* You likely have started too many long-running Spark Apps like SparkShell, PySparkShell, PySpark (Jupyter/iPython Notebook), Zeppelin Notebook, Spark Streaming, etc.

* Below is a sample screenshot of the Spark Admin UI when you are in this state

* You need to kill some of the long-running Spark Apps through the Spark Admin UI - usually one of the PySpark (Jupyter/iPython) apps

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