Hi Mohammed,
just for completeness and to publicy document how this issue was resolved:
Starting the Spark Jobserver must be done through the boot scripts which we provide. If you follow the installation guide PDF (see [1]) you should have it installed under /etc/init.d/spark-job-server
We currently provide boot scripts for RedHat 6.x/7.x, CentOS 6.x/7.x and Ubuntu 14 (SysV boot).
Starting and stopping the jobserver can the be done via the /etc/init.d/spark-job-server script (more details are also in [1]).
Best,
Björn
[1] https://www.knime.org/knime-spark-executor#install