320 lines
13 KiB
Python
320 lines
13 KiB
Python
import logging
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import torch
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from torch import nn
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import triton
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import triton.language as tl
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from flash_attn.flash_attn_interface import flash_attn_varlen_func, flash_attn_with_kvcache
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from nanovllm.utils.context import get_context
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logger = logging.getLogger(__name__)
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@triton.jit
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def store_kvcache_kernel(
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key_ptr,
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key_stride,
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value_ptr,
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value_stride,
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k_cache_ptr,
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v_cache_ptr,
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slot_mapping_ptr,
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D: tl.constexpr,
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):
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idx = tl.program_id(0)
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slot = tl.load(slot_mapping_ptr + idx)
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if slot == -1: return
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key_offsets = idx * key_stride + tl.arange(0, D)
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value_offsets = idx * value_stride + tl.arange(0, D)
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key = tl.load(key_ptr + key_offsets)
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value = tl.load(value_ptr + value_offsets)
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cache_offsets = slot * D + tl.arange(0, D)
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tl.store(k_cache_ptr + cache_offsets, key)
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tl.store(v_cache_ptr + cache_offsets, value)
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def store_kvcache(key: torch.Tensor, value: torch.Tensor, k_cache: torch.Tensor, v_cache: torch.Tensor, slot_mapping: torch.Tensor):
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N, num_heads, head_dim = key.shape
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D = num_heads * head_dim
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assert key.stride(-1) == 1 and value.stride(-1) == 1
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assert key.stride(1) == head_dim and value.stride(1) == head_dim
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assert k_cache.stride(1) == D and v_cache.stride(1) == D
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assert slot_mapping.numel() == N
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store_kvcache_kernel[(N,)](key, key.stride(0), value, value.stride(0), k_cache, v_cache, slot_mapping, D)
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class Attention(nn.Module):
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def __init__(
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self,
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num_heads,
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head_dim,
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scale,
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num_kv_heads,
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):
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super().__init__()
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self.num_heads = num_heads
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self.head_dim = head_dim
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self.scale = scale
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self.num_kv_heads = num_kv_heads
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self.k_cache = self.v_cache = torch.tensor([])
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# Layer ID set by model_runner after model creation
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self.layer_id: int = -1
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def forward(self, q: torch.Tensor, k: torch.Tensor, v: torch.Tensor):
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context = get_context()
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k_cache, v_cache = self.k_cache, self.v_cache
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if k_cache.numel() and v_cache.numel():
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store_kvcache(k, v, k_cache, v_cache, context.slot_mapping)
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if context.is_prefill:
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if context.is_chunked_prefill:
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# Chunked prefill: merge attention from previous KV
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o = self._chunked_prefill_attention(q, k, v, context)
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elif context.block_tables is not None: # prefix cache
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k, v = k_cache, v_cache
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o = flash_attn_varlen_func(q, k, v,
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max_seqlen_q=context.max_seqlen_q, cu_seqlens_q=context.cu_seqlens_q,
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max_seqlen_k=context.max_seqlen_k, cu_seqlens_k=context.cu_seqlens_k,
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softmax_scale=self.scale, causal=True, block_table=context.block_tables)
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else:
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o = flash_attn_varlen_func(q, k, v,
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max_seqlen_q=context.max_seqlen_q, cu_seqlens_q=context.cu_seqlens_q,
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max_seqlen_k=context.max_seqlen_k, cu_seqlens_k=context.cu_seqlens_k,
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softmax_scale=self.scale, causal=True, block_table=context.block_tables)
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else: # decode
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if context.is_chunked_prefill:
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# Chunked decode: need to load all KV from CPU+GPU
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o = self._chunked_decode_attention(q, k, v, context)
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else:
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o = flash_attn_with_kvcache(q.unsqueeze(1), k_cache, v_cache,
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cache_seqlens=context.context_lens, block_table=context.block_tables,
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softmax_scale=self.scale, causal=True)
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return o
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def _chunked_prefill_attention(
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self,
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q: torch.Tensor,
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k: torch.Tensor,
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v: torch.Tensor,
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context,
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) -> torch.Tensor:
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"""
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Compute attention with three-region GPU buffer for chunked prefill.
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For chunked prefill:
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1. Load previous KV from CPU using Compute/Prefetch region (if any previous chunks)
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2. Compute attention against previous KV chunks (no causal mask)
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3. Compute attention against current chunk's KV (causal)
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4. Merge all results using online softmax
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Three-region design guarantees: current chunk's KV is in Compute region, previous KV is loaded
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from CPU to Prefetch region, so write and load regions never overlap.
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"""
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from nanovllm.kvcache.chunked_attention import flash_attn_with_lse, merge_attention_outputs
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# q, k, v shape: [total_tokens, num_heads, head_dim]
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# Reshape for flash attention: [batch, seq, heads, dim]
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q_batched = q.unsqueeze(0) # [1, total_tokens, heads, dim]
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k_batched = k.unsqueeze(0)
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v_batched = v.unsqueeze(0)
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o_acc = None
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lse_acc = None
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# Load previous KV from CPU using Compute/Prefetch region
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kvcache_manager = context.kvcache_manager
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seq = context.chunked_seq if hasattr(context, 'chunked_seq') else None
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if kvcache_manager is not None and seq is not None and self.layer_id >= 0:
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# Get prefilled CPU blocks (blocks already written in previous chunks)
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cpu_block_table = kvcache_manager.get_prefilled_cpu_blocks(seq)
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if cpu_block_table:
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offload_engine = kvcache_manager.offload_engine
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# For prefill: ONLY use Prefetch region to avoid conflict with
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# current chunk's KV being written to Compute region slots
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# Use synchronous per-layer loading (async would conflict with writes)
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chunk_size = offload_engine.num_prefetch_blocks
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num_chunks = (len(cpu_block_table) + chunk_size - 1) // chunk_size
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for chunk_idx in range(num_chunks):
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start = chunk_idx * chunk_size
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end = min(start + chunk_size, len(cpu_block_table))
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num_blocks_in_chunk = end - start
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chunk_ids = cpu_block_table[start:end]
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# Load to Prefetch region (per-layer, sync)
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offload_engine.load_to_prefetch_layer(self.layer_id, chunk_ids)
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offload_engine.wait_prefetch_layer(self.layer_id)
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prev_k, prev_v = offload_engine.get_kv_for_prefetch(
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self.layer_id, num_blocks_in_chunk
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)
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# Compute attention against this chunk (no causal mask)
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prev_o, prev_lse = flash_attn_with_lse(
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q_batched,
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prev_k,
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prev_v,
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softmax_scale=self.scale,
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causal=False,
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)
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# Merge with accumulated
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if o_acc is None:
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o_acc, lse_acc = prev_o, prev_lse
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else:
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o_acc, lse_acc = merge_attention_outputs(o_acc, lse_acc, prev_o, prev_lse)
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# Compute attention against current chunk's KV (with causal mask)
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current_o, current_lse = flash_attn_with_lse(
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q_batched,
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k_batched,
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v_batched,
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softmax_scale=self.scale,
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causal=True,
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)
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# Merge with accumulated
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if o_acc is None:
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final_o = current_o
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else:
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final_o, _ = merge_attention_outputs(o_acc, lse_acc, current_o, current_lse)
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# Remove batch dimension: [1, total_tokens, heads, dim] -> [total_tokens, heads, dim]
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return final_o.squeeze(0)
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def _chunked_decode_attention(
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self,
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q: torch.Tensor,
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k: torch.Tensor,
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v: torch.Tensor,
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context,
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) -> torch.Tensor:
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"""
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Compute decode attention with async double-buffering using Compute and Prefetch regions.
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Pipeline design:
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- Compute region: holds current chunk being computed
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- Prefetch region: async loads next chunk while current is computing
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- After computation, swap roles of the two regions
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Timeline:
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┌─────────────┐ ┌─────────────┐ ┌─────────────┐
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│Load C0→Comp │ │Load C1→Pref │ │Load C2→Comp │ ...
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└─────────────┘ └─────────────┘ └─────────────┘
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↘ ↘ ↘
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┌─────────────┐ ┌─────────────┐ ┌─────────────┐
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│ Compute C0 │ │ Compute C1 │ │ Compute C2 │
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└─────────────┘ └─────────────┘ └─────────────┘
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"""
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from nanovllm.kvcache.chunked_attention import flash_attn_with_lse, merge_attention_outputs
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# q shape: [batch_size, num_heads, head_dim] (single decode token per sequence)
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q_batched = q.unsqueeze(1) # [batch, 1, heads, dim]
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kvcache_manager = context.kvcache_manager
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seq = context.chunked_seq
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# Get all CPU blocks for this sequence
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cpu_block_table, _ = kvcache_manager.get_all_cpu_blocks(seq)
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if self.layer_id == 0:
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logger.debug(f"Decode attention: cpu_block_table={cpu_block_table}, seq.block_table={list(seq.block_table)}")
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if not cpu_block_table:
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raise RuntimeError("Chunked decode attention failed: no CPU blocks available")
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offload_engine = kvcache_manager.offload_engine
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# Use prefetch_size as chunk size for double buffering
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# This ensures both Compute and Prefetch regions can hold a full chunk
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chunk_size = offload_engine.num_prefetch_blocks
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num_chunks = (len(cpu_block_table) + chunk_size - 1) // chunk_size
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o_acc = None
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lse_acc = None
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# Double buffering state: True = use Compute region, False = use Prefetch region
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use_compute = True
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# Pre-load first chunk to Compute region (async)
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first_chunk_ids = cpu_block_table[:min(chunk_size, len(cpu_block_table))]
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offload_engine.load_to_compute_layer(self.layer_id, first_chunk_ids)
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for chunk_idx in range(num_chunks):
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start = chunk_idx * chunk_size
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end = min(start + chunk_size, len(cpu_block_table))
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num_blocks_in_chunk = end - start
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# Wait for current buffer to be ready
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if use_compute:
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offload_engine.wait_compute_layer(self.layer_id)
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else:
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offload_engine.wait_prefetch_layer(self.layer_id)
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# Trigger async prefetch of next chunk to the OTHER buffer
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# This overlaps transfer with current chunk's computation
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if chunk_idx + 1 < num_chunks:
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next_start = end
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next_end = min(next_start + chunk_size, len(cpu_block_table))
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next_chunk_ids = cpu_block_table[next_start:next_end]
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if use_compute:
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# Current in Compute, prefetch next to Prefetch region
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offload_engine.load_to_prefetch_layer(self.layer_id, next_chunk_ids)
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else:
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# Current in Prefetch, prefetch next to Compute region
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offload_engine.load_to_compute_layer(self.layer_id, next_chunk_ids)
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# Get KV from current buffer
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if use_compute:
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k_chunk, v_chunk = offload_engine.get_kv_for_compute(
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self.layer_id, num_blocks_in_chunk
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)
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else:
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k_chunk, v_chunk = offload_engine.get_kv_for_prefetch(
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self.layer_id, num_blocks_in_chunk
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)
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# Compute attention for this chunk
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o_chunk, lse_chunk = flash_attn_with_lse(
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q_batched, k_chunk, v_chunk,
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softmax_scale=self.scale,
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causal=False,
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)
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# Merge with accumulated
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if o_acc is None:
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o_acc, lse_acc = o_chunk, lse_chunk
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else:
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o_acc, lse_acc = merge_attention_outputs(o_acc, lse_acc, o_chunk, lse_chunk)
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# Swap buffers for next iteration
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use_compute = not use_compute
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# Now attend to Decode region (contains accumulated decode tokens)
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pos_in_block = context.decode_pos_in_block
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start_pos = context.decode_start_pos_in_block
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num_accumulated = pos_in_block - start_pos + 1
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if num_accumulated > 0:
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decode_k = offload_engine.k_cache_gpu[self.layer_id, offload_engine.decode_slot, start_pos:pos_in_block+1]
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decode_v = offload_engine.v_cache_gpu[self.layer_id, offload_engine.decode_slot, start_pos:pos_in_block+1]
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decode_k = decode_k.unsqueeze(0)
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decode_v = decode_v.unsqueeze(0)
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decode_o, decode_lse = flash_attn_with_lse(
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q_batched, decode_k, decode_v,
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softmax_scale=self.scale,
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causal=False,
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)
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if o_acc is None:
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o_acc = decode_o
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else:
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o_acc, _ = merge_attention_outputs(o_acc, lse_acc, decode_o, decode_lse)
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if o_acc is None:
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raise RuntimeError("Chunked decode attention failed: no KV available")
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return o_acc
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