reuse JVM in Hadoop mapreduce jobs

I know we can set the property "mapred.job.reuse.jvm.num.tasks" to re-use JVM. My questions are: (1) how to decide the number of tasks to be set here, -1 or some other positive integers? (2) is it a good idea to already reuse JVMs and set this property to the value of -1 in mapreduce jobs? Thank you very much!

[imai-factory]

小さなファイルをたくさん処理したい時にはこれ使えばいいのね。s3distcpでファイルをconcatするときとかによさげ。