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

dougqh · 2026-06-22T21:22:34Z

What This Does

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)

Correctness — gated by a characterization test

First commit adds PropagationTagsChildSpanTest, which pins the load-bearing behavior: a non-root (local child) span still carries the inbound distributed _dd.p.* tags on injection. Both tests pass, confirming this is pure allocation waste, not a latent correctness bug. The test is the gate + safety net for the change.

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

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>

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	14.06 s	13.97 s	[-0.1%; +1.4%] (no difference)
startup:insecure-bank:tracing:Agent	13.00 s	13.11 s	[-1.7%; -0.1%] (maybe better)
startup:petclinic:appsec:Agent	16.91 s	16.78 s	[-0.2%; +1.7%] (no difference)
startup:petclinic:iast:Agent	16.93 s	16.97 s	[-1.2%; +0.7%] (no difference)
startup:petclinic:profiling:Agent	16.83 s	16.99 s	[-2.1%; +0.3%] (no difference)
startup:petclinic:sca:Agent	16.98 s	16.57 s	[+1.5%; +3.4%] (significantly worse)
startup:petclinic:tracing:Agent	16.06 s	16.05 s	[-0.8%; +0.9%] (no difference)

Commit: e4542743 · 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.

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

dd-octo-sts · 2026-06-24T15:30:29Z

Hi! 👋 Thanks for your pull request! 🎉

To help us review it, please make sure to:

Add at least one type, and one component or instrumentation label to the pull request

If you need help, please check our contributing guidelines.

amarziali

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

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>

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.

…gment

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. 🙏

dougqh and others added 2 commits June 22, 2026 17:22

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

This comment has been minimized.

Sign in to view

dougqh mentioned this pull request Jun 22, 2026

Decouple W3C lastParentId from shared propagation-tag mutation #11702

Draft

dougqh commented Jun 23, 2026

View reviewed changes

Comment thread dd-trace-core/src/main/java/datadog/trace/core/CoreTracer.java Outdated

dougqh and others added 2 commits June 23, 2026 12:01

Simplify the share-PropagationTags comment (review feedback)

1860ad6

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

Merge branch 'master' into dougqh/ptags-per-segment

f8b3095

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

View reviewed changes

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

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

1d03e0a

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>

Merge remote-tracking branch 'origin/master' into dougqh/ptags-per-se…

ba8cdc2

…gment

dougqh added this to the 1.64.0 milestone Jun 30, 2026

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

Merge branch 'master' into dougqh/ptags-per-segment

e454274

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

Share parent's PropagationTags for local child spans#11701

Share parent's PropagationTags for local child spans#11701
dougqh wants to merge 8 commits into
masterfrom
dougqh/ptags-per-segment

dougqh commented Jun 22, 2026 •

edited

Loading

Uh oh!

This comment has been minimized.

dd-octo-sts Bot commented Jun 22, 2026 •

edited

Loading

Uh oh!

Uh oh!

dd-octo-sts Bot commented Jun 24, 2026

Uh oh!

amarziali left a comment

Uh oh!

dougqh commented Jun 29, 2026

Uh oh!

dougqh commented Jun 30, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

Uh oh!

Conversation

dougqh commented Jun 22, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

What This Does

Motivation

Additional Notes

Correctness — gated by a characterization test

Safety

Uh oh!

This comment has been minimized.

dd-octo-sts Bot commented Jun 22, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

🟢 Java Benchmark SLOs — All performance SLOs passed

Uh oh!

Uh oh!

dd-octo-sts Bot commented Jun 24, 2026

Uh oh!

amarziali left a comment

Choose a reason for hiding this comment

Uh oh!

dougqh commented Jun 29, 2026

Uh oh!

dougqh commented Jun 30, 2026

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

2 participants

dougqh commented Jun 22, 2026 •

edited

Loading

dd-octo-sts Bot commented Jun 22, 2026 •

edited

Loading