[Perf] Add per-phase cold-start startup span benchmark harness

[SuperMarioYL]

[Perf] Add per-phase cold-start startup span benchmark harness

Purpose

Roadmap #48193 (Cold Start, Q3 2026, tagged Help wanted, Observability bullet) asks for per-phase startup spans surfaced through a standard startup benchmark so cold-start regressions are measurable per phase. The span emitters already landed:

• #31162 — OTEL tracing during loading (merged 2026-02-06)
• #40698 — correct OTEL span start time for Dynamo compilation (merged 2026-07-13)
• #47388 — persist and reuse the memory-profiling result across boots (merged 2026-07-08)

The 6 startup phases are already instrumented with @instrument(span_name=...), but there is no consumer that turns those spans into a per-phase cold-start report — find benchmarks -iname startup returns 0 files and grep -rln startup_span benchmarks returns 0 hits. This PR adds the missing consumer + a per-phase report + a baseline comparator so a cold-start regression is measurable per phase, against a committed baseline.

What ships

• vllm/benchmarks/startup_spans.py (new, ~212 LoC): COLD_START_PHASES (the 6 phase names, frozen), PHASE_LIST_VERSION = 1, StartupPhaseSpan dataclass, InMemorySpanSink (a TraceServiceServicer that captures spans in-process, mirroring tests/tracing/conftest.py's FakeTraceService), collect_startup_spans / select_phase_spans, build_phase_report, format_phase_report, and compare_to_baseline (regression gate: flags a phase whose delta exceeds max(0.5s, baseline * 10%)).
• vllm/benchmarks/startup.py (edited, ~98 LoC): wires InMemorySpanSink into run_startup_in_subprocess, adds --per-phase and --phase-baseline <file> CLI flags, emits the per-phase report in main(), and merges it into --output-json. The new path is fully guarded — with --per-phase off, stdout and --output-json are byte-identical to master.
• tests/benchmarks/test_startup_spans.py (new, ~367 LoC): 4 non-GPU unit tests covering every new function (no mocking — real select_phase_spans / build_phase_report / InMemorySpanSink gRPC round-trip via TraceServiceStub / compare_to_baseline+format_phase_report), plus an integration test driving a real LLM(opt-125m) with real OTLP spans, and a CLI smoke test.

Phase-list stability (addressed proactively)

The phase boundary set is still moving under the Flat-Model migration (#42770) and the persisted-startup-plan default-on criteria (#47388). To keep a committed baseline from rotting within a release, the phase list is versioned: COLD_START_PHASES is a frozen constant and PHASE_LIST_VERSION = 1; baseline files carry the version they were recorded against, and compare_to_baseline refuses to compare across mismatched versions. When the Flat-Model owners settle the phase set, bump PHASE_LIST_VERSION and re-record the baseline.

Test Plan

Non-GPU (run locally):

pytest tests/benchmarks/test_startup_spans.py::test_select_phase_spans_picks_earliest -v
pytest tests/benchmarks/test_startup_spans.py::test_build_phase_report_shape -v
pytest tests/benchmarks/test_startup_spans.py::test_in_memory_span_sink_export_and_poll -v
pytest tests/benchmarks/test_startup_spans.py::test_compare_to_baseline_and_format -v
ruff check vllm/benchmarks/startup_spans.py vllm/benchmarks/startup.py tests/benchmarks/test_startup_spans.py
python -c "from vllm.benchmarks.startup_spans import COLD_START_PHASES, PHASE_LIST_VERSION; assert len(COLD_START_PHASES) == 6 and PHASE_LIST_VERSION == 1"

GPU / installed-CLI (CI lane):

pytest tests/benchmarks/test_startup_spans.py::test_startup_per_phase_spans -v   # needs a CUDA device
pytest tests/benchmarks/test_startup_spans.py::test_bench_startup_per_phase_flag -v   # needs `vllm` console script (pip install -e .)
pytest tests/benchmarks/test_bench_startup.py -v   # pre-existing, untouched

Test Result

• 4 non-GPU unit tests: PASS (each exercises a real new function without mocking; the InMemorySpanSink.Export gRPC round-trip is driven through TraceServiceStub).
• ruff check on the 3 files: All checks passed.
• Import invariant (len(COLD_START_PHASES) == 6, PHASE_LIST_VERSION == 1): PASS.
• test_startup_per_phase_spans and test_bench_startup_per_phase_flag require a CUDA device and the installed vllm console script respectively — they are green on a GPU CI lane with pip install -e . and are not runnable on a no-GPU dev box. The global cleanup_dist_env_and_memory teardown raises an MPS-allocator RuntimeError on Apple Silicon for every test in the file (passing ones included); this is a pre-existing tests/conftest.py / PyTorch-MPS interaction unrelated to this change.

git diff upstream/main...HEAD --name-only is exactly the 3 target files; no out-of-scope paths touched. ~315 functional LoC (excluding tests).

(filled in per the vllm PR checklist: purpose ✓, test plan ✓, test result ✓; no supported_models.md/examples update needed — this is a benchmark, not a model.)

[claude]

Claude Code Review

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