[dougqh]

What Does This Do

#12397 made every root probability decision build an OtelTraceState, inside the sampling lock. Only W3C tracestate injection and OTLP export ever read it.

This PR defers building it for the common case: a fresh probability decision with no inherited ot tracestate member, not rejected by the rate limiter.

• tryUpdateProbabilitySamplingDecision installs a new ProbabilitySamplingState. It keeps only the trace id low-order bits and the rate, and builds the ot member on the first getOtelTraceState().
• Racing readers agree on a single instance through a CAS. Duplicates would still be correct, because the state is immutable. One instance keeps the identity-based "unchanged" checks from installing a new state or clearing the W3C header cache for nothing. It also means publication doesn't depend on OtelTraceState keeping its fields final. The CAS runs once per injected or exported trace, uncontended.
• PropagationTags.SamplingState is no longer final. preservesInheritedOtelPosition now reads through the getter.
• The decision-maker computation moves out of installSamplingState into nextDecisionMakerTagValue, so the new path can share it.

Every other path is unchanged:

• states derived from an inherited ot member
• rate-limiter rejection
• priority-only decisions
• forceKeep

Motivation

This is part of the follow-ups to #12397's per-trace sampling overhead (APMLP-2048).

Stacked on #12981. Without #12981, moving the decision-maker computation into its own method made the remaining "-" + mechanism concat escape. That added 24 B/op to priority-only decisions. With #12981 the concat is gone and that path is unchanged.

Benchmark

Decision and header paths: SamplingDecisionBenchmark

Before = #12981; after = this branch. -prof gc, B/op.

arm	before	after	Δ
ptagsProbabilityKeep (decision only)	216	168	−48
ptagsProbabilityKeepDatadogHeader	440	392	−48
ptagsProbabilityKeepW3CHeader	752	768	+16
ptagsProbabilityKeepOtlpTracestate	960	1072	+112, see below
ptagsPriorityKeep	152	152	0
spanDefaultSamplerKeep	1048	1000	−48
spanRuleSamplerKeep	1048	1000	−48
spanManualKeep	888	888	0

• Never read (Datadog-only propagation): the full 48 B saving holds end to end.
• W3C inject: the OtelTraceState is still built, plus 16 B for the two inputs ProbabilitySamplingState carries.
• OTLP tracestate: with -XX:-DoEscapeAnalysis the gap is +16 B (1080 → 1096) at equal throughput. The rest is a scalar-replacement win the old code got in this direct-call arm and this code doesn't. It only applies to OTLP export with rule sampling: under OTLP export the default sampler is ParentBasedAlwaysOnSampler, a priority-only decision that never creates a ProbabilitySamplingState. Real export goes through OtlpTraceJson.writeSpan, so whether the JIT effect carries over there isn't measured.

Scenarios: PropagationScenariosBenchmark

A scenario benchmark: each arm is a whole request driven through the tracer's public API, with a default-configured tracer (datadog,tracecontext,baggage):

• headNoPropagation, headPropagation: a root that decides, with or without an outbound inject
• leaf, intermediate: extract an upstream decision, with or without relaying it

It's here to put this PR in context of #12397's overall cost. It only uses API unchanged since v1.66.0, so it builds onto both commits.

#12397 alone (79a1710fcd^ vs 79a1710fcd, B/op):

• head, no propagation: +32 to +104 B
• head, propagating: +48 to +288 B
• leaf: +160 to +260 B
• intermediate: +256 to +495 B

This PR on that benchmark (default sampler, v1.66.0 → master → this branch):

scenario	v1.66.0	master	this branch
head, no propagation	976	1096	1000
head, propagating	3688	3656	3656
leaf	2109	2342	2339
intermediate	4642	5056	4976

• Precompute decision maker tag values per sampling mechanism #12981 + this PR recover ~80% of the non-propagating head's growth.
• No measurable change on the inject path. Propagating head and intermediate with -f 3: Δ from −104 to +39 B, within ±50–150 B error. The expected +16 B is below resolution.
• Leaf and intermediate are unchanged, by design. Most of Support OpenTelemetry consistent tracestate sampling #12397's cost is on extract (parsing and reconciling the inherited ot member), which this PR doesn't touch. That's tracked separately in APMLP-2049.

B/op on the larger scenario arms varies by ±100 B between forks, so treat small deltas there as noise.

Additional Notes

• New ProbabilitySamplingStateTest covers:
  • the deferred value matches the eager conversion (keep, drop, rate 1.0, rate 0.0)
  • repeated reads return a single instance
  • rate-limiter rejection records no otel state
  • a later forceKeep starts from the deferred state
• The full dd-trace-core test suite passes, apart from:
  • 2 Docker-dependent TracerConnectionReliabilityTest cases (no Docker locally)
  • DDAgentWriterCombinedTest and PendingTraceBufferTest, which failed under full-suite load and pass when run on their own

🤖 Generated with Claude Code

[dd-octo-sts]

🟢 Java Benchmark SLOs — All performance SLOs passed

Suite	Status
Startup	🟢 pass

SLO thresholds are defined here based on automatically generated metrics. A warning is raised when results are within 5% of the threshold.

PR vs. master results

Scenario	Candidate	master	Δ (95% CI of mean)
startup:insecure-bank:iast:Agent	14.00 s	13.95 s	[-0.5%; +1.3%] (no difference)
startup:insecure-bank:tracing:Agent	12.87 s	12.99 s	[-1.7%; -0.1%] (maybe better)
startup:petclinic:appsec:Agent	17.06 s	16.90 s	[-0.0%; +1.9%] (no difference)
startup:petclinic:iast:Agent	16.33 s	16.95 s	[-8.0%; +0.7%] (no difference)
startup:petclinic:profiling:Agent	16.47 s	16.10 s	[-1.9%; +6.6%] (no difference)
startup:petclinic:sca:Agent	16.96 s	16.85 s	[-0.4%; +1.6%] (no difference)
startup:petclinic:tracing:Agent	15.67 s	16.20 s	[-7.3%; +0.7%] (no difference)

Commit: 1d4880bb · CI Pipeline · Benchmarking Platform UI

---

Load and DaCapo benchmarks can be triggered manually in the GitLab pipeline. Results will appear in the Benchmarking Platform UI after completion.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-21T19:40:22.753290Z	1d4880b	Draft marked ready

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[datadog-prod-us1-6]

More details

The static review found no actionable regressions in sampling overrides, concurrent publication, propagation, or header caching from deferring OpenTelemetry state construction.

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[mcculls]

could consider adding a multi-threaded test, since this just checks serial access?

[mcculls]

LGTM