Store known span tags densely in TagMap by tag-id (phase 2)#12045
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Store known span tags densely in TagMap by tag-id (phase 2)#12045dougqh wants to merge 1 commit into
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Known tags (keyOf resolves to a stored id) are held in insertion-ordered parallel arrays (knownIds/knownValues) with NO per-tag Entry object — the allocation lever. Lazily allocated on the first known-tag write; custom tags stay in the hash buckets. Disjoint by construction (known-ness is global), so read-through shadow checks stay within-region and the bucket path is unchanged. - KnownTagCodec (id encoding + resolver) + hand-written KnownTags (keyOf substrate over StringIndex). Off-by-default: dormant until a resolver registers, so production is byte-identical. - CoreTracer flips it live behind `-Ddd.trace.dense.tags.enabled` for A/B. - Sizing is a generous fixed stopgap (KNOWN_INIT_CAP=12, the per-type max); exact per-type sizing comes with the tag registry. Reconciled onto the level-split stack (fold + read-through + StringIndex); built on the folded final-class TagMap. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Bits has a CI fix ready🟢 Investigated · 🟢 Fix prepared · ⚪ Validation skipped · 🟠 Ready
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This was referenced Jul 23, 2026
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What
Adds a dense store for known span tags in
TagMap: tags resolved to a stable tag-id viaKnownTagCodec.keyOfare held in insertion-ordered parallel arrays (knownIds/knownValues) with no per-tagEntryobject — eliminating theTagMap$Entryallocation that macro JFR profiling flagged as the #1 tracer allocation lever.KnownTags), so production is byte-identical until the generator lands — all-known maps allocate zero buckets.EntryReadingHelper) — no per-entryEntryalloc on the read/serialize path either.parentDenseVisible(mirrorsparentEntryVisible), nearest-level-wins with tombstone/shadow checks. Disjointness (known tags never bucket) keeps the two stores independent by construction.Why
Removes
TagMap$Entryallocation for known tags (the macro alloc win — see the tracer-overhead JFR profiling). CPU is neutral/parity; the headline is allocation on the app thread.Stack
Sits on
dougqh/tagset(StringIndex / #11660 base). Supersedes the old dense PR #11814.Test
TagMapDenseForkedTest,TagMapDenseFuzzForkedTest,KnownTagsTestgreen;spotbugsMain+spotlessJavaCheckclean.🤖 Generated with Claude Code