Poor performance with large keys using RocksDB and MapState

[ירון שני]

Hello,
I have a poor throughput issue, and I think I managed to reproduce it using
the following code:

val conf = new Configuration()
conf.set(TaskManagerOptions.MANAGED_MEMORY_SIZE,
MemorySize.ofMebiBytes(6 * 1000))
conf.set(TaskManagerOptions.TOTAL_PROCESS_MEMORY,
MemorySize.ofMebiBytes(8 * 1000))
conf.set(TaskManagerOptions.NETWORK_MEMORY_MIN, MemorySize.ofMebiBytes(256))
conf.set(RocksDBConfigurableOptions.BLOCK_SIZE, new MemorySize(8 * 1000))

val be = new RocksDBStateBackend("file:///tmp")
val env = StreamExecutionEnvironment.createLocalEnvironmentWithWebUI(conf)
  .setStateBackend(be)

env.setParallelism(3)
env.getConfig.enableObjectReuse()

val r = new scala.util.Random(31)
    val randStr = r.nextString(4992)
    val s = env.fromElements(1).process((value: Int, ctx:
_root_.org.apache.flink.streaming.api.functions.ProcessFunction[Int,
_root_.scala.Predef.String]#Context, out:
_root_.org.apache.flink.util.Collector[_root_.scala.Predef.String]) =>
{
      for (a <- 1 to 1000 * 1000 * 10) {
        out.collect( randStr + r.nextString(8) )

      }
    }).keyBy(a=>a).process(new ProcessFunction[String, String] {
      private var someState: MapState[String, String] = _

      override def open(parameters: Configuration): Unit = {
        someState = getRuntimeContext.getMapState(
          new MapStateDescriptor[String, String]("someState",
createTypeInformation[String], createTypeInformation[String])
        )
      }

      override def processElement(value: _root_.scala.Predef.String,
ctx: 
_root_.org.apache.flink.streaming.api.functions.ProcessFunction[_root_.scala.Predef.String,
_root_.scala.Predef.String]#Context, out:
_root_.org.apache.flink.util.Collector[_root_.scala.Predef.String]):
Unit = {
        if(!someState.contains(value)) {
          someState.put(value, value)
        }
      }
})

env.execute()

This has really poor throughput.
Now changing
out.collect( randStr + r.nextString(8) )

to
out.collect( r.nextString(8) + randStr)
Solves the issue.
Is there any way easy to fix this?
I tried to use hash index, but it required rocks db option called "prefix
extractor" which I don't know how to fill yet, and no idea if it will fix
it.
If anyone encountered that before, I would really use some advice/help.
Thanks!