spark-submit --master local[n] cannot create multi-threads
Just find the problem: there is another user running his own spark task on the same instance which occupying resources.
I write pyspark code to deal with some spark-sql data. 
Last month, it worked perfectly when I ran spark-submit --master local[25]. From top command, I could see 25 python threads.
However, nothing change but today the spark-submit only create one thread. I wonder what kind of things can cause such problem.
This is on a ubuntu server on AWS, which has 16 CPU cores. The Spark version is 2.2.1 and Python is 3.6