我们有一个为文件中的每一行生成唯一键的场景。我们有一个timestamp列，但在少数情况下，同一时间戳有多行可用。
我们决定将唯一值作为时间戳，并在下面的程序中提到它们各自的计数。
mapper只会发出时间戳作为key，整行作为它的值，在reducer中生成key。
问题是map输出大约236行，其中只有230条记录作为reducer的输入，reducer输出相同的230条记录。

public class UniqueKeyGenerator extends Configured implements Tool {

    private static final String SEPERATOR = "\t";
    private static final int TIME_INDEX = 10;
    private static final String COUNT_FORMAT_DIGITS = "%010d";

    public static class Map extends Mapper<LongWritable, Text, Text, Text> {

        @Override
        protected void map(LongWritable key, Text row, Context context)
                throws IOException, InterruptedException {
            String input = row.toString();
            String[] vals = input.split(SEPERATOR);
            if (vals != null && vals.length >= TIME_INDEX) {
                context.write(new Text(vals[TIME_INDEX - 1]), row);
            }
        }
    }

    public static class Reduce extends Reducer<Text, Text, NullWritable, Text> {

        @Override
        protected void reduce(Text eventTimeKey,
                Iterable<Text> timeGroupedRows, Context context)
                throws IOException, InterruptedException {
            int cnt = 1;
            final String eventTime = eventTimeKey.toString();
            for (Text val : timeGroupedRows) {
                final String res = SEPERATOR.concat(getDate(
                        Long.valueOf(eventTime)).concat(
                        String.format(COUNT_FORMAT_DIGITS, cnt)));
                val.append(res.getBytes(), 0, res.length());
                cnt++;
                context.write(NullWritable.get(), val);
            }
        }
    }

    public static String getDate(long time) {
        SimpleDateFormat utcSdf = new SimpleDateFormat("yyyyMMddhhmmss");
        utcSdf.setTimeZone(TimeZone.getTimeZone("America/Los_Angeles"));
        return utcSdf.format(new Date(time));
    }

    public int run(String[] args) throws Exception {
        conf(args);
        return 0;
    }

    public static void main(String[] args) throws Exception {
        conf(args);
    }

    private static void conf(String[] args) throws IOException,
            InterruptedException, ClassNotFoundException {
        Configuration conf = new Configuration();
        Job job = new Job(conf, "uniquekeygen");
        job.setJarByClass(UniqueKeyGenerator.class);

        job.setOutputKeyClass(Text.class);
        job.setOutputValueClass(Text.class);

        job.setMapperClass(Map.class);
        job.setReducerClass(Reduce.class);

        job.setInputFormatClass(TextInputFormat.class);
        job.setOutputFormatClass(TextOutputFormat.class);
        // job.setNumReduceTasks(400);

        FileInputFormat.addInputPath(job, new Path(args[0]));
        FileOutputFormat.setOutputPath(job, new Path(args[1]));

        job.waitForCompletion(true);
    }

}

对于较高的行数，这是一致的，对于20855982行的输入，差异相当于208969条记录。减少减速机输入的原因可能是什么？