hadoop mapreduce经常OOM

<property>
  <name>mapred.child.java.opts</name>
  <value>-Xmx512m</value>//默认200m
  <description>Java opts for the task tracker child processes. 
  The following symbol, if present, will be interpolated: @taskid@ is replaced
  by current TaskID. Any other occurrences of '@' will go unchanged.
  For example, to enable verbose gc logging to a file named for the taskid in
  /tmp and to set the heap maximum to be a gigabyte, pass a 'value' of:
        -Xmx1024m -verbose:gc -Xloggc:/tmp/@taskid@.gc
 
  The configuration variable mapred.child.ulimit can be used to control the
  maximum virtual memory of the child processes.
  </description>
</property>