web concurrency auto

config/puma.rb

@@ -33,8 +33,14 @@ environment ENV.fetch('RAILS_ENV', 'development')
 # the concurrency of the application would be max `threads` * `workers`.
 # Workers do not work on JRuby or Windows (both of which do not support
 # processes).
-#
-workers ENV.fetch('WEB_CONCURRENCY', 0)
+
+if ENV['WEB_CONCURRENCY_AUTO'] == 'true'
+  require 'etc'
+
+  workers Etc.nprocessors
+else
+  workers ENV.fetch('WEB_CONCURRENCY', 0)
+end
 
 # Use the `preload_app!` method when specifying a `workers` number.
 # This directive tells Puma to first boot the application and load code