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https://issues.apache.org/jira/browse/SPARK-59653?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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ASF GitHub Bot updated SPARK-59653:
-----------------------------------
Labels: pull-request-available (was: )
> Lazily materialize UnsafeMapData key/value arrays to reduce allocations
> -----------------------------------------------------------------------
>
> Key: SPARK-59653
> URL: https://issues.apache.org/jira/browse/SPARK-59653
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 4.1.0
> Reporter: David Mollitor
> Priority: Minor
> Labels: pull-request-available
>
> h2. Problem
> {{UnsafeRow.getMap}} / {{UnsafeArrayData.getMap}} create a fresh
> {{UnsafeMapData}} per access, and its constructor eagerly allocates two
> nested {{UnsafeArrayData}} objects ({{{}keys{}}} and {{{}values{}}}).
> For a transient, count-only map access – {{{}size(map){}}},
> {{{}cardinality{}}}, and the {{size(x) > 0}} filter that
> {{InferFiltersFromGenerate}} inserts below every non-outer
> {{{}explode{}}}/{{{}inline{}}} – this nested object generates a wrapper plus
> two sub-wrappers are allocated per row purely to read an element count that
> is already stored inline in the layout.
> By contrast, {{UnsafeArrayData}} is a flat object (primitive fields plus a
> base-object reference after {{{}pointTo{}}}), which the JIT already
> scalar-replaces – so {{size(array_col)}} allocates nothing. Only maps carry
> this per-row garbage.
> h2. Change
> Make {{UnsafeMapData}} flat so a count-only access is scalar-replaceable by
> the JIT:
> * Build the {{{}keys{}}}/{{{}values{}}} {{UnsafeArrayData}} views lazily in
> {{{}keyArray(){}}}/{{{}valueArray() {}}}instead of eagerly in the constructor.
> * Compute {{numElements()}} by reading the key-array header directly
> ({{{}Platform.getLong(baseObject, baseOffset + 8){}}}), which equals
> {{keyArray().numElements() }}without materializing any view.
> * Store the key-array byte size as a primitive field in {{pointTo}} and drop
> the eager
> {{keys.numElements() == values.numElements()}} debug assertion.
> There is no storage-format change and no call-site changes:
> {{{}keyArray(){}}}/{{{}valueArray(){}}} keep their signatures, and
> serialization ({{{}Externalizable{}}}/Kryo), {{{}copy(){}}}, and
> {{MapData.foreach}}
> are unaffected (they go through the accessors or operate on the raw bytes).
> h2. Impact
> On a representative {{size(map_col)}} workload (whole-stage codegen on,
> assertions disabled to match production), the per-row {{getMap}} wrapper
> allocation dropped from ~4022 to ~94 JFR allocation samples (~98%
> eliminated), and {{UnsafeMapData}} leaves the top of the allocation-by-class
> profile.
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