three jobs in my hadoop cluster have stopped at state 4 for five hours!!!

my hadoop become more and more slow sice i begin setup it, i think mybe
because the hadoop filesystem become more and more
larger? here is bin/hadoop fs -count result:
    401141       824522       659714994647 hdfs://hadoop01:9000/user/hadoop

things become very bad since three days ago when i begin runing  five more
jobs simultaneously. a job need two minutes now need
eight more minutes, and i noticed that the jobs submited to hadoop stay at
state 4 in the job queue for long time, about one minute and more.
now , three jobs stay at state 4 for five hours because the last completed
job finished at five hours ago!
here is the bin/hadoop job -list result:

 3 jobs currently running
JobId   State   StartTime       UserName        Priority
SchedulingInfo
job_201001221137_7050   4       1264453449856   hadoop  NORMAL
job_201001221137_7051   4       1264453919364   hadoop  NORMAL
job_201001221137_7052   4       1264453927418   hadoop  NORMAL


can any body help me?
-- 
Regards
Junyong

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