@Override
  public long getMemSize(long partitionId) {
    if (memCostForEachPartition.containsKey(partitionId)) {
      return memCostForEachPartition.get(partitionId);
    }
    return 0;
  }
record memory cost of map for each partitionId
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/HashLastFlushTimeMap.java