[GitHub] spark pull request #16909: [SPARK-13450] Introduce ExternalAppendOnlyUnsafeR...

[tejasapatil <gi...@git.apache.org>]

Github user tejasapatil commented on a diff in the pull request:

    https://github.com/apache/spark/pull/16909#discussion_r103749494
  
    --- Diff: sql/core/src/main/scala/org/apache/spark/sql/execution/ExternalAppendOnlyUnsafeRowArray.scala ---
    @@ -0,0 +1,218 @@
    +/*
    + * Licensed to the Apache Software Foundation (ASF) under one or more
    + * contributor license agreements.  See the NOTICE file distributed with
    + * this work for additional information regarding copyright ownership.
    + * The ASF licenses this file to You under the Apache License, Version 2.0
    + * (the "License"); you may not use this file except in compliance with
    + * the License.  You may obtain a copy of the License at
    + *
    + *    http://www.apache.org/licenses/LICENSE-2.0
    + *
    + * Unless required by applicable law or agreed to in writing, software
    + * distributed under the License is distributed on an "AS IS" BASIS,
    + * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    + * See the License for the specific language governing permissions and
    + * limitations under the License.
    + */
    +
    +package org.apache.spark.sql.execution
    +
    +import java.util.ConcurrentModificationException
    +
    +import scala.collection.mutable.ArrayBuffer
    +
    +import org.apache.spark.{SparkEnv, TaskContext}
    +import org.apache.spark.internal.Logging
    +import org.apache.spark.sql.catalyst.expressions.UnsafeRow
    +import org.apache.spark.sql.execution.ExternalAppendOnlyUnsafeRowArray.DefaultInitialSizeOfInMemoryBuffer
    +import org.apache.spark.util.collection.unsafe.sort.{UnsafeExternalSorter, UnsafeSorterIterator}
    +
    +/**
    + * An append-only array for [[UnsafeRow]]s that spills content to disk when there a predefined
    + * threshold of rows is reached.
    + *
    + * Setting spill threshold faces following trade-off:
    + *
    + * - If the spill threshold is too high, the in-memory array may occupy more memory than is
    + *   available, resulting in OOM.
    + * - If the spill threshold is too low, we spill frequently and incur unnecessary disk writes.
    + *   This may lead to a performance regression compared to the normal case of using an
    + *   [[ArrayBuffer]] or [[Array]].
    + */
    +private[sql] class ExternalAppendOnlyUnsafeRowArray(numRowsSpillThreshold: Int) extends Logging {
    --- End diff --
    
    Update: I created a test version by removing the `ArrayBuffer` and let alone `ExternalUnsafeSorter` be used to back the data. Over prod queries, this turned out to be slightly slower unlike what the micro-benchmark results showed. I will stick with the current approach.


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