Can not checkpoint Graph object's vertices but could checkpoint edges

[dash]

I'm trying to workaround the StackOverflowError when an object have a long
dependency chain, someone said I should use checkpoint to cuts off
dependencies. I write a sample code to test it, but I can only checkpoint
edges but not vertices. I think I do materialize vertices and edges after
calling checkpoint, why only edge been checkpointed?

Here is my code, really appreciate if you can point out what I did wrong.

def main(args: Array[String]) {
    val conf = new SparkConf().setAppName("Test")
      .setMaster("local[4]")
    val sc = new SparkContext(conf)
    sc.setCheckpointDir("./checkpoint")
    val v = sc.parallelize(Seq[(VertexId, Long)]((0L, 0L), (1L, 1L), (2L,
2L)))
    val e = sc.parallelize(Seq[Edge[Long]](Edge(0L, 1L, 0L), Edge(1L, 2L,
1L), Edge(2L, 0L, 2L)))
    var g = Graph(v, e)
    val vertexIds = Seq(0L, 1L, 2L)
    var prevG: Graph[VertexId, Long] = null
    for (i <- 1 to 100000) {
      vertexIds.toStream.foreach(id => {
        println("generate new graph")
        prevG = g
        g = Graph(g.vertices, g.edges)

        println("uncache vertices")
        prevG.unpersistVertices(blocking = false)
        println("uncache edges")
        prevG.edges.unpersist(blocking = false)

        //Third approach, do checkpoint
        //Vertices can not be checkpointed, still have StackOverflowError
        g.vertices.checkpoint()
        g.edges.checkpoint()
        println(g.vertices.count()+g.edges.count())
        println(g.vertices.isCheckpointed+" "+g.edges.isCheckpointed)

      })

      println(" iter " + i + " finished")
    }
  }





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