我使用级联做一个哈希连接两个300mb的文件。我执行以下级联工作流：

// select the field which I need from the first file
Fields f1 = new Fields("id_1");
docPipe1 = new Each( docPipe1, scrubArguments, new ScrubFunction( f1 ), Fields.RESULTS );   

// select the fields which I need from the second file 
Fields f2 = new Fields("id_2","category");
docPipe2 = new Each( docPipe2, scrubArguments, new ScrubFunction( f2), Fields.RESULTS ); 

// hashJoin
Pipe tokenPipe = new HashJoin( docPipe1, new Fields("id_1"), 
                     docPipe2, new Fields("id_2"), new LeftJoin());

// count the number of each "category" based on the id_1 matching id_2
Pipe pipe = new Pipe(tokenPipe );
pipe = new GroupBy( pipe , new Fields("category"));
pipe = new Every( pipe, Fields.ALL, new Count(), Fields.ALL );

我在hadoop集群上运行这个级联程序，这个集群有3个datanode，每个datanode有8个ram和4个内核（我将mapred.child.java.opts设置为4096mb）；但我花了大约30分钟才得到最终结果。我觉得太慢了，但是我觉得我的程序和集群都没有问题。如何使此级联联接更快？