How to stabilize spark streaming application with a handful of super big sessions? - hadoop

Question

I am running a Spark Streaming application based on mapWithState DStream function . The application transforms input records into sessions based on a session ID field inside the records.
A session is simply all of the records with the same ID . Then I perform some analytics on a session level to find an anomaly score.
I couldn't stabilize my application because a handful of sessions are getting bigger at each batch time for extended period ( more than 1h) . My understanding is a single session (key - value pair) is always processed by a single core in spark . I want to know if I am mistaken , and if there is a solution to mitigate this issue and make the streaming application stable.
I am using Hadoop 2.7.2 and Spark 1.6.1 on Yarn . Changing batch time, blocking interval , partitions number, executor number and executor resources didn't solve the issue as one single task makes the application always choke. However, filtering those super long sessions solved the issue.
Below is a code updateState function I am using :
val updateState = (batchTime: Time, key: String, value: Option[scala.collection.Map[String,Any]], state: State[Seq[scala.collection.Map[String,Any]]]) => {
val session = Seq(value.getOrElse(scala.collection.Map[String,Any]())) ++ state.getOption.getOrElse(Seq[scala.collection.Map[String,Any]]())
if (state.isTimingOut()) {
Option(null)
} else {
state.update(session)
Some((key,value,session))
}
}
and the mapWithStae call :
def updateStreamingState(inputDstream:DStream[scala.collection.Map[String,Any]]): DStream[(String,Option[scala.collection.Map[String,Any]], Seq[scala.collection.Map[String,Any]])] ={//MapWithStateDStream[(String,Option[scala.collection.Map[String,Any]], Seq[scala.collection.Map[String,Any]])] = {
val spec = StateSpec.function(updateState)
spec.timeout(Duration(sessionTimeout))
spec.numPartitions(192)
inputDstream.map(ds => (ds(sessionizationFieldName).toString, ds)).mapWithState(spec)
}
Finally I am applying a feature computing session foreach DStream , as defined below :
def computeSessionFeatures(sessionId:String,sessionRecords: Seq[scala.collection.Map[String,Any]]): Session = {
val features = Functions.getSessionFeatures(sessionizationFeatures,recordFeatures,sessionRecords)
val resultSession = new Session(sessionId,sessionizationFieldName,sessionRecords)
resultSession.features = features
return resultSession
}