Phusion Passenger Optimization.

If you faceing same as following issue.
Exception NameError in PhusionPassenger::Rack::ApplicationSpawner (uninitialized constant MongoMapper)

Please do the settings. It will really help you.
Also, check your Database Query and cusumption of Memroy on server.
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http {
     ..
     PassengerMaxPoolSize 3
     keepalive_timeout  120;
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Please let me know, if still you have an issue.