How to configure nutch to crawl parallelly

[xiao yang]

Hi, All

I'm using Nutch-1.0 on a 12 nodes cluster, and configure
conf/hadoop-site.xml as follow:
  ...
  <property>
    <name>mapred.tasktracker.map.tasks.maximum</name>
    <value>20</value>
  </property>
  <property>
    <name>mapred.tasktracker.reduce.tasks.maximum</name>
    <value>20</value>
  </property>
  ...
but the "Running Jobs" section in page
http://cluster0:50030/jobtracker.jsp never has more than one item.

Thanks!
Xiao