我是hadoop的新手，我有一个4节点的集群，它有32个map插槽和16个reduce插槽。job使用761个Map和2个还原器处理接近100GB的数据。
我的问题是为什么它只使用2个减速器。请让我如果我错过了任何有关减速器的配置或它是预期的。
我在mapreduce配置中设置了以下属性，但它仍然使用2个reducer。
每个作业的默认减少任务数
mapred.reduce.tasks=8
日志：

15/12/30 14:58:56 INFO mapred.JobClient: Job complete: job_201512301313_0002
15/12/30 14:58:56 INFO mapred.JobClient: Counters: 33
15/12/30 14:58:56 INFO mapred.JobClient:   File System Counters
15/12/30 14:58:56 INFO mapred.JobClient:     FILE: Number of bytes read=11711801793
15/12/30 14:58:56 INFO mapred.JobClient:     FILE: Number of bytes written=24324166884
15/12/30 14:58:56 INFO mapred.JobClient:     FILE: Number of read operations=0
15/12/30 14:58:56 INFO mapred.JobClient:     FILE: Number of large read operations=0
15/12/30 14:58:56 INFO mapred.JobClient:     FILE: Number of write operations=0
15/12/30 14:58:56 INFO mapred.JobClient:     HDFS: Number of bytes read=101855418108
15/12/30 14:58:56 INFO mapred.JobClient:     HDFS: Number of bytes written=821001518
15/12/30 14:58:56 INFO mapred.JobClient:     HDFS: Number of read operations=1536
15/12/30 14:58:56 INFO mapred.JobClient:     HDFS: Number of large read operations=0
15/12/30 14:58:56 INFO mapred.JobClient:     HDFS: Number of write operations=2
15/12/30 14:58:56 INFO mapred.JobClient:   Job Counters
15/12/30 14:58:56 INFO mapred.JobClient:     Launched map tasks=761
15/12/30 14:58:56 INFO mapred.JobClient:     Launched reduce tasks=2
15/12/30 14:58:56 INFO mapred.JobClient:     Data-local map tasks=753
15/12/30 14:58:56 INFO mapred.JobClient:     Rack-local map tasks=8
15/12/30 14:58:56 INFO mapred.JobClient:     Total time spent by all maps in occupied slots (ms)=10467348
15/12/30 14:58:56 INFO mapred.JobClient:     Total time spent by all reduces in occupied slots (ms)=936182
15/12/30 14:58:56 INFO mapred.JobClient:     Total time spent by all maps waiting after reserving slots (ms)=0
15/12/30 14:58:56 INFO mapred.JobClient:     Total time spent by all reduces waiting after reserving slots (ms)=0