[TRTLLM-11628][perf] Batch the beam-search finish-reason reduction - #17494
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In `@tensorrt_llm/_torch/pyexecutor/sampler/sampler_strategy.py`:
- Line 1344: In the strategy handling around rows_per_request, bind
strategies[0] to a local variable before narrowing it, then use that variable
for the tag check and row_stride access so strict mypy preserves the narrowed
type; alternatively apply the existing BeamSearch cast pattern used near line
342.
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WalkthroughBeam-search completion now computes finished-beam prefix lengths for all sequence slots in one reduction. Request beam widths determine completion. Tests cover varying beam widths, ignored columns, request states, and finish reasons. ChangesBeam completion handling
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🔵 Low · up to The change batches beam-finish processing without changing runtime semantics, but the current code may fail the configured type-check gate because a cast was removed; merge is reasonable with owner awareness or a type-check confirmation. Suggested reviewers: 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
✨ Finishing Touches🧪 Generate unit tests (beta)
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Hi @lori-ren @YihuiLu512 could you help to review this PR, thanks a lot. |
_handle_finish_reasons_impl reduced one request's row of first_finish_reasons at a time, so every beam-search request in the batch paid a slice, a compare, a sum and a tensor-to-bool conversion to answer a question about at most max_beam_width integers. The dispatch overhead of those per-request ATen calls dominates the arithmetic they perform, and it lands on the host critical path of every decode step. Reduce the whole tensor once instead: count, per slot, how many leading beams have finished. The per-request check then reduces to comparing that count against the request's own beam width, which is a plain list lookup, and only the request updates -- the state assignment and the per-beam set_finished_reason calls, both of which cross into pybind -- keep looping. Counting the finished prefix rather than the finished beams keeps the result independent of what the columns past a request's beam width hold, so slots with differing beam widths share one reduction without relying on the padding columns being reset. Signed-off-by: ZhaoyangWang <zhaoyangw@nvidia.com>
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…VIDIA#17494) Signed-off-by: ZhaoyangWang <zhaoyangw@nvidia.com>
…VIDIA#17494) Signed-off-by: ZhaoyangWang <zhaoyangw@nvidia.com>
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Dev Engineer Review
_finished_beam_prefix_lengthscounts leading finished beams in one tensor operation.QA Engineer Review
tests/integration/test_lists/test-db/orqa/.Description
Reduce the beam-search finish reasons for the whole batch in one operation instead of once
per request.
_handle_finish_reasons_implreduced a single request's row offirst_finish_reasonsat atime, so every beam-search request paid a slice, a compare, a sum and a tensor-to-bool
conversion to answer a question about at most
max_beam_widthintegers. The dispatchoverhead of those per-request ATen calls dominates the arithmetic they perform, and it sits
on the host critical path of every decode step.
TorchSampler._finished_beam_prefix_lengths, which counts per slot how many leadingbeams have finished, over the whole
(max_batch_size, max_beam_width)tensor at once, andcall it once per
_update_requestsinstead of once per request._handle_first_finish_reasonsto a list lookup (prefix_length >= beam_width) plusthe request updates that must stay in the loop: the state assignment and the per-beam
set_finished_reasoncalls, both of which cross into pybind.what the columns past a request's beam width hold and slots with differing beam widths can
share one reduction.
sampler_strategy.py:row_strideis alreadyintat the call site, andstrategies[0]is already narrowed to
BeamSearchby the tag check above it.Test Coverage
tests/unittest/_torch/sampler/test_torch_sampler.py::TestFinishReasons:test_finished_beam_prefix_lengths_matches_per_request_reduction-- exhaustive equivalenceagainst the reduction this replaces, over all 4-column/4-reason rows and every beam width.
test_finished_beam_prefix_lengths_ignores_columns_past_beam_width-- columns past arequest's beam width must neither complete it nor mask an unfinished beam.
test_handle_first_finish_reasons_completes_only_fully_finished_requests-- mixed beamwidths (2/4/1) in one batch; asserts which requests complete and the exact per-beam reasons
recorded.
Existing coverage for the modified path:
tests/unittest/_torch/sampler/test_beam_search.pyand
test_beam_search_speculative_d2h.py.PR Checklist
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