There could be several scenario Pig want to submit multiple DAG:
1. a blocking operator in grunt
2. a blocking operator in script (exec, fs)
3. break a DAG into two for possible inner DAG optimization
Pig should use TezSession so that we can reuse AM. TezSession should be a thread local variable. For those who use Pig Java API, they will use the same AM within a thread, but different AM in another thread.