2.28 MapReduce在实际应用中常见的优化

一、优化的点

• Reduce Task Number
• Map Task输出压缩
• Shuffle Phase 参数
• map、reduce分配的虚拟CPU

二、Reduce Task Number

Reduce Task 默认是一个；

Reduce Task的数目也不是越多越好，实际中需要测试调整，以调整到最优的个数, 如下；

job.setNumReduceTasks(2);

 

三、Map Task输出压缩

上一节已经讲到了；

四、Shuffle Phase 参数

具体可参考：mapred-default.xml

可调的有如下几点：

mapreduce.task.io.sort.factor:

<property>
  <name>mapreduce.task.io.sort.factor</name>
  <value>10</value>
  <description>The number of streams to merge at once while sorting
  files.  This determines the number of open file handles.</description>
</property>

mapreduce.task.io.sort.mb：

<property>
  <name>mapreduce.task.io.sort.mb</name>
  <value>100</value>
  <description>The total amount of buffer memory to use while sorting 
  files, in megabytes.  By default, gives each merge stream 1MB, which
  should minimize seeks.</description>
</property>

mapreduce.map.sort.spill.percent：

<property>
  <name>mapreduce.map.sort.spill.percent</name>
  <value>0.80</value>
  <description>The soft limit in the serialization buffer. Once reached, a
  thread will begin to spill the contents to disk in the background. Note that
  collection will not block if this threshold is exceeded while a spill is
  already in progress, so spills may be larger than this threshold when it is
  set to less than .5</description>
</property>

五、map、reduce分配的虚拟CPU

默认都是一个虚拟CPU，实际中也可以调整；

1、map

mapreduce.map.cpu.vcores：

<property>
  <name>mapreduce.map.cpu.vcores</name>
  <value>1</value>
  <description>
      The number of virtual cores required for each map task.
  </description>
</property>

2、reduce

mapreduce.reduce.cpu.vcores：

<property>
  <name>mapreduce.reduce.cpu.vcores</name>
  <value>1</value>
  <description>
      The number of virtual cores required for each reduce task.
  </description>
</property>