Question

我有一个MapReduce程序

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));
        }
    }

我想运行仅 map任务，其输出应为<number, 1>格式当我从命令行运行时，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，我看到像

这样的条目

这意味着数据正在聚合

我怎么能不运行reducer？

Answer 1

只有在你实现ToolRunner.run方法并在main方法中传递参数时，这才有效。

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

如果您不想尝试设置

job.setNumReduceTasks(0);

或另一个选项是在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");

Answer 2

在-D选项后面添加一个空格，它应该可以工作;）

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

即使在命令行上告知-D mapred.reduce.tasks = 0之后，hadoop也会减少任务运行

2 个答案: