 | ✓ |
| bi100_float32_plus_o4 | 512 | 16 | 32768 | ✓ |
| bi100_float64_plus_o4 | 512 | 12 | 24576 | ✓ |
| bi100_int64_plus_o4 | 384 | 16 | 24576 | ✓ |
| bi100_int64_plus_o8 | 384 | 16 | 24576 | ✓ |
## Non-Hot-Path Algorithms (20 missing)
These 20 CCCL algorithms have muh/schema/*.yaml but no tuning header.
They are NOT on the vllm inference hot path for Qwen3.6 decode.
If any competition test case triggers them, they will use CCCL defaults
which may cause SMEM overflow on BI-V100 for large types.
Priority to add (by SMEM overflow risk):
1. `radix_sort` (89KB tuning, 161 type dispatches) — HIGH risk
2. `reduce_by_key` (72KB, 134 dispatches) — HIGH risk
3. `select_if` (107KB, 2729 lines) — MEDIUM risk
4. `scan_by_key` (88KB) — MEDIUM risk
5. `unique_by_key` (61KB) — LOW risk
6. Others: LOW risk (small tile sizes, unlikely SMEM overflow)