[DSv4.1][PCP] Compress ratio-2 groups rank-locally (proposal for vllm-project/vllm#59857)

[LucasWilkinson]

Overview

Proposal against vllm-project#59857: keep the DeepSeek-V4.1 ratio-2 compressor rank-local under PCP, which removes the InputBatch.pcp_metadata / PCPBatchMetadata plumbing so vllm/v1/worker/gpu/ is unchanged relative to main.

Claims

• No core changes: input_batch.py and pcp_manager.py match main; PCP chunking is untouched (arbitrary chunk boundaries still supported).
• Each rank compresses only its own tokens. Ratio-2 pairs split across PCP chunks are closed with the predecessor row gathered from the owning rank.
• Ratio-2 compressor communication drops from all-gathering kv_score (FP32 × 1024 per token) to the BF16 latent gather already used by ratio 1, plus one kv_score row per local request.
• Ratio 1 and ratio 2 share a single PCP cache-insert path in attention.py.

Validation

Unit tests, B300, chg run -g 1:

.venv/bin/python -m pytest tests/kernels/test_compressor_kv_cache.py tests/v1/worker/test_gpu_pcp_manager.py -q

239 passed, 13 skipped (skips are ROCm/gfx950-only). ruff, ruff-format and mypy (3.10 / 3.12) pass.

test_v41_pcp_compressor_preserves_pairs_and_decode_state (PCP=2/4) simulates every rank with main's chunking, including prefills starting at odd positions (pairs split across ranks) and decodes whose owner rank changes each step. It asserts bit-exact latents against the non-PCP kernel and identical rings on all ranks. Removing the gathered predecessor rows makes it fail (33% / 99.8% mismatched elements).

Not yet done: end-to-end GSM8K / prefill benchmark on DeepSeek-V4.1-Flash. I don't have the checkpoint locally; the vllm-project#59857 GSM8K and vllm bench serve setup should be rerun on this branch before adopting it.

Details

• CompressorMetadataBuilder (PCP only) all-gathers one int64 per local request: ring block << 32 | position of its last token. The ring block identifies a request on every rank. After sorting, each rank finds (a) the gathered row holding the token before each local chunk's first token, when that token closes a pair, and (b) each request's newest gathered row, which is written to the ring if it leaves a pair open.
• DeepseekCompressor.forward gathers kv_score of those last tokens. fused_save_compress_norm takes optional prev_rows / prev_row_indices: the request program reads its predecessor from the gathered rows instead of the ring, and skips its own ring stores. save_ring_rows then writes the open rows so every rank keeps the same ring.
• Under PCP the ring holds only the open row, which is all ratio 2 reads without speculative decoding; the builder asserts speculative_config is None (already rejected by the model for PCP).
• The builder runs one small integer all-gather per step, like the existing positions all-gather in DeepseekV41SparseSWAMetadataBuilder. No host syncs were added.
• The engram lookback fix and the doubled lookback buffer from [Feature][PCP] Add DeepSeek-V4.1-Flash support vllm-project/vllm#59857 are kept.

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Pull Request Checklist

• I used vLLM's /pr-checklist skill. (Mandatory for agents, optional for humans).

• AI assistance was used during the creation of this PR.

• Design Fit: Minimizes impact on core components, reuses existing functionality, and justifies added complexity.

• Testing and Validation: Validates the change and ensures any added tests are meaningful and reliable, with CI coverage or documented CI resource constraints and validation performed outside CI.

• Code Quality and Style: Keeps code and comments clear and concise, and updates relevant documentation and examples.

• Pull Request Contents: Includes a brief summary and relevant links, supports claims with evidence, explains root causes and implementation trade-offs, and follows the contributing guide.

This is a suggestion PR into vllm-project#59857's branch, not a separate change to main. AI assistance (Claude Code) was used to write it.

🤖 Generated with Claude Code

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