David Mollitor created SPARK-59558:
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Summary: Use ArrayDeque instead of LinkedList for the whole-stage
codegen row buffer
Key: SPARK-59558
URL: https://issues.apache.org/jira/browse/SPARK-59558
Project: Spark
Issue Type: Improvement
Components: SQL
Affects Versions: 4.1.0
Reporter: David Mollitor
h2. Summary
{{BufferedRowIterator.currentRows}} is the output buffer for whole-stage
codegen: every generated iterator ({{GeneratedIteratorForCodegenStageN extends
BufferedRowIterator}}) pushes its output rows into it. It is declared as a
{{java.util.LinkedList<InternalRow>}} and used purely as a FIFO queue:
* {{append(row)}} -> {{currentRows.add(row)}}
* {{next()}} -> {{currentRows.remove()}}
* {{hasNext()}} / {{shouldStop()}} -> {{currentRows.isEmpty()}}
Because {{shouldStop()}} returns as soon as the buffer is non-empty, the
steady-state pattern is "append one row, poll one row" per output row, so
{{LinkedList}} allocates and discards one {{LinkedList$Node}} for every row
every codegen pipeline emits.
This changes the field to a {{java.util.ArrayDeque}} (referenced through the
{{Queue}}
interface). {{ArrayDeque}} allocates its backing array once and reuses it
across add/remove, eliminating the per-row node allocation, with better cache
locality. Its JavaDoc notes it is "likely to be faster than {{LinkedList}} when
used as a queue."
h2. Why
JFR allocation profiling of {{JoinBenchmark}} showed
{{java.util.LinkedList.linkLast}} as a significant allocation site on the
codegen output path -- ~25% of sampled allocation across the full suite
(amplified by high-fan-out operators), and ~4.8% in a focused duplicated
broadcast-hash-join case. This is per-row garbage on the path shared by
essentially every query that uses whole-stage codegen, so removing it reduces
GC pressure fleet-wide.
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