public void startWalking(Collection<Node> startNodes,
      HashMap<Node, Object> nodeOutput) throws SemanticException {
    toWalk.addAll(startNodes);
    while (toWalk.size() > 0) {
      Node nd = toWalk.remove(0);
      walk(nd);
      if (nodeOutput != null) {
        nodeOutput.put(nd, retMap.get(nd));
      }
    }
  }
starting point for walking.

@throws SemanticException
ql/src/java/org/apache/hadoop/hive/ql/lib/TaskGraphWalker.java