[rdtsc]

(1) In the normal case it just counts a certain number of VM instructions each process runs before it gets to rescheduled. But it gets interesting as well with internal modules or C modules (for example regex matcher), in that case the C module periodicaly as it works through the data, reports that it consumed some number of reductions and is possibly told to yield now. (On that note: in 19.0 we'll have dirty schedulers by default so in that case blocking long running C code will be handled much better).

(2) Erlang VM instance (it is called a node) is an OS process (plus some helper processes, but they are not important in this case) is of course one heap from kernel's point of view. But internal allocator keep spaces separated for BEAM's data. It is not always as basic as in some cases for binary blocks and sub-blocks it can actually share and ref-count them.

In the new release I like that it could have mailbox outside the main heap of each Erlang process. That could be an interesting parameter to play with.