sm12x: multi-head prefill accumulate kernel + drop fp8 einsum autotune#6
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jasl merged 1 commit intoMay 16, 2026
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Two prefill performance fixes for SM12x DeepSeek V4: 1. Add _accumulate_indexed_attention_chunk_multihead_kernel (HEAD_BLOCK=8) that loads KV once per candidate and reuses across 8 heads, reducing L2 traffic in the prefill accumulate phase. Same pattern as the existing decode _finish_materialized_scores_with_sink_kernel. Prefill throughput on 2× RTX PRO 6000 WS, TP=2, MTP=2: - 1K tokens: +49% (2,746 → 4,100 tok/s) - 4.5K tokens: +37% (3,122 → 4,271 tok/s) - 18K tokens: +36% (2,474 → 3,360 tok/s) - 64K tokens: +28% (1,679 → 2,146 tok/s) Tuned config: HEAD_BLOCK=8, num_warps=4, num_stages=2. Benchmarked against HEAD_BLOCK=4 and num_warps=8 variants — HEAD_BLOCK=8 with num_warps=4 wins at all sizes. 2. Drop @triton.autotune from _deepseek_v4_sm12x_fp8_einsum_kernel and pin num_warps=4, num_stages=3. The autotune key included num_tokens which varies per request, causing ~200 unique keys with zero cache hits — re-benchmarking 4 configs at ~1s each on every request. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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jasl
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May 16, 2026
Run dequantize_and_gather_k_cache for the compressed + SWA caches on aux_stream[1] while the indexer forward runs on aux_stream[0], so the gather is hidden behind the indexer instead of serialising before _forward_prefill. The workspace allocation at the wrapper places kv_workspace at offset 0 of the same per-ubatch workspace buffer that _forward_prefill would otherwise allocate kv from; _reserve_prefill_workspace during warmup already grew the buffer to fit the full prefill spec list, so the kv-only request cannot trigger a resize that orphans kv_workspace mid-forward. A gather_done_event joins the aux stream back before mla_attn runs. CUDA-graph-safe: both aux streams join (event.wait()) before the attention boundary, and the gather is gated on num_prefills > 0 and num_prefills <= PREFILL_CHUNK_SIZE (single-chunk only). Multi-chunk prefill or non-C128A paths fall through to the existing per-chunk gather in _forward_prefill. Original implementation by aabbccddwasd in their dsv4-sm120-opt-v2 branch (commit 6ff395e). This re-applies only the gather-overlap half of that commit; the multi-head prefill kernel half is dropped because the canonical tip already has alex's HEAD_BLOCK=8 version (671958e / vllm-project#41834 PR #6) which was empirically tuned for this hardware. Signed-off-by: jasl <jasl9187@hotmail.com>
jasl
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May 16, 2026
Run dequantize_and_gather_k_cache for the compressed + SWA caches on aux_stream[1] while the indexer forward runs on aux_stream[0], so the gather is hidden behind the indexer instead of serialising before _forward_prefill. The workspace allocation at the wrapper places kv_workspace at offset 0 of the same per-ubatch workspace buffer that _forward_prefill would otherwise allocate kv from; _reserve_prefill_workspace during warmup already grew the buffer to fit the full prefill spec list, so the kv-only request cannot trigger a resize that orphans kv_workspace mid-forward. A gather_done_event joins the aux stream back before mla_attn runs. CUDA-graph-safe: both aux streams join (event.wait()) before the attention boundary, and the gather is gated on num_prefills > 0 and num_prefills <= PREFILL_CHUNK_SIZE (single-chunk only). Multi-chunk prefill or non-C128A paths fall through to the existing per-chunk gather in _forward_prefill. Original implementation by aabbccddwasd in their dsv4-sm120-opt-v2 branch (commit 6ff395e). This re-applies only the gather-overlap half of that commit; the multi-head prefill kernel half is dropped because the canonical tip already has alex's HEAD_BLOCK=8 version (671958e / vllm-project#41834 PR #6) which was empirically tuned for this hardware. Signed-off-by: jasl <jasl9187@hotmail.com>
jasl
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May 18, 2026
Run dequantize_and_gather_k_cache for the compressed + SWA caches on aux_stream[1] while the indexer forward runs on aux_stream[0], so the gather is hidden behind the indexer instead of serialising before _forward_prefill. The workspace allocation at the wrapper places kv_workspace at offset 0 of the same per-ubatch workspace buffer that _forward_prefill would otherwise allocate kv from; _reserve_prefill_workspace during warmup already grew the buffer to fit the full prefill spec list, so the kv-only request cannot trigger a resize that orphans kv_workspace mid-forward. A gather_done_event joins the aux stream back before mla_attn runs. CUDA-graph-safe: both aux streams join (event.wait()) before the attention boundary, and the gather is gated on num_prefills > 0 and num_prefills <= PREFILL_CHUNK_SIZE (single-chunk only). Multi-chunk prefill or non-C128A paths fall through to the existing per-chunk gather in _forward_prefill. Original implementation by aabbccddwasd in their dsv4-sm120-opt-v2 branch (commit 6ff395e). This re-applies only the gather-overlap half of that commit; the multi-head prefill kernel half is dropped because the canonical tip already has alex's HEAD_BLOCK=8 version (671958e / vllm-project#41834 PR #6) which was empirically tuned for this hardware. Signed-off-by: jasl <jasl9187@hotmail.com>
jasl
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May 19, 2026
Run dequantize_and_gather_k_cache for the compressed + SWA caches on aux_stream[1] while the indexer forward runs on aux_stream[0], so the gather is hidden behind the indexer instead of serialising before _forward_prefill. The workspace allocation at the wrapper places kv_workspace at offset 0 of the same per-ubatch workspace buffer that _forward_prefill would otherwise allocate kv from; _reserve_prefill_workspace during warmup already grew the buffer to fit the full prefill spec list, so the kv-only request cannot trigger a resize that orphans kv_workspace mid-forward. A gather_done_event joins the aux stream back before mla_attn runs. CUDA-graph-safe: both aux streams join (event.wait()) before the attention boundary, and the gather is gated on num_prefills > 0 and num_prefills <= PREFILL_CHUNK_SIZE (single-chunk only). Multi-chunk prefill or non-C128A paths fall through to the existing per-chunk gather in _forward_prefill. Original implementation by aabbccddwasd in their dsv4-sm120-opt-v2 branch (commit 6ff395e). This re-applies only the gather-overlap half of that commit; the multi-head prefill kernel half is dropped because the canonical tip already has alex's HEAD_BLOCK=8 version (671958e / vllm-project#41834 PR #6) which was empirically tuned for this hardware. Signed-off-by: jasl <jasl9187@hotmail.com>
jasl
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May 19, 2026
Run dequantize_and_gather_k_cache for the compressed + SWA caches on aux_stream[1] while the indexer forward runs on aux_stream[0], so the gather is hidden behind the indexer instead of serialising before _forward_prefill. The workspace allocation at the wrapper places kv_workspace at offset 0 of the same per-ubatch workspace buffer that _forward_prefill would otherwise allocate kv from; _reserve_prefill_workspace during warmup already grew the buffer to fit the full prefill spec list, so the kv-only request cannot trigger a resize that orphans kv_workspace mid-forward. A gather_done_event joins the aux stream back before mla_attn runs. CUDA-graph-safe: both aux streams join (event.wait()) before the attention boundary, and the gather is gated on num_prefills > 0 and num_prefills <= PREFILL_CHUNK_SIZE (single-chunk only). Multi-chunk prefill or non-C128A paths fall through to the existing per-chunk gather in _forward_prefill. Original implementation by aabbccddwasd in their dsv4-sm120-opt-v2 branch (commit 6ff395e). This re-applies only the gather-overlap half of that commit; the multi-head prefill kernel half is dropped because the canonical tip already has alex's HEAD_BLOCK=8 version (671958e / vllm-project#41834 PR #6) which was empirically tuned for this hardware. Signed-off-by: jasl <jasl9187@hotmail.com>
jasl
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May 20, 2026
Run dequantize_and_gather_k_cache for the compressed + SWA caches on aux_stream[1] while the indexer forward runs on aux_stream[0], so the gather is hidden behind the indexer instead of serialising before _forward_prefill. The workspace allocation at the wrapper places kv_workspace at offset 0 of the same per-ubatch workspace buffer that _forward_prefill would otherwise allocate kv from; _reserve_prefill_workspace during warmup already grew the buffer to fit the full prefill spec list, so the kv-only request cannot trigger a resize that orphans kv_workspace mid-forward. A gather_done_event joins the aux stream back before mla_attn runs. CUDA-graph-safe: both aux streams join (event.wait()) before the attention boundary, and the gather is gated on num_prefills > 0 and num_prefills <= PREFILL_CHUNK_SIZE (single-chunk only). Multi-chunk prefill or non-C128A paths fall through to the existing per-chunk gather in _forward_prefill. Original implementation by aabbccddwasd in their dsv4-sm120-opt-v2 branch (commit 6ff395e). This re-applies only the gather-overlap half of that commit; the multi-head prefill kernel half is dropped because the canonical tip already has alex's HEAD_BLOCK=8 version (671958e / vllm-project#41834 PR #6) which was empirically tuned for this hardware. Signed-off-by: jasl <jasl9187@hotmail.com>
jasl
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May 22, 2026
Run dequantize_and_gather_k_cache for the compressed + SWA caches on aux_stream[1] while the indexer forward runs on aux_stream[0], so the gather is hidden behind the indexer instead of serialising before _forward_prefill. The workspace allocation at the wrapper places kv_workspace at offset 0 of the same per-ubatch workspace buffer that _forward_prefill would otherwise allocate kv from; _reserve_prefill_workspace during warmup already grew the buffer to fit the full prefill spec list, so the kv-only request cannot trigger a resize that orphans kv_workspace mid-forward. A gather_done_event joins the aux stream back before mla_attn runs. CUDA-graph-safe: both aux streams join (event.wait()) before the attention boundary, and the gather is gated on num_prefills > 0 and num_prefills <= PREFILL_CHUNK_SIZE (single-chunk only). Multi-chunk prefill or non-C128A paths fall through to the existing per-chunk gather in _forward_prefill. Original implementation by aabbccddwasd in their dsv4-sm120-opt-v2 branch (commit 6ff395e). This re-applies only the gather-overlap half of that commit; the multi-head prefill kernel half is dropped because the canonical tip already has alex's HEAD_BLOCK=8 version (671958e / vllm-project#41834 PR #6) which was empirically tuned for this hardware. Signed-off-by: jasl <jasl9187@hotmail.com>
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Summary
_accumulate_indexed_attention_chunk_multihead_kernel(HEAD_BLOCK=4) that loads KV once per candidate and reuses across 4 heads, reducing L2 traffic in the prefill accumulate phase. Same pattern as the existing decode_finish_materialized_scores_with_sink_kernel.@triton.autotunefrom_deepseek_v4_sm12x_fp8_einsum_kernel— the key includednum_tokenswhich varies per request, so autotune never got a cache hit (re-benchmarked 4 configs at ~1s each on every request).Benchmarks
Prefill throughput on 2× RTX PRO 6000 WS Edition, TP=2, MTP=2, 524K max_model_len:
Test plan
🤖 Generated with Claude Code