[Perf] Skip detokenization in offline beam search - #50333
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Signed-off-by: samuelkim7 <samuelmwkim@gmail.com>
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vllm-project#50333 disabled detokenization on both offline beam-search paths but added no test, so a regression would be silent: the search loop only reads token IDs and logprob values, and nothing fails loudly if the per-step params start detokenizing again. Add GPU-free tests asserting detokenize is False on the plain per-step sampling params and on the per-beam params built for the structured-output path. Signed-off-by: pjdurden <prajjwalchittori1@gmail.com>
Signed-off-by: samuelkim7 <samuelmwkim@gmail.com>
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What & why
Follow-up to #46422, which skipped detokenization in online beam search and noted the offline path has the identical pattern.
Offline beam search asks for
logprobs = 2 * beam_widthon every internal per-step request. The engine detokenizes all of those token ids into strings on every step, but beam search never reads them. It ranks beams bycum_logproband decodes the final text itself. This PR setsdetokenize=Falseon the two internalSamplingParams, same as #46422 did for the online path.As with #46422, the per-step
Logprobentries in the returned sequences no longer carrydecoded_tokenstrings. Final output text is unchanged.Benchmark
A100, Qwen/Qwen3-1.7B (same model as #46422), offline
LLM.beam_search, 8 prompts of 256 tokens, 32 output tokens, temperature 0,enforce_eager. One arm per process. Output token ids are sha256-compared across arms as a hard gate.The win grows with beam width, matching the O(beam_width) per-step cost #46422 measured on the online path (1.80x at width 20 there; eager mode inflates the per-step GPU time here, so these numbers are conservative).
Correctness
Output token ids are bit-identical between arms at all three beam widths on the A100 run (sha256 gate). I also verified output equality on CPU (TinyLlama-1.1B, beam widths 4 and 8, 3 interleaved rounds each). The existing
tests/samplers/test_beam_search.pysuite covers the offline path against HF reference outputs in CI.Duplicate check
No open PR applies this to the offline path. #47630 touches the same file for
allowed_token_idshandling and does not conflict with this change.Done with Claude Code assistance, mostly on the benchmark harness and runs. I reviewed the change and validated the results.