[https://nvbugs/6758245][fix] Cherry-pick #20429: Keep /metrics from blocking the serving event loop by JunyiXu-nv · Pull Request #20698 · NVIDIA/TensorRT-LLM

JunyiXu-nv · 2026-10-20T18:29:02Z

Description

Cherry-pick of #20429 (merged into release/1.3 as ae62a63) to main. Fixes https://nvbugs/6758245: turning enable_iter_perf_stats off made serving slower, because each GET /metrics scrape blocked the server's event loop on a stats read of up to 2 s.

Stats off (commit by Bowen Fu, from [https://nvbugs/6758245][fix] Avoid metrics queue polling when iteration stats are disabled #19517): /metrics returns [] at once instead of waiting for stats that never arrive.
Stats on without the background collector:
- Dedicated thread: the stats read runs on a one-thread ThreadPoolExecutor, so streaming continues while it waits.
- One read at a time: a scrape that arrives while a read is in flight gets [] at once instead of queueing a new read.
- Cancelled scrapes: the stats a cancelled scrape's read drained go to the next scrape.
- Shutdown: lifespan shuts the executor down after the generator.
RPC failures: an RPCError from the read returns [] instead of a 500.
Behavior change: each statistic is returned to only one caller. With two clients polling at once, one of them may get an empty list. The /metrics section of trtllm-serve.rst says so.

tests/unittest/llmapi/apps/test_openai_server_iteration_stats.py is registered in l0_cpu.yml, as in #20429.

Conflict resolution: the only conflict was the import block of tensorrt_llm/serve/openai_server.py: main imports tensorrt_llm.executor.utils, which release/1.3 lacks, and this change adds tensorrt_llm.executor.rpc.RPCError. Both imports are kept. The change to openai_server.py is otherwise the same as in #20429 (63 changed lines in both).

Test Coverage

tests/unittest/llmapi/apps/test_openai_server_iteration_stats.py (CPU-Generic stages): all 13 tests pass locally on this branch, three runs in a row. They cover:
- the stats-off early return;
- event-loop responsiveness;
- use of the metrics executor;
- one read in flight under sustained scraping;
- the cancellation and completion races;
- RPCError handling.
[https://nvbugs/6758245][fix] Keep /metrics from blocking the serving event loop #20429 passed the full release/1.3 pipeline, multi-GPU stages included (pipeline [Question]How do I check whether kv cache int8 is effective? #394).

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Dev Engineer Review

/metrics now avoids blocking the serving event loop during iteration-stat reads. A single-thread executor prevents overlapping reads, preserves drained stats after cancellation, handles RPCError as an empty result, and shuts down during lifespan cleanup. No material API or configuration risks are identified.

QA Engineer Review

tests/unittest/llmapi/apps/test_openai_server_iteration_stats.py covers slow reads, executor usage, disabled stats, concurrent reads, cancellation races, executor errors, and RPCError. The eager-start race test is restricted to Python 3.12 and newer. The test is listed in tests/integration/test_lists/test-db/l0_cpu.yml.

The author reports 13 passing tests across three local runs. A reported CI pipeline failed, and a later run has no completion result. Coverage verdict: needs follow-up.

Per-File QA Perspective

tensorrt_llm/serve/openai_server.py: Verify non-blocking reads, single-read enforcement, cancellation reuse, executor shutdown, and RPCError handling.
tests/unittest/llmapi/apps/test_openai_server_iteration_stats.py: Covers the new metrics-read paths and failure cases. It is listed in the CPU CI test list.
tests/integration/test_lists/test-db/l0_cpu.yml: Adds the iteration-stats test to CPU CI.
docs/source/commands/trtllm-serve/trtllm-serve.rst: Documents different /metrics results for concurrent clients, including empty lists.

… event loop (NVIDIA#20429) Signed-off-by: Bowen Fu <5812640+BowenFu@users.noreply.github.com> Signed-off-by: JunyiXu-nv <219237550+JunyiXu-nv@users.noreply.github.com> Co-authored-by: Bowen Fu <5812640+BowenFu@users.noreply.github.com> (cherry picked from commit ae62a63)

JunyiXu-nv · 2026-10-20T18:29:12Z

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coderabbitai · 2026-10-20T18:33:48Z

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tests/unittest/llmapi/apps/test_openai_server_iteration_stats.py

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Changes

Iteration statistics metrics

Layer / File(s)	Summary
Executor-backed metrics reads `tensorrt_llm/serve/openai_server.py`, `docs/source/commands/trtllm-serve/trtllm-serve.rst`	`/metrics` dispatches blocking reads to a dedicated executor, skips reads when statistics are disabled, prevents overlapping reads, and handles cancellation and executor shutdown. The documentation describes concurrent polling behavior.
Metrics read validation `tests/unittest/llmapi/apps/test_openai_server_iteration_stats.py`, `tests/integration/test_lists/test-db/l0_cpu.yml`	Tests cover executor use, non-blocking responses, serialized reads, cancellation, RPC errors, disabled polling, and route behavior. The test file is added to the CPU test list.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Suggested reviewers: juney-nvidia

Merge Risk: ⚪ Minimal · up to cd17b

The change moves /metrics stats reads off the serving event loop, and the test coverage appears sufficient. No actionable merge-blocking risk remains in the supplied review context.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 27.27% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 22 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title follows the required ticket and type format and clearly identifies the main change: preventing /metrics from blocking the serving event loop.
Description check		The description includes complete Description, Test Coverage, and PR Checklist sections. It explains the problem, solution, behavior changes, test coverage, and reported CI results.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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@tests/unittest/llmapi/apps/test_openai_server_iteration_stats.py:
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tensorrt-cicd · 2026-10-20T18:43:17Z

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/bot run --disable-fail-fast

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…n 3.12+ only asyncio.Task(eager_start=...) exists from Python 3.12, and the package supports 3.10, where the test would fail with an IndexError instead of testing the race. Signed-off-by: JunyiXu-nv <219237550+JunyiXu-nv@users.noreply.github.com>

JunyiXu-nv · 2026-10-21T04:18:45Z

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LGTM for infra part

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Main now keeps /metrics stats reads off the default executor (NVIDIA#20698), on a dedicated metrics thread. The multi-frontend launcher's 1 s idle poll of the engine stats queue used asyncio.to_thread, i.e. that same default executor; run it on the metrics thread instead, which is idle while the collector loop owns the queue (/metrics then serves from the collector's buffer). Signed-off-by: Lance Liao <108499334+lancelly@users.noreply.github.com>

JunyiXu-nv requested review from a team as code owners October 20, 2026 18:29

JunyiXu-nv requested review from BowenFu, asfiyab-nvidia, kaiyux, laikhtewari, mikeiovine, schetlur-nv, tburt-nv and yuanjingx87 October 20, 2026 18:29

JunyiXu-nv mentioned this pull request Oct 20, 2026

[https://nvbugs/6758245][fix] Keep /metrics from blocking the serving event loop #20429

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Comment thread tests/unittest/llmapi/apps/test_openai_server_iteration_stats.py Resolved

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trtllm-agent added the ci: full pre-merge approved label Oct 21, 2026

JunyiXu-nv merged commit 679ef13 into NVIDIA:main Oct 23, 2026

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