question: @ Override public long freeMemory ( long windowId ) throws IOException { long size = keyStream . dataSizeUpToWindow ( windowId ) + valueStream . dataSizeUpToWindow ( windowId ) ; windowsForFreeMemory . add ( windowId ) ; return size ; }
target:  Free memory up to the given windowId This method will be called by another thread. Adding concurrency control to Stream would impact the performance. This method only calculates the size of the memory that could be released and then sends free memory request to the operator thread