A Rack application running on Heroku can improve performance by using different application servers with different performance characteristics.
The exact configuration settings will vary by application and how much memory each app takes. A default Rails app with a basic scaffold can easily support 4 unicorn workers or 4 puma threads. A small Sinatra app can support several times that number.
Here are the
Procfile settings you can use for each:
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Use the New Relic RPM Add-on to monitor your application resources.