I have an Ubuntu system with 512MB physical RAM and 512MB swap. Running my four Rails 3.2.12 apps on Passenger 4.0.45 server (Apache module) using single-user (my user) RVM Ruby 2.1.2, it uses about 30% more memory than when I configure Passenger to use the older system (root) Ruby 1.9.3. That is the only difference in the Passenger config.
This causes Passenger to move other applications to swap when one is accessed, which means each time a different application is accessed, it has to be loaded in from swap, resulting in a delay. This means that I almost run out of memory if multiple apps are being used concurrently, and they react slowly to users.
I can't understand why using RVM causes Passenger to use more memory. I doubt it is because of Ruby 2.1.2 unless there are memory parameters I need to tune.
The out of the box settings for ruby 2.1.2 do increase memory use compared to 2.0.0 (which I believe was on par with 1.9.3 if you don't consider the copy on write improvements in 2.0.0)
The reason is that Ruby 2.1 introduced a new garbage collection algorithm. In a nutshell the algorithm assumes that while some objects live for a long time (eg the objects representing your code) others are very short lived. Older rubies would spend a long time trying to see whether all objects were ready to be garbage collected, whereas ruby 2.1 switches between minor collections (only try to collect the short lived objects) and major collections (try to collect the long lived ones).
This increases performance (minor collections are much faster) at the expense of some memory usage.
You can tune this, using (among others) the RUBY_GC_HEAP_OLDOBJECT_LIMIT_FACTOR environment variable. The default is 2, a setting of 0.9 turns off the the generational garbage collector and numbers in between will trade off memory for performance.
You can also use the jemalloc library (on any ruby version) to gain a little performance and reduce memory usage slightly.
Lastly part of the problem with rails apps is the many apps have what one might call medium lived objects that last for a whole request - Ruby's attempt to split objects into just 2 generations isn't quite sufficient. Ruby 2.2 is slated to improve on this.
Sam Saffron has a great post on this if you want to read more
In addition ruby 2.1.3 made some changes to gc timing that reduces memory uses in most cases compared to 2.1.2
