GEMFILE that starts mongrel
gem 'rails', '3.1.0'
gem 'mongrel', '>=1.2.0.pre2'
GEMFILE that start WEBrick (not mongrel)
gem 'rails', '3.2.0.rc2'
gem 'mongrel', '>=1.2.0.pre2'
Starting on Rails 3.2 you must specify the server you want to use, otherwise Webrick will be used.
So, to use mongrel, you must place the gem at your Gemfile AND start the server like this:
$ rails s mongrel
来源:https://stackoverflow.com/questions/8853338/why-doesnt-mongrel-start-in-rails-3-2-rc