Process Heaps ELI5

Allocation in Erlang happens inside so called “carriers”. They look like “zone memory”, used by games — big chunks of system heap pre-allocated beforehand. Inside carriers the real allocation is happening. For simplicity you can imagine simple malloc/realloc, that’s how its done. This is done to fight memory fragmentation and is not important for the understanding. You can see erts/emulator/beam/erl_*alloc.c (many those files, one per strategy of allocation).

Garbage Collection ELI5

When a heap comes to its capacity threshold (something like 75%), the process triggers garbage collection. A new bigger heap may be allocated. Scanning generational garbage collection algorithm runs on the heap. The algorithm takes “roots” — all known to be live values on stack and in registers, and moves them to a new heap. Then it scans the remaining parts of source heap and extracts more values, referred by the roots. After the scanning, source heap contains only dead values and algorithm drops it.

In reality the picture is a bit more complicated. There can be one or two heaps with different placement logic applied to them. Also there is mature mark for values which survived previous GC.

Having garbage collector run per process heap allows Erlang to keep collection latency low. Also it doesn’t pause or affect other processes on other schedulers. This is not a simple topic, but theory is all here: http://gchandbook.org/