perf: skip snapshot rebuild in mergeDuplicates when names are unique - #2441
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zeitlinger merged 2 commits intoSep 15, 2026
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September 1, 2026 19:07
MetricSnapshots is always sorted by prometheus name, so duplicate names are adjacent. Detect duplicates in a single allocation-free pass and, when there are none (the common case), return the input unchanged instead of rebuilding it through a LinkedHashMap, an ArrayList per group, a MetricSnapshots.Builder and a freshly sorted MetricSnapshots. The merge path for actual duplicates is unchanged. Output is byte-identical (verified by the existing exposition-format tests, including DuplicateNamesExpositionTest). Signed-off-by: David Mollitor <david.mollitor@databricks.com>
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Signed-off-by: Gregor Zeitlinger <gregor.zeitlinger@grafana.com>
zeitlinger
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What
TextFormatUtil.mergeDuplicatescurrently only short-circuits when there is a single snapshot. For every scrape with two or more metric families it unconditionally builds aLinkedHashMap, anArrayListper group, aMetricSnapshots.Builder(with its own list + set) and a freshly sortedMetricSnapshots— even when there is nothing to merge, which is the common case.Since
MetricSnapshotsis always sorted by prometheus name (see its constructor), any duplicate names are adjacent. This adds a single allocation-free pass to detect duplicates; when there are none, the input is returned unchanged.The merge path for actual duplicates is unchanged.
Why
It removes the map / per-group list / builder / re-sort allocations on every scrape that has no duplicate metric names. Measured on a JMH benchmark of a histogram-heavy scrape (collect + serialize to the Prometheus text format), allocation on the serialize path dropped by ~880 B/op, scaling with the number of metric families.
Correctness
Output is byte-identical. The existing exposition-format tests pass, including
DuplicateNamesExpositionTest, which exercises the merge path with real duplicate names.