Puma 2.0.0.b4 is run as rails app backend server by the means of upstart as:

bundle exec puma -C config/puma.rb -e staging >> log/puma.log 2>&1

If there are no workers 1 line in the config file puma uses cpu quite ordinary (less than 1% if there is not load. No problem for now).

As soon as I add workers 1 (or any higher number) to the puma's config file (effectively enabling puma cluster mode), puma master process consumes 100% CPU (under no load). The rails app is perfectly operatable at the same time. Don't know what will happend under high load though. The puma's stdout and stderr report no errors of any kind:

[25912] Puma 2.0.0.b4 starting in cluster mode... [25912] * Process workers: 1 [25912] * Min threads: 1, max threads: 16 [25912] * Environment: staging [25912] * Listening on unix:///srv/projects/projectname/current/tmp/sockets/puma.sock [25912] Use Ctrl-C to stop