There are some brilliant help documents over at the AWS Documentation server. I'm not able to link them from here, but the steps you should take using their documentation are:
Step 1: Gather data about the issue
Step 2: Check the environment
Step 3: Examine the log files
Step 4: Check cluster and instance health
Step 5: Check for suspended groups
Step 6: Review configuration settings
Step 7: Examine input data
Every Amazon EMR cluster reports metrics to CloudWatch. You should use this tool to troubleshoot.
I hope this answers your question.
Thanks.
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