hadoop-2.0.5-alpha userlogs dir is empty

[haiwei.xie-soulinfo]

hi, 

   I used hadoop-2.0.5-alpha to run mapreduce tasks, but 
hadoop-2.0.5-alpha/logs/userlogs dir is empty,
No task log files exist, so it's hard to trace failed task.  

   How can I get task log files? Thanks.

--terrs