activity monitor – "0 bytes" swap used always reported
Is this actually an error in Activity Monitor, or is it correct?


That is how it suppose to work

If it's not an error, what mechanism is being used to maintain the
RAM in place of the swapfile?


Mavericks active memory management

Is this mechanism verifiable faster than using a swapfile, and is
there a way to disable it and use the swapfile by default?


it is faster then using actual swap to HDD, it also uses something called memory compression. It always tries to use all of the memory to handle its tasks, and will rearrange the memory according to your usage. Once that is no longer possible it will use the swap.