Dispatcher disp = new DefaultRuleDispatcher(new GenMROperator(), opRules, procCtx);
GraphWalker ogw = new GenMapRedWalker(disp);
ArrayList<Node> topNodes = new ArrayList<Node>();
topNodes.addAll(this.topOps.values());
ogw.startWalking(topNodes, null);
// reduce sink does not have any kids - since the plan by now has been broken up into multiple
// tasks, iterate over all tasks.
// For each task, go over all operators recursively
for(Task<? extends Serializable> rootTask: rootTasks)