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[#22020][perf] Search only new streamed text for stop strings - #22021
Incremental streaming responses now search from the prior text length, with overlap for stop strings that cross response boundaries. Full-text search remains for non-incremental decoding and when a post-processor hook can rewrite text. Non-string stop reasons are skipped. The change preserves stop-string truncation and inclusion behavior in the covered cases.
The implementation is scoped to stop-string matching and adds no public API. The reported benchmark shows lower processing time for long streamed outputs with stop strings. The author also reports identical results in a 26,000-stream differential check.
QA Engineer Review
The modified unit test covers text stop strings across varied stop values and chunk sizes, both include_stop_str_in_output settings, matching streamed output against non-streamed output, boundary overlap, and text emitted by a post-processor hook. It also exhaustively compares _find_stop_string with full search for prefixes known to have no match. The test is listed in tests/integration/test_lists/test-db/l0_cpu.yml. Coverage verdict: sufficient.
The author reports 167 tests passed and 1 deselected in the specified stop-string and post-processor test run, with native binding imports stubbed. The author also reports that pre-commit passed for both changed files. Other tests and GPU or end-to-end serving tests were not run locally.
Per-File QA Perspective
tensorrt_llm/executor/result.py: Verify streamed stop-string matches at response boundaries and confirm that post-processor rewrites are still found on subsequent responses. Confirm stop truncation, finish reason, and stop reason remain correct.
tests/unittest/executor/test_stop_word_streaming_text.py: Covers streamed versus non-streamed text-stop behavior, search-boundary overlap, and post-processor output. Listed in the CPU CI test list; no manual-QA list entry was found.
DetokenizedGenerationResultBase._handle_response searched the whole
accumulated text for every string stop sequence on every streamed response.
With the default stream_interval=1, a streamed request with stop strings
spent time quadratic in its output length on the proxy's asyncio thread (the trtllm-serve event loop by default) or in a postprocess worker.
Incremental detokenization only appends to text, and earlier responses
already searched the text before _last_text_len, so the first match must end
in the appended part. _find_stop_string starts the search len(stop) - 1
characters before that point and returns the same index as text.find(stop).
The full search stays for outputs that are decoded again from all their tokens
(non-streaming, beam search, a trimmed stop word) and for results with a
post-processor hook, since the hook can rewrite the last chunk after it was
searched. Stop-string order, truncation, include_stop_str_in_output and empty
stop strings behave as before.
Benchmark
Intel Xeon Platinum 8352Y, CPython 3.12.3, transformers 5.5.4, tokenizers
0.22.2, one pinned core. The benchmark drives the repository's DetokenizedGenerationResultBase._handle_response with the Qwen3-0.6B
tokenizer (HF incremental detokenization), one response per generated token,
on English text from docs/source. The stop strings do not occur, so every
response is searched, as happens until a stop string appears. One warm-up
stream, then five runs per row; medians. Stop strings: ["</answer>"] and ["Question:", "</s>", "<|im_end|>"]. Final text and finish reason are
identical before and after in every row.
Output tokens (chars)
stop
Whole stream, main
This PR
Per response, last 1,000 responses
8,192 (28K)
none
91.8 ms
90.4 ms
11.4 -> 11.4 us
8,192 (28K)
1 string
109.4 ms
100.1 ms
15.3 -> 12.2 us
8,192 (28K)
3 strings
151.5 ms
105.7 ms
25.6 -> 13.0 us
16,384 (58K)
none
202.9 ms
204.8 ms
12.9 -> 13.1 us
16,384 (58K)
1 string
461.4 ms
218.8 ms
53.0 -> 13.9 us
16,384 (58K)
3 strings
796.0 ms
231.4 ms
98.1 -> 15.0 us
32,768 (125K)
none
465.2 ms
451.6 ms
16.7 -> 16.6 us
32,768 (125K)
1 string
1767.0 ms
475.6 ms
102.2 -> 17.6 us
32,768 (125K)
3 strings
3356.5 ms
513.8 ms
214.4 -> 18.6 us
At 32K output tokens this saves 1.3 s (one string) to 2.8 s (three strings) of
CPU per streamed request. Near 16K tokens each response carried an extra
39-83 us, which across 64 concurrent streams is 2.5-5.3 ms of serial work per
decode step.
Test Coverage
tests/unittest/executor/test_stop_word_streaming_text.py (listed in l0_cpu.yml; CPU-Generic x86 and ARM stages):
streamed text-level stop strings that span responses (chunk sizes 1, 2, 3
and 5, with and without include_stop_str_in_output, including multibyte
characters) end with the same text, finish reason and stop reason as the
non-streamed path;
_find_stop_string matches str.find for every text of up to 8 characters
over a two-letter alphabet, every stop string of up to 3 characters
(including the empty string) and every searched prefix that holds no match;
a stop string that a post-processor hook emits is still found on the next
response.
The stream and hook tests also pass on main, so they pin the current
behavior. Dropping the len(stop) - 1 overlap makes 78 of the new test cases
fail, and using the shorter search with a hook makes the hook test fail.
Differential check: 26,000 randomized streams on main and with this change
(HF and TRTLLM incremental detokenization, chunk sizes 1-6, stream_interval
up to 32, n > 1 and best_of > n, beam search, non-streaming, token-level
stop-word trims, post-processor hooks that rewrite, suppress or inject stop
strings, empty stop strings, byte-fallback multibyte tokens) produced
identical per-response text deltas, final text, finish reasons and stop
reasons.
pytest -q tests/unittest/executor/test_stop_word_streaming_text.py tests/unittest/executor/test_stop_word_side_output_trim.py tests/unittest/executor/test_postprocessor_hook.py -k "not harmony":
167 passed, 1 deselected. Run against the checkout with the native binding
imports stubbed; the changed code is CPU-only Python.
pre-commit run --files <two changed files>: all hooks passed, including
yapf, Ruff, legacy lint and codespell; Pyrefly reported no diagnostics.
Not run locally:
test_harmony_model_rejects_post_processor_hook, which imports the serving
stack (uvicorn) that the stubbed environment does not provide.
tests/unittest/llmapi/test_executor.py, which imports the full tensorrt_llm.executor package and needs a native build.
GPU and end-to-end serving tests.
PR Checklist
PR description clearly explains what and why.
PR follows the TensorRT-LLM coding guidelines.
Test cases are provided for the changed path and are registered in CPU CI.
No public API, dependency, or configuration change is introduced.
Documentation and CODEOWNERS do not need updates.
Please check this after reviewing the above items as appropriate for this PR.
Reviewing files that changed from the base of the PR and between 26f7ab4 and adcf0f5.
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Walkthrough
Stop-string detection now uses a bounded search during incremental decoding without a post-processor hook. Tests compare streamed and non-streamed results and cover boundary matches and post-processor output.
A new helper searches from the earliest position that could contain a new or boundary-spanning match. Incremental decoding without a post-processor hook uses this search; other cases search from the start. Non-string stop reasons are skipped.
Streaming and search tests tests/unittest/executor/test_stop_word_streaming_text.py
Tests compare streamed and non-streamed text stops across chunk sizes and output settings. They also check helper results and a post-processor that emits a stop string.
No actionable merge-blocking risk is identified for streamed stop-string detection after normal checks.
Pre-merge checks: 4 · 1
Failed checks: 1 warning
Check name
Status
Explanation
Resolution
Docstring Coverage
Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documenta…
Write docstrings for the functions missing them to satisfy the coverage threshold.
Passed checks: 4 passed
Check name
Status
Explanation
Title check
The title clearly identifies issue #22020 and summarizes the performance improvement to streamed stop-string searches.
Description check
The description explains the issue and solution, gives detailed test coverage and benchmark results, and addresses the PR checklist.
Linked Issues check
Issue #22020 requests reducing repeated full-text searches for stop strings during incremental streaming while preserving matching behavior. result.py adds _find_stop_string and searches from `_la…
Out of Scope Changes check
The changes are limited to streamed stop-string matching in result.py and tests for that behavior. The helper and test additions directly support issue #22020. No unrelated changes appear in the rev…
Full details: Linked Issues check
Explanation
Issue #22020 requests reducing repeated full-text searches for stop strings during incremental streaming while preserving matching behavior. result.py adds _find_stop_string and searches from _last_text_len when decoding is incremental and no post-processor hook is configured. Other paths retain a full search. The new tests check overlap matches, streamed versus non-streamed results, empty stop strings, and stop strings emitted by a hook. The reported benchmark and differential checks support the performance and behavior claims.
Full details: Out of Scope Changes check
Explanation
The changes are limited to streamed stop-string matching in result.py and tests for that behavior. The helper and test additions directly support issue #22020. No unrelated changes appear in the reviewed diff.
Full details: Docstring Coverage
Explanation
Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 5 functions across 2 files.
Resolution
Write docstrings for the functions missing them to satisfy the coverage threshold.
Finishing Touches
Generate unit tests (beta)
Create a new PR
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Could you please rebase this branch onto the latest upstream main to include the CI hang fix in #22101 (6edbe383497a)? The current branch still contains the regression that can cause pipelines to hang for an extended period and worsen CI queueing
Incremental detokenization only appends to an output's text, and earlier
responses already searched the old text for stop strings, so a new match
must end in the appended part. Start the search len(stop) - 1 characters
before it instead of searching the whole output on every streamed
response. Outputs decoded again from all their tokens, and results with a
post-processor hook, keep the full search.
AI assistance (Cursor) was used for this change.
Co-authored-by: Cursor <cursoragent@cursor.com>
Signed-off-by: YangHaoooooo <233765393+Hang-get@users.noreply.github.com>
Rebased onto current main (efec60ea5), which includes the CI hang fix from #22101 (6edbe383). This branch is now adcf0f563. The patch is unchanged.
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Dev Engineer Review
Incremental streaming responses now search from the prior text length, with overlap for stop strings that cross response boundaries. Full-text search remains for non-incremental decoding and when a post-processor hook can rewrite text. Non-string stop reasons are skipped. The change preserves stop-string truncation and inclusion behavior in the covered cases.
The implementation is scoped to stop-string matching and adds no public API. The reported benchmark shows lower processing time for long streamed outputs with stop strings. The author also reports identical results in a 26,000-stream differential check.
QA Engineer Review
The modified unit test covers text stop strings across varied stop values and chunk sizes, both
include_stop_str_in_outputsettings, matching streamed output against non-streamed output, boundary overlap, and text emitted by a post-processor hook. It also exhaustively compares_find_stop_stringwith full search for prefixes known to have no match. The test is listed intests/integration/test_lists/test-db/l0_cpu.yml. Coverage verdict: sufficient.The author reports 167 tests passed and 1 deselected in the specified stop-string and post-processor test run, with native binding imports stubbed. The author also reports that pre-commit passed for both changed files. Other tests and GPU or end-to-end serving tests were not run locally.
Per-File QA Perspective
tensorrt_llm/executor/result.py: Verify streamed stop-string matches at response boundaries and confirm that post-processor rewrites are still found on subsequent responses. Confirm stop truncation, finish reason, and stop reason remain correct.tests/unittest/executor/test_stop_word_streaming_text.py: Covers streamed versus non-streamed text-stop behavior, search-boundary overlap, and post-processor output. Listed in the CPU CI test list; no manual-QA list entry was found.Description
Fixes #22020.
DetokenizedGenerationResultBase._handle_responsesearched the wholeaccumulated text for every string stop sequence on every streamed response.
With the default
stream_interval=1, a streamed request withstopstringsspent time quadratic in its output length on the proxy's asyncio thread (the
trtllm-serveevent loop by default) or in a postprocess worker.Incremental detokenization only appends to
text, and earlier responsesalready searched the text before
_last_text_len, so the first match must endin the appended part.
_find_stop_stringstarts the searchlen(stop) - 1characters before that point and returns the same index as
text.find(stop).The full search stays for outputs that are decoded again from all their tokens
(non-streaming, beam search, a trimmed stop word) and for results with a
post-processor hook, since the hook can rewrite the last chunk after it was
searched. Stop-string order, truncation,
include_stop_str_in_outputand emptystop strings behave as before.
Benchmark
Intel Xeon Platinum 8352Y, CPython 3.12.3, transformers 5.5.4, tokenizers
0.22.2, one pinned core. The benchmark drives the repository's
DetokenizedGenerationResultBase._handle_responsewith the Qwen3-0.6Btokenizer (HF incremental detokenization), one response per generated token,
on English text from
docs/source. The stop strings do not occur, so everyresponse is searched, as happens until a stop string appears. One warm-up
stream, then five runs per row; medians. Stop strings:
["</answer>"]and["Question:", "</s>", "<|im_end|>"]. Final text and finish reason areidentical before and after in every row.
stopmainAt 32K output tokens this saves 1.3 s (one string) to 2.8 s (three strings) of
CPU per streamed request. Near 16K tokens each response carried an extra
39-83 us, which across 64 concurrent streams is 2.5-5.3 ms of serial work per
decode step.
Test Coverage
tests/unittest/executor/test_stop_word_streaming_text.py(listed inl0_cpu.yml; CPU-Generic x86 and ARM stages):and 5, with and without
include_stop_str_in_output, including multibytecharacters) end with the same text, finish reason and stop reason as the
non-streamed path;
_find_stop_stringmatchesstr.findfor every text of up to 8 charactersover a two-letter alphabet, every stop string of up to 3 characters
(including the empty string) and every searched prefix that holds no match;
response.
The stream and hook tests also pass on
main, so they pin the currentbehavior. Dropping the
len(stop) - 1overlap makes 78 of the new test casesfail, and using the shorter search with a hook makes the hook test fail.
Differential check: 26,000 randomized streams on
mainand with this change(HF and TRTLLM incremental detokenization, chunk sizes 1-6,
stream_intervalup to 32, n > 1 and best_of > n, beam search, non-streaming, token-level
stop-word trims, post-processor hooks that rewrite, suppress or inject stop
strings, empty stop strings, byte-fallback multibyte tokens) produced
identical per-response text deltas, final text, finish reasons and stop
reasons.
pytest -q tests/unittest/executor/test_stop_word_streaming_text.py tests/unittest/executor/test_stop_word_side_output_trim.py tests/unittest/executor/test_postprocessor_hook.py -k "not harmony":167 passed, 1 deselected. Run against the checkout with the native binding
imports stubbed; the changed code is CPU-only Python.
pre-commit run --files <two changed files>: all hooks passed, includingyapf, Ruff, legacy lint and codespell; Pyrefly reported no diagnostics.
Not run locally:
test_harmony_model_rejects_post_processor_hook, which imports the servingstack (
uvicorn) that the stubbed environment does not provide.tests/unittest/llmapi/test_executor.py, which imports the fulltensorrt_llm.executorpackage and needs a native build.PR Checklist
PR description clearly explains what and why.
PR follows the TensorRT-LLM coding guidelines.
Test cases are provided for the changed path and are registered in CPU CI.
No public API, dependency, or configuration change is introduced.
Documentation and CODEOWNERS do not need updates.
Please check this after reviewing the above items as appropriate for this PR.
AI assistance (Cursor) was used for this change.