I am making some test on odoo 11 and I just want to use the command line to run my odoo test server

 python odoo -d odoo11 -r odoo --xmlrpc-port 8071 --addons-path="/opt/odoo11/odoo/addons,/opt/odoo11/odoo/community" --workers 9 --log-level debug --proxy-mode --longpolling-port 8079 --max-cron-threads 3

I got this error every few seconds. I search on internet but I can not find any solution. BTW I can not see the speed difference between with and without workers :(

 File "/opt/odoo11/odoo/addons/bus/controllers/main.py", line 37, in poll
    raise Exception("bus.Bus unavailable")
Exception: bus.Bus unavailable