hadoop RehashPartitioner 源码
haddop RehashPartitioner 代码
文件路径：/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapreduce/lib/partition/RehashPartitioner.java
/**
* 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.hadoop.mapreduce.lib.partition;
import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;
import org.apache.hadoop.mapreduce.Partitioner;
/**
* This partitioner rehashes values returned by {@link Object#hashCode()}
* to get smoother distribution between partitions which may improve
* reduce reduce time in some cases and should harm things in no cases.
* This partitioner is suggested with Integer and Long keys with simple
* patterns in their distributions.
* @since 2.0.3
*/
@InterfaceAudience.Public
@InterfaceStability.Stable
public class RehashPartitioner<K, V> extends Partitioner<K, V> {
/** prime number seed for increasing hash quality */
private static final int SEED = 1591267453;
/** Rehash {@link Object#hashCode()} to partition. */
public int getPartition(K key, V value, int numReduceTasks) {
int h = SEED ^ key.hashCode();
h ^= (h >>> 20) ^ (h >>> 12);
h = h ^ (h >>> 7) ^ (h >>> 4);
return (h & Integer.MAX_VALUE) % numReduceTasks;
}
}
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