technology from back to front

Erlang’s GC only runs when you’re running

Recently, as part of RabbitMQ server development, we ran into an interesting issue regarding Erlang’s per-process garbage collection. If a process is idle — not doing any work at all, simply waiting for an external event — then its garbage-collector will not run until it starts working again. The solution is to hibernate idle processes, which causes a very aggressive garbage-collection run and puts the process into a suspended state, from which it will wake when a message next arrives.

We were implementing a form of flow control in RabbitMQ server, essentially equivalent to XON/XOFF, and when a channel was paused, we found that its memory usage wasn’t decreasing in proportion to its decreased activity. As soon as we started hibernating processes, the memory usage dropped right back.


eight + 9 =

2000-14 LShift Ltd, 1st Floor, Hoxton Point, 6 Rufus Street, London, N1 6PE, UK+44 (0)20 7729 7060   Contact us