public int getSafetyLine() {
    int min = safetyPiles[0];
    for (int i = 1; i < size; ++i) {
      min = Math.min(safetyPiles[i], min);
    }
    return min;
  }
Get the safe index.

@return the index of the first element that cannot be evicted. in other words, elements whose
    index are <b>less than</b> the return value can be evicted.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/transformation/dag/memory/SafetyLine.java