44e4f6f947
Two changes based on CCCL source reading:
1. PARTITION_SIZE 512→1024 in paged_attention_v2_pytorch.py
From dispatch_scan.cuh: grid_size = num_tiles = ceil(N / tile_size).
Optimal tile_size balances parallelism vs overhead:
- BI-V100: 16 SMs, max ~32 concurrent CTAs
- Need num_partitions >= 32 to fill one wave
- 100K tokens / 1024 = 98 partitions (3 waves) ✓
- 100K tokens / 512 = 195 partitions (6 waves) — twice the Phase 2 cost
Note: only affects V2 (PyTorch path). V1 (ixformer) has its own partition size.
2. Fix V2 import path in _custom_ops.py
paged_attention_v2_pytorch.py is in repo root, not vllm package.
Added sys.path manipulation to find it at runtime.
Also read: cccl_upstream/thrust/examples/expand.cu (variable-length
replication pattern — maps to GQA expansion, but our broadcast approach
is already more efficient than physical replication).
Source: cccl_upstream/cub/cub/device/dispatch/dispatch_scan.cuh lines 350-380
cccl_upstream/thrust/examples/expand.cu
232 lines
11 KiB
Python
232 lines
11 KiB
Python
"""
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paged_attention_v2_pytorch.py — BI-V100 PagedAttention V2 (CCCL-informed)
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===========================================================================
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Fills the `raise NotImplementedError()` hole in vllm/_custom_ops.py.
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Algorithm: Partitioned attention with log-sum-exp reduction.
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Architecture informed by CCCL patterns:
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- summary_statistics.cu: fuse multiple statistics in a single reduction pass
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- warp_reduce_shfl.cuh: accumulate (max, sum, weighted_output) as one compound type
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- block_reduce_warp_reductions.cuh: reduce across partitions via shared accumulators
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Key optimization: Batched partition attention via reshaped 3D bmm.
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Instead of looping over P partitions with P × torch.bmm calls,
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reshape KV into [H, P*part_len, d] and Q into [H, 1, d], then
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slice scores into [H, P, part_len] for partition-wise softmax.
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This gives ONE bmm launch for all partitions.
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For seq_len=100K, PARTITION_SIZE=512:
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Before: 195 × bmm([H,1,d] @ [H,d,512]) = 195 kernel launches
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After: 1 × bmm([H,1,d] @ [H,d,100K]) + reshape = 1 kernel launch
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The partition-wise softmax is then a reshape + per-chunk operation:
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scores: [H, 100K] → [H, P, 512] → max/exp/sum per partition
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Phase 2 reduction (cross-partition combine) follows CCCL's summary_statistics
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binary_op pattern: combine (max_a, sum_a, out_a) with (max_b, sum_b, out_b)
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using the numerically stable log-sum-exp rescaling.
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"""
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import torch
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from typing import Optional
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_PARTITION_SIZE = 1024 # CCCL dispatch_scan.cuh insight: tile_size balances
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# parallelism (num_partitions >= SM_count * 2 to fill one wave) vs overhead
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# (fewer partitions = smaller Phase 2 reduction).
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# BI-V100: 16 SMs, max ~32 concurrent CTAs.
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# For 100K tokens: 1024 → 98 partitions (3 waves), 512 → 195 (6 waves).
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# 98 > 32 so parallelism is sufficient; halving partitions halves Phase 2 cost.
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def paged_attention_v2_pytorch(
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output: torch.Tensor, # [num_seqs, num_heads, head_size]
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exp_sums: torch.Tensor, # [num_seqs, num_heads, max_num_partitions]
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max_logits: torch.Tensor, # [num_seqs, num_heads, max_num_partitions]
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tmp_output: torch.Tensor, # [num_seqs, num_heads, max_num_partitions, head_size]
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query: torch.Tensor, # [num_seqs, num_heads, head_size]
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key_cache: torch.Tensor, # [num_blocks, num_kv_heads, head_size/x, block_size, x]
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value_cache: torch.Tensor, # [num_blocks, num_kv_heads, head_size, block_size]
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num_kv_heads: int,
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scale: float,
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block_tables: torch.Tensor, # [num_seqs, max_blocks_per_seq]
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seq_lens: torch.Tensor, # [num_seqs]
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block_size: int,
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max_seq_len: int,
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alibi_slopes: Optional[torch.Tensor],
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kv_cache_dtype: str = "auto",
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k_scale: float = 1.0,
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v_scale: float = 1.0,
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tp_rank: int = 0,
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blocksparse_local_blocks: int = 0,
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blocksparse_vert_stride: int = 0,
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blocksparse_block_size: int = 64,
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blocksparse_head_sliding_step: int = 0,
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) -> None:
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num_seqs, num_heads, head_size = query.shape
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gqa_ratio = num_heads // num_kv_heads
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max_num_partitions = tmp_output.shape[2]
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# Initialize unused slots
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max_logits.fill_(float('-inf'))
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exp_sums.zero_()
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tmp_output.zero_()
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for seq_idx in range(num_seqs):
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seq_len = int(seq_lens[seq_idx].item())
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if seq_len == 0:
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output[seq_idx].zero_()
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continue
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num_blocks_seq = (seq_len + block_size - 1) // block_size
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num_partitions = (seq_len + _PARTITION_SIZE - 1) // _PARTITION_SIZE
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# =============================================================
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# Batched KV gather: ONE index_select, ONE reshape
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# Pattern: avoid per-block Python loop (CCCL does this via
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# block-cooperative load, we do it via batched indexing)
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# =============================================================
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blk_ids = block_tables[seq_idx, :num_blocks_seq]
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# Key: [nblk, kv_h, d/x, blk_sz, x] → [nblk*blk_sz, kv_h, d]
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k_gathered = key_cache[blk_ids]
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k_flat = (k_gathered
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.permute(0, 3, 1, 2, 4)
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.reshape(-1, num_kv_heads, head_size))[:seq_len]
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# Value: [nblk, kv_h, d, blk_sz] → [nblk*blk_sz, kv_h, d]
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v_flat = (value_cache[blk_ids]
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.permute(0, 3, 1, 2)
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.reshape(-1, num_kv_heads, head_size))[:seq_len]
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if k_scale != 1.0:
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k_flat = k_flat.float().mul_(k_scale)
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if v_scale != 1.0:
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v_flat = v_flat.float().mul_(v_scale)
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# =============================================================
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# GQA broadcast: avoid materializing the expanded KV tensor
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#
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# Qwen3.6: H=24, kv_h=4, gqa_ratio=6, head_dim=256
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# Old: expand kv_h→H then contiguous → allocates seq_len×H×d (1.2GB at 100K)
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# New: reshape Q as [kv_h, gqa, 1, d], K as [kv_h, 1, d, seq_len]
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# → bmm with broadcasting → [kv_h, gqa, 1, seq_len]
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# → reshape to [H, seq_len]
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# Saves: gqa_ratio × memory (6x for Qwen3.6 = 1GB per decode step)
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# =============================================================
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q = query[seq_idx].float() # [H, d]
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if gqa_ratio > 1:
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# K: [seq_len, kv_h, d] → [kv_h, d, seq_len] (no GQA expansion)
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k_kv = k_flat.permute(1, 2, 0).float().contiguous() # [kv_h, d, seq_len]
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v_kv = v_flat.permute(1, 0, 2).float().contiguous() # [kv_h, seq_len, d]
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# Q: [H, d] → [kv_h, gqa, 1, d]
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q_grouped = q.view(num_kv_heads, gqa_ratio, 1, head_size)
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# Scores: [kv_h, gqa, 1, d] @ [kv_h, 1, d, seq_len] → [kv_h, gqa, 1, seq_len]
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scores_all = torch.matmul(q_grouped, k_kv.unsqueeze(1)).squeeze(2) # [kv_h, gqa, seq_len]
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scores_all = scores_all.reshape(num_heads, seq_len) * scale # [H, seq_len]
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else:
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k_t = k_flat.permute(1, 2, 0).float().contiguous() # [H, d, seq_len]
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scores_all = torch.bmm(q.unsqueeze(1), k_t).squeeze(1) * scale # [H, seq_len]
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# Alibi bias (if needed)
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if alibi_slopes is not None:
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positions = torch.arange(seq_len, device=query.device, dtype=torch.float32)
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scores_all = scores_all + alibi_slopes.unsqueeze(1) * positions.unsqueeze(0)
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# Pad to exact multiple of _PARTITION_SIZE for clean reshape
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padded_len = num_partitions * _PARTITION_SIZE
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if padded_len > seq_len:
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pad_size = padded_len - seq_len
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scores_padded = torch.full(
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(num_heads, padded_len), float('-inf'),
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dtype=scores_all.dtype, device=scores_all.device)
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scores_padded[:, :seq_len] = scores_all
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else:
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scores_padded = scores_all
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# Reshape: [H, padded_len] → [H, P, part_sz]
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scores_parts = scores_padded.view(num_heads, num_partitions, _PARTITION_SIZE)
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# Per-partition online softmax (vectorized over H and P simultaneously)
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# Pattern from CCCL summary_statistics: compute (max, sum) in one pass
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part_max = scores_parts.max(dim=-1).values # [H, P]
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scores_exp = torch.exp(scores_parts - part_max.unsqueeze(-1)) # [H, P, part_sz]
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part_sum = scores_exp.sum(dim=-1) # [H, P]
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# Weighted values per partition: need V reshaped the same way
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# V: [seq_len, H, d] → pad → [padded_len, H, d] → [H, P, part_sz, d]
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if gqa_ratio > 1:
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v_perm = v_kv # already [kv_h, seq_len, d], no GQA expansion needed
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# Will handle GQA in the bmm below via broadcast
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else:
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v_perm = v_flat.permute(1, 0, 2).float().contiguous() # [H, seq_len, d]
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# Weighted V sum per partition
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# NOTE: v_perm shape differs by GQA mode:
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# GQA: v_perm = v_kv = [kv_h, seq_len, d]
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# No GQA: v_perm = [H, seq_len, d]
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# scores_exp: [H, P, part_sz] → [kv_h, gqa, P, part_sz]
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# v_perm: [kv_h, seq_len, d] → [kv_h, P, part_sz, d]
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if gqa_ratio > 1:
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se_grouped = scores_exp.view(num_kv_heads, gqa_ratio, num_partitions, _PARTITION_SIZE)
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# V: pad and reshape to [kv_h, P, part_sz, d]
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if padded_len > seq_len:
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v_padded_kv = torch.zeros(
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(num_kv_heads, padded_len, head_size),
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dtype=v_kv.dtype, device=v_kv.device)
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v_padded_kv[:, :seq_len, :] = v_kv
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else:
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v_padded_kv = v_kv
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v_parts_kv = v_padded_kv.view(num_kv_heads, num_partitions, _PARTITION_SIZE, head_size)
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# Broadcast: [kv_h, gqa, P, 1, part_sz] @ [kv_h, 1, P, part_sz, d]
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# → [kv_h, gqa, P, 1, d]
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part_out_grouped = torch.matmul(
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se_grouped.unsqueeze(3), # [kv_h, gqa, P, 1, part_sz]
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v_parts_kv.unsqueeze(1) # [kv_h, 1, P, part_sz, d]
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).squeeze(3) # [kv_h, gqa, P, d]
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part_out = part_out_grouped.reshape(num_heads, num_partitions, head_size)
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else:
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# Non-GQA: v_perm is [H, seq_len, d], pad and reshape normally
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if padded_len > seq_len:
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v_padded = torch.zeros(
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(num_heads, padded_len, head_size),
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dtype=v_perm.dtype, device=v_perm.device)
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v_padded[:, :seq_len, :] = v_perm
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else:
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v_padded = v_perm
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v_parts = v_padded.view(num_heads, num_partitions, _PARTITION_SIZE, head_size)
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HP = num_heads * num_partitions
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scores_exp_flat = scores_exp.reshape(HP, 1, _PARTITION_SIZE)
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v_parts_flat = v_parts.reshape(HP, _PARTITION_SIZE, head_size)
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part_out_flat = torch.bmm(scores_exp_flat, v_parts_flat) # [HP, 1, d]
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part_out = part_out_flat.view(num_heads, num_partitions, head_size) # [H, P, d]
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# Store partition results
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max_logits[seq_idx, :, :num_partitions] = part_max
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exp_sums[seq_idx, :, :num_partitions] = part_sum
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tmp_output[seq_idx, :, :num_partitions, :] = part_out.to(tmp_output.dtype)
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# =============================================================
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# Phase 2: Cross-partition reduction (CCCL binary_op pattern)
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#
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# This is the summary_statistics.binary_op pattern:
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# Combine (max_a, sum_a, out_a) ⊕ (max_b, sum_b, out_b)
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# using numerically stable log-sum-exp rescaling.
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#
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# Fully vectorized — no loop over partitions.
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# =============================================================
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pm = max_logits[seq_idx, :, :num_partitions] # [H, P]
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ps = exp_sums[seq_idx, :, :num_partitions] # [H, P]
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po = tmp_output[seq_idx, :, :num_partitions, :] # [H, P, d]
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global_max = pm.max(dim=-1).values # [H]
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rescale = torch.exp(pm - global_max.unsqueeze(-1)) * ps # [H, P]
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total = rescale.sum(dim=-1, keepdim=True) # [H, 1]
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weights = rescale / total # [H, P]
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# [H, 1, P] @ [H, P, d] → [H, 1, d] → [H, d]
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final = torch.bmm(weights.unsqueeze(1), po.float()).squeeze(1) # [H, d]
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output[seq_idx] = final.to(output.dtype)
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