Share parent's PropagationTags for local child spans by dougqh · Pull Request #11701 · DataDog/dd-trace-java

dougqh · 2026-06-22T21:22:34Z

What Does This Do

Local child spans now share the parent's (trace-level) PropagationTags instead of each allocating a fresh empty(). Collapses N+1 → 1 PropagationTags per N-span trace.

Motivation

CoreTracer span-build allocated propagationTagsFactory.empty() for every local child span (the common in-process case); only the distributed ExtractedContext path reused the parent's. A non-root child's own PropagationTags, however, is never read — DDSpanContext.getPropagationTags() routes to the root (getRootSpanContextOrThis().propagationTags). So the per-span empty() was pure allocation waste.

Additional Notes

50s petclinic run under JMeter multi-endpoint load (Zulu 17, alloc JFR):

5,481 DDSpanContext allocations, 311 PTags allocations — 1 PTags per ~17.6 spans (vs 1:1 before)
The 311 are all root spans (expected — no parent to share from); child spans share the root's instance
PTagsFactory$PTags no longer appears in the top 30 allocating classes (below 0.5% of sampled allocation)

Allocation measured (controlled toggle, via #11915)

Using #11915's TraceAssemblyBenchmark (web-shape root + N children), isolating just this one-line change (share vs empty()-per-child, all else identical), -prof gc, tracer logging forced to WARN:

arm	`empty()`/child	shared	Δ B/op	Δ/child
bareStartSpan (control)	899.7	898.3	~0	—
trace[1]	2202.3	2177.5	24.8	(noise floor)
trace[5]	4705.5	4263.8	441.7	88
trace[20]	14032.1	12000.5	2031.6	102

A slope reduction, not an intercept shift: the saving scales with fan-out — ~2 KB/op (≈14.5%) at 20 children — while the single-span control stays flat (roots always allocate their own PropagationTags, so they can't benefit). JOL puts one empty() PTags at 96 B shallow (compressed oops), matching the ~88–106 B/child slope on the resolvable arms. (trace[1] is one 96 B object in ~2.2 KB/op — at the -f1 noise floor.)

Correctness — gated by a characterization test

PropagationTagsChildSpanTest pins the load-bearing behavior on both sides of the shared instance:

Injection (read side) — a non-root local child still injects the inbound distributed _dd.p.*, because getPropagationTags() routes reads to the root. Safe with or without sharing.
Serialization (emit side) — sharing surfaced a real regression. DDSpanContext.processTagsAndBaggage emits each span's PropagationTags into its own metadata; pre-PR that was gated only implicitly by children holding an empty() instance. With the shared (non-empty) root instance, _dd.p.dm and other trace-level _dd.p.* leaked onto every child span — caught by the test_sampling_span_tags parametric test. Fixed by gating the emit on span position (local root only), which exactly restores pre-PR behavior; localChildDoesNotEmitTraceLevelDdpTags guards it (fails without the gate, passes with it).

All three trace mappers (v0.4 / v0.5 / v1) read per-span _dd.p.* from Metadata.getBaggage(), so the one gate covers all of them; v1's chunk-level samplingMechanism reads the first span only, so it was never a per-span leak.

Safety

The ctor's updateTraceIdHighOrderBits stamp is guard-idempotent (if (traceIdHighOrderBits != highOrderBits)) and a no-op on the already-stamped shared root instance (same trace ⇒ same high-order bits).
The full dd-trace-core propagation + span-build suite passes (W3C/Datadog extractors+injectors, W3CPropagationTags, OPM round-trip, OrgGuard, CoreSpanBuilderTest, DDSpanContextTest) — 0 failures.

The lastParentId/OPM inject-time mutation race (a transient per-injection value parked in the shared object) is pre-existing — injection already routes to the root's shared instance via getPropagationTags() — so it is a separate concurrency concern, not introduced or worsened by this change. Tracked in #11702.

tag: no release note
tag: ai generated

🤖 Generated with Claude Code

dd-octo-sts · 2026-06-22T21:53:40Z

🟢 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	13.97 s	14.00 s	[-1.1%; +0.6%] (no difference)
startup:insecure-bank:tracing:Agent	12.99 s	13.00 s	[-0.8%; +0.6%] (no difference)
startup:petclinic:appsec:Agent	16.80 s	15.98 s	[+0.7%; +9.5%] (maybe worse)
startup:petclinic:iast:Agent	16.36 s	16.85 s	[-7.1%; +1.3%] (no difference)
startup:petclinic:profiling:Agent	16.58 s	16.66 s	[-1.6%; +0.5%] (no difference)
startup:petclinic:sca:Agent	16.80 s	16.81 s	[-0.9%; +0.8%] (no difference)
startup:petclinic:tracing:Agent	16.07 s	16.36 s	[-2.8%; -0.8%] (maybe better)

Commit: 76851be3 · 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.

dd-octo-sts · 2026-06-24T15:30:29Z

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amarziali

Doug the intent is ok to me. It looks some tests are still failing

dougqh · 2026-06-29T21:55:12Z

Doug the intent is ok to me. It looks some tests are still failing

@amarziali Thanks. It looks like a rename got merged in last week that broke the tests that I ported from Groovy. It should be better now.

dougqh · 2026-06-30T17:44:48Z

Synced with master (the branch was ~57 commits behind) and confirmed :dd-trace-core compiles + PropagationTags* tests pass locally on the merge. The reds you saw were the flaky parametric/system-tests jobs, not this change — a fresh run is underway. Targeted at the 1.64.0 milestone. 🙏

amarziali · 2026-07-02T10:48:26Z

Synced with master (the branch was ~57 commits behind) and confirmed :dd-trace-core compiles + PropagationTags* tests pass locally on the merge. The reds you saw were the flaky parametric/system-tests jobs, not this change — a fresh run is underway. Targeted at the 1.64.0 milestone. 🙏

I see that parametric system tests fails and seem indeed related to this change:

 AssertionError: non-root spans that are not first in a chunk should never includ the _dd.p.dm tag

dougqh · 2026-07-02T11:36:16Z

Synced with master (the branch was ~57 commits behind) and confirmed :dd-trace-core compiles + PropagationTags* tests pass locally on the merge. The reds you saw were the flaky parametric/system-tests jobs, not this change — a fresh run is underway. Targeted at the 1.64.0 milestone. 🙏

I see that parametric system tests fails and seem indeed related to this change:
 AssertionError: non-root spans that are not first in a chunk should never includ the _dd.p.dm tag

@amarziali Yeah, usually, the system test failures have been flakes, but I agree that looks like a legitimate failure.
Looks like like I'll have to figure out a different approach to solving this.

CoreTracer span-build allocates a fresh propagationTagsFactory.empty() per local child span; only the distributed ExtractedContext path reuses the parent's. Before optimizing that per-span allocation away, this test pins the load-bearing behavior: that a non-root (local child) span still carries the inbound distributed _dd.p.* tags when it injects. Verdict (both tests pass): it does. DDSpanContext.getPropagationTags() routes to the root (getRootSpanContextOrThis), so a non-root child's own propagationTags field is never read for injection. The per-span empty() is pure allocation waste, not a latent correctness bug. This test is the gate + safety net for the planned "share the parent's PropagationTags for local children" allocation fix. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CoreTracer span-build allocated a fresh propagationTagsFactory.empty() for every local child span, reused from the parent only on the distributed ExtractedContext path — N+1 PropagationTags per N-span trace, N of them needless empties. A non-root child's own PropagationTags is never read for injection: DDSpanContext.getPropagationTags() routes to the root (getRootSpanContextOrThis), confirmed by PropagationTagsChildSpanTest (inbound _dd.p.* survive child injection). So local children can share the parent's (trace-level) instance instead of allocating their own, collapsing N+1 -> 1 per trace. Safe: the ctor's updateTraceIdHighOrderBits stamp is guard-idempotent and a no-op on the already-stamped shared root instance (same trace => same high-order bits). Full dd-trace-core propagation + span-build suite passes. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

The change landed, so drop the pre-implementation 'open question / planned fix' framing, the scratch-doc reference (ptags-allocation-findings.md), and the brittle CoreTracer line numbers. State what it guards: local children share the root's PropagationTags, and reads route to the root, so a non-root span still injects the inbound _dd.p.* tags. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

The master merge renamed AgentSpan.context() to spanContext(); update the two call sites so :dd-trace-core:compileTestJava passes. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Local children now share the root's PropagationTags, so DDSpanContext.processTagsAndBaggage emitted _dd.p.* (including the _dd.p.dm decision-maker) into every child span's metadata -- the emit was previously gated only implicitly by children holding an empty() instance. Gate it on span position (local root only), which exactly restores pre-PR behavior. All three trace mappers read per-span _dd.p.* from Metadata.getBaggage(), so the one gate covers them. Caught by the test_sampling_span_tags parametric test; add a serialization-side case to PropagationTagsChildSpanTest that fails without the gate and passes with it. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

dougqh changed the title ~~Characterize non-root span _dd.p.* propagation (gate for PTags per-span alloc fix)~~ Share parent's PropagationTags for local child spans Jun 22, 2026

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dougqh mentioned this pull request Jun 22, 2026

Decouple W3C lastParentId from shared propagation-tag mutation #11702

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dougqh commented Jun 23, 2026

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Comment thread dd-trace-core/src/main/java/datadog/trace/core/CoreTracer.java Outdated

dougqh marked this pull request as ready for review June 24, 2026 15:30

dougqh requested a review from a team as a code owner June 24, 2026 15:30

dougqh requested a review from amarziali June 24, 2026 15:30

dd-octo-sts Bot added the tag: ai generated Largely based on code generated by an AI or LLM label Jun 24, 2026

amarziali reviewed Jun 29, 2026

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dougqh added tag: performance Performance related changes type: feature Enhancements and improvements comp: context propagation Trace context propagation labels Jun 29, 2026

dougqh added this to the 1.64.0 milestone Jun 30, 2026

dougqh requested a review from amarziali June 30, 2026 17:44

dougqh marked this pull request as draft July 2, 2026 11:36

mhlidd removed this from the 1.64.0 milestone Jul 7, 2026

dougqh mentioned this pull request Jul 14, 2026

Add span-creation JMH benchmarks (front-half allocation) #11915

Open

dougqh and others added 6 commits July 14, 2026 14:29

Simplify the share-PropagationTags comment (review feedback)

233ffac

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Fix PropagationTagsChildSpanTest compile: context() -> spanContext()

e5aa0f6

The master merge renamed AgentSpan.context() to spanContext(); update the two call sites so :dd-trace-core:compileTestJava passes. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

dougqh force-pushed the dougqh/ptags-per-segment branch from 57bed99 to 76851be Compare July 14, 2026 18:31

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Share parent's PropagationTags for local child spans#11701

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What Does This Do

Motivation

Additional Notes

Allocation measured (controlled toggle, via #11915)

Correctness — gated by a characterization test

Safety

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🟢 Java Benchmark SLOs — All performance SLOs passed

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