When running a simple app that just loads classes and throws away the class loader the VM invokes a VM_CollectForMetadataAllocation when the metadata usage has increased over a threshold.
If I'm running with CMS and without setting +CMSClassUnloadingEnabled then the CMS cycles that the VM operation triggers to not result in any classes being unloaded.
Before NPG this was "solved" by the VM doing a full gc (serial mark sweep) when perm gen was full but currently if +CMSClassUnloadingEnabled is not set then CMS will never be able to unload classes (unless you call System.gc())