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Wire mergedTracerTags as a read-through parent at span build (level-split phase 1)#11932

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Wire mergedTracerTags as a read-through parent at span build (level-split phase 1)#11932
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@dougqh

@dougqh dougqh commented Jul 13, 2026 •
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What Does This Do

Splits trace-level tags & span-level tags into two separate TagMap-s
To maintain full semantic compatibility, this is done via the read-through parent capability introduced in #11789.

Motivation

Reduces some allocation by avoiding the cloning of the BucketGroup collision chain
Reduces span creation time by replacing O(n) map-walk with O(1) referencing of parent TagMap

Additional Notes

Stacked on #11789 (TagMap read-through mechanism) — review that first.

Level-split phase 1: the wiring. Attaches mergedTracerTags as a read-through parent at span build (gated on !mergedTracerTagsNeedsIntercept) instead of copying it into every span's storage. This is the change that actually activates read-through — #11789 alone is inert (parent == null).

Commits: config-version handling out of the trace-level bundle, the read-through flip (DDSpanContext.parentTags / CoreTracer copy-vs-share gate), and a TagMapReadThroughBenchmark.

Draft — description to be refined; posting now so review can start this week ahead of the release cut.

🤖 Generated with Claude Code

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dd-octo-sts Bot commented Jul 13, 2026 •
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🟢 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 13.98 s 13.95 s [-0.4%; +0.9%] (no difference)
startup:insecure-bank:tracing:Agent 12.93 s 12.97 s [-1.1%; +0.4%] (no difference)
startup:petclinic:appsec:Agent 16.94 s 16.66 s [+0.7%; +2.7%] (maybe worse)
startup:petclinic:iast:Agent 16.92 s 16.93 s [-1.0%; +0.8%] (no difference)
startup:petclinic:profiling:Agent 16.48 s 16.81 s [-3.2%; -0.8%] (maybe better)
startup:petclinic:sca:Agent 16.96 s 16.22 s [+0.2%; +8.9%] (maybe worse)
startup:petclinic:tracing:Agent 15.69 s 16.00 s [-6.1%; +2.2%] (no difference)

Commit: d2aa48fe · 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.

@dougqh dougqh force-pushed the dougqh/tagmap-read-through-consumer branch 2 times, most recently from 7bb20f4 to e65e58f Compare July 13, 2026 18:24
@dougqh dougqh force-pushed the dougqh/tagmap-read-through branch from 3a0a318 to e692601 Compare July 15, 2026 18:20
@dougqh dougqh force-pushed the dougqh/tagmap-read-through-consumer branch from e65e58f to 3b9156e Compare July 15, 2026 18:24
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@dougqh

dougqh commented Jul 15, 2026

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Directional span-creation benchmark: master vs this branch

Grafted SpanCreationBenchmark (from #11915) onto both master and this branch and ran a directional comparison with a realistic trace-level bundle. Not committed to the branch — a one-off directional check.

Config: -f 2 -wi 3 -i 3 -w 5 -r 5 -prof gc, @Threads(8), JDK 17, quiet box. Trace-level bundle via -jvmArgsAppend "-Ddd.env=bench -Ddd.version=1.0.0 -Ddd.tags=team:apm,tier:web,region:us1,shard:7,build:1234" (7 non-interceptable tags → a non-empty mergedTracerTags for read-through to fall through to). Alloc = gc.alloc.rate.norm.

method master B/op branch B/op Δ Δ%
bareStartSpan 52,511 52,457 −54 −0.10%
bareBuildSpan 52,514 52,447 −67 −0.13%
webServerSpan 62,871 62,872 +1 ~0%
webServerSpanViaBuilder 69,543 69,484 −59 −0.08%
jdbcClientSpan 64,508 64,356 −153 −0.24%

Throughput deltas were all within noise (≤1.6%, no trend) — directional only, not quoted.

Read: the deltas are small but real — a consistent ~50–150 B/op reduction (the mergedTracerTags bucket structure read-through shares instead of copying), visible even on bare spans since the bundle applies to every span. But it's <0.25% of total span-creation alloc, which is dominated by a ~52–69 KB/op baseline (span/context/PendingTrace object graph). So this benchmark confirms read-through does what it should but is the wrong lens for its alloc win — that shows in the isolated TagMapReadThroughBenchmark (~300–600 B/op, −13% to −48% by bundle size).

Consistent with the settled framing: read-through is structural groundwork (dense-store enabler), not a standalone span-creation alloc headline.

From Claude: directional check requested during the fold/read-through rebase; grafted benches, not part of the branch diff.

🤖 Generated with Claude Code

dougqh added a commit that referenced this pull request Jul 15, 2026
@dougqh @claude
Add StringIndex: a generic open-addressed string set
1d8fd80 
StringIndex is a compact open-addressed string→index structure (the keyOf
substrate the dense tag store builds on): parallel hash/name arrays, linear
probing, on par with HashSet on lookup at a smaller footprint. Includes unit
tests, a footprint test (jol), and comparison benchmarks (vs HashSet/switch).

No TagMap changes — standalone util. Rebased onto the level-split stack
(consumer #11932) as the layer dense-store sits on.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
@dougqh @claude
Wire mergedTracerTags as a read-through parent at span build (level-s…
d2aa48f 
…plit phase 1)

Attach the trace's merged tracer tags to each span's TagMap as a frozen
read-through parent (via TagMap.createFromParent) at span construction,
instead of copying them into every span. The span sees the shared tags on
read and only stores its own local tags, so the common trace-level bundle
is held once per trace rather than duplicated per span.

- CoreTracer builds the frozen merged-tracer-tags parent once; config
  version is kept out of that bundle.
- DDSpanContext attaches the parent at construction (fixed, no re-parenting).
- Adds TagMapReadThroughBenchmark (copy-down vs read-through, -prof gc).

Stacked on the read-through mechanism (#11789), which builds on the folded
final-class TagMap (#11967).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
@dougqh dougqh force-pushed the dougqh/tagmap-read-through-consumer branch from 3b9156e to d2aa48f Compare July 15, 2026 21:19
dougqh added a commit that referenced this pull request Jul 15, 2026
@dougqh @claude
Add StringIndex: a generic open-addressed string set
36bb222 
StringIndex is a compact open-addressed string→index structure (the keyOf
substrate the dense tag store builds on): parallel hash/name arrays, linear
probing, on par with HashSet on lookup at a smaller footprint. Includes unit
tests, a footprint test (jol), and comparison benchmarks (vs HashSet/switch).

No TagMap changes — standalone util. Rebased onto the level-split stack
(consumer #11932) as the layer dense-store sits on.

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