[Spare Core] function SparkContext.cancelJobGroup(groupId) doesn't work

[Tao Li]

Hi all:

    In my application, I start SparkContext sc and execute some task on sc.
(Each task is a thread, which execute some transform and action on RDDs)
    For each task, I use "sc.setJobGroup(JOB_GROUPID, JOB_DESCRIPTION)" to
set jobGroup for each task.
    But when I call "sc.cancelJobGroup(JOB_GROUPID)" to try to cancel the
task, it doesn't work.
    I want to know it's my usage mistake, or there exist some bugs?

Thanks