java - 即使在命令行上告诉为 -D mapred.reduce.tasks=0 后，hadoop reduce 任务仍在运行

Question

我有一个 MapReduce编程为

public static class MapClass extends MapReduceBase implements Mapper<Text, Text, IntWritable, IntWritable> {

        private final static IntWritable uno = new IntWritable(1);
        private IntWritable citationCount = new IntWritable();

        public void map(Text key, Text value, OutputCollector<IntWritable, IntWritable> output, Reporter reporter) throws IOException {
            citationCount.set(Integer.parseInt(value.toString()));
            output.collect(citationCount, uno);
        }
    }

    public static class Reduce extends MapReduceBase implements Reducer<IntWritable, IntWritable, IntWritable, IntWritable> {

        public void reduce(IntWritable key, Iterator<IntWritable> values, OutputCollector<IntWritable, IntWritable> output, Reporter reporter) throws IOException {
            int count = 0;
            while (values.hasNext()) {
                count += values.next().get();
            }

            output.collect(key, new IntWritable(count));
        }
    }

我想仅运行map任务，输出应为 <number, 1> 形式 当我从命令行运行它时，我说

$ hadoop jar Hadoop-programs.jar com/hadoop/patent/CitationHistogram input output -Dmapred.reduce.tasks=0

但这是我在命令行输出中看到的

12/07/30 06:13:14 INFO mapred.JobClient:  map 50% reduce 0%
12/07/30 06:13:23 INFO mapred.JobClient:  map 58% reduce 0%
12/07/30 06:13:26 INFO mapred.JobClient:  map 60% reduce 8%
12/07/30 06:13:29 INFO mapred.JobClient:  map 68% reduce 8%
12/07/30 06:13:32 INFO mapred.JobClient:  map 76% reduce 8%
12/07/30 06:13:35 INFO mapred.JobClient:  map 85% reduce 16%
12/07/30 06:13:38 INFO mapred.JobClient:  map 93% reduce 16%
12/07/30 06:13:41 INFO mapred.JobClient:  map 98% reduce 16%
12/07/30 06:13:44 INFO mapred.JobClient:  map 100% reduce 16%
12/07/30 06:13:55 INFO mapred.JobClient:  map 100% reduce 69%
12/07/30 06:13:58 INFO mapred.JobClient:  map 100% reduce 78%
12/07/30 06:14:01 INFO mapred.JobClient:  map 100% reduce 94%
12/07/30 06:14:08 INFO mapred.JobClient:  map 100% reduce 100%

当我看到作业的输出时，我看到类似

的条目

1       2
13      2
24      1
29      1
31      2
42      3
6796    7
6799    1
6806    1
6815    1
6824    2

这意味着数据正在聚合

我怎么能完全不运行 reducer？

Answer 1

只有当您实现 ToolRunner.run 方法并在您的 main 方法中传递参数时，它才会起作用。

ToolRunner.run(new Configuration(), new YourClasImplmentingToolRunner(), args);

如果你不想做尝试设置

job.setNumReduceTasks(0);

或者另一个选项是在 conf 中设置值并在作业中使用该配置。

Configuration conf = new Configuration();
conf.set("mapred.reduce.tasks", "0");
Job job = new Job(conf, "My job Name");