[dougqh]

What Does This Do

PTags.installSamplingState built the _dd.p.dm value with TagValue.from("-" + samplingMechanism) on every keep decision. That allocated a String, then ran validation, hashing and a lookup in TagValue's 128-entry cache. All of this happens inside samplingStateLock.

This PR builds the TagValues for mechanisms 0–15 once, at class init, and indexes them by mechanism. Larger mechanisms still go through TagValue.from. The negative/UNKNOWN guard is unchanged.

Motivation

This is the first of several small follow-ups on per-trace sampling overhead in propagation tags. The others are the eager OtelTraceState from #12397 (APMLP-2048), the per-PTags lock object, and a race in the single-entry Knuth sampling-rate cache. Those will come as separate PRs.

The concat itself is older than #12397. #12397 moved it inside the sampling lock.

Benchmark

This PR adds SamplingDecisionBenchmark:

• span* arms run a bare root span through start → sample → finish.
• ptags* arms make the decision on fresh PropagationTags with no span around it.

Before = master's PTagsFactory; after = this branch. Both runs used -f 1 -wi 3 -w 2s -i 5 -r 3s -t 8 -prof gc on JDK 17 / M-series laptop. B/op is the primary signal. Throughput comes from a single fork and is directional only.

arm	B/op before	B/op after	Δ B/op	ops/µs before	ops/µs after
ptagsPriorityKeep	176	152	−24	259 ± 17	319 ± 36
ptagsProbabilityKeep	240	216	−24	185 ± 16	284 ± 47
spanDefaultSamplerKeep	1074 ± 94	1048	~−26	3.53 ± 0.50	3.51 ± 0.89
spanRuleSamplerKeep	1096	1048	−48	2.87 ± 0.29	2.92 ± 0.12
spanManualKeep	936	840	−96	3.34 ± 1.26	3.31 ± 0.69

• Each decision-maker update saved 24 B. The span arms save a multiple of that because a trace goes through more than one decision-maker update.
• The baseline spanDefaultSamplerKeep B/op varied run to run (±94), so treat its Δ as approximate.
• spanManualKeep also varies between runs: a later run of the same post-change jar measured 888 B/op, not 840. Its Δ could be −48 rather than −96. The ptags* arms gave the same values in every run.
• The ptags* arms still allocate 150–220 B per trace, for PTags itself, its lock object, SamplingState and the eager OtelTraceState. Those are the targets of the follow-up PRs.

Additional Notes

• The table holds 16 entries rather than more because each entry also takes a slot in TagValue's shared 128-entry cache.
• New DecisionMakerTagValueTest covers values inside and beyond the table, and instance reuse.
• datadog.trace.core.propagation.*, DDSpanContextPropagationTagsTest and CoreSpanBuilderTest pass locally.

🤖 Generated with Claude Code

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Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-21T13:29:50.222445Z	4ddb5d9	Draft marked ready

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[datadog-prod-us1-4]

More details

The precomputed decision-maker values preserve existing formatting, the negative-mechanism guard, and fallback behavior for mechanisms outside the table.

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[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	13.93 s	13.90 s	[-0.5%; +1.0%] (no difference)
startup:insecure-bank:tracing:Agent	12.90 s	12.94 s	[-1.1%; +0.4%] (no difference)
startup:petclinic:appsec:Agent	17.08 s	16.88 s	[+0.2%; +2.2%] (maybe worse)
startup:petclinic:iast:Agent	16.31 s	17.06 s	[-8.6%; -0.2%] (maybe better)
startup:petclinic:profiling:Agent	16.68 s	16.63 s	[-0.5%; +1.1%] (no difference)
startup:petclinic:sca:Agent	17.07 s	16.98 s	[-0.3%; +1.3%] (no difference)
startup:petclinic:tracing:Agent	15.68 s	16.27 s	[-7.7%; +0.5%] (no difference)

Commit: 4ddb5d9d · CI Pipeline · Benchmarking Platform UI

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