crunch-core/src/main/java/org/apache/crunch/util/PartitionUtils.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.crunch.util;

import org.apache.crunch.PCollection;
import org.apache.hadoop.conf.Configuration;

/**
 *  Helper functions and settings for determining the number of reducers to use in a pipeline
 *  job created by the Crunch planner.
 */
public class PartitionUtils {
  public static final String BYTES_PER_REDUCE_TASK = "crunch.bytes.per.reduce.task";
  public static final long DEFAULT_BYTES_PER_REDUCE_TASK = 1000L * 1000L * 1000L;

  /**
   * Set an upper limit on the number of reducers the Crunch planner will set for an MR
   * job when it tries to determine how many reducers to use based on the input size.
   */
  public static final String MAX_REDUCERS = "crunch.max.reducers";
  public static final int DEFAULT_MAX_REDUCERS = 500;

  public static <T> int getRecommendedPartitions(PCollection<T> pcollection) {
    Configuration conf = pcollection.getPipeline().getConfiguration();
    return getRecommendedPartitions(pcollection, conf);
  }

  public static <T> int getRecommendedPartitions(PCollection<T> pcollection, Configuration conf) {
    long bytesPerTask = conf.getLong(BYTES_PER_REDUCE_TASK, DEFAULT_BYTES_PER_REDUCE_TASK);
    int recommended = 1 + (int) (pcollection.getSize() / bytesPerTask);
    int maxRecommended = conf.getInt(MAX_REDUCERS, DEFAULT_MAX_REDUCERS);
    if (maxRecommended > 0 && recommended > maxRecommended) {
      return maxRecommended;
    } else {
      return recommended;
    }
  }
}