Github user zecevicp commented on a diff in the pull request: https://github.com/apache/spark/pull/21109#discussion_r193753823 --- Diff: sql/core/src/main/scala/org/apache/spark/sql/execution/InMemoryUnsafeRowQueue.scala --- @@ -0,0 +1,183 @@ +/* + * 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 +import scala.collection.mutable.ArrayBuffer + +import org.apache.spark.{SparkEnv, TaskContext} +import org.apache.spark.memory.TaskMemoryManager +import org.apache.spark.serializer.SerializerManager +import org.apache.spark.sql.catalyst.expressions.UnsafeRow +import org.apache.spark.sql.execution.ExternalAppendOnlyUnsafeRowArray.DefaultInitialSizeOfInMemoryBuffer +import org.apache.spark.storage.BlockManager + +/** + * An append-only array for [[UnsafeRow]]s that strictly keeps content in an in-memory array + * until [[numRowsInMemoryBufferThreshold]] is reached post which it will switch to a mode which + * would flush to disk after [[numRowsSpillThreshold]] is met (or before if there is + * excessive memory consumption). Setting these threshold involves following trade-offs: + * + * - If [[numRowsInMemoryBufferThreshold]] is too high, the in-memory array may occupy more memory + * than is available, resulting in OOM. + * - If [[numRowsSpillThreshold]] is too low, data will be spilled frequently and lead to + * excessive disk writes. This may lead to a performance regression compared to the normal case + * of using an [[ArrayBuffer]] or [[Array]]. + */ +private[sql] class InMemoryUnsafeRowQueue( --- End diff -- A queue is needed here because it's a moving window instead of a fixed block of rows. Maybe I missed an existing class that could do this easily so I'll take another look. But, I believe any alternative would indeed be messier.
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