[erlang-questions] garbage collection: when?

Roberto Ostinelli <>
Sun Jun 5 16:21:54 CEST 2011


2011/6/1 Richard Carlsson <>

> Garbage collection is not triggered by any particular event (except an
> explicit call to garbage_collect()), but rather, when the code tries to do
> something that requires more memory, e.g., to create a tuple or cons cell,
> than what is currently easily available on the heap. It then calls the
> garbage collector to try to get some more free space from the newest
> generation - this moves the used memory to one end and all the free memory
> to the other end. If this creates enough contiguous space, the code can
> continue with the allocation. Otherwise, the system will try to garbage
> collect the next older generation, and so on. If all generations have been
> garbage collected and there's still not enough memory for the allocation,
> the Erlang runtime system will enlarge the process' heap (by allocating more
> memory from the operating system).
>
> Thus, how often garbage collection is triggered depends on how quickly you
> create tuples and other data structures, and the size of the process heap
> depends on whether or not it allocates new data faster than it releases old
> data. If it releases data at the same speed or faster, then it will stay at
> the same size (or even shrink), because garbage collection will always be
> able to reclaim enough space from the existing heap.
>
> In your example above, the original list has no more references to it after
> the call to lists:keyreplace(), so it might get collected at that point, or
> at any later point, depending on whether your program needs to allocate more
> data structures and how much space is currently free on the process heap. A
> process that does not try to allocate more data does not waste time doing
> garbage collection either.
>
>    /Richard
>

thank you richard and jesper for these very helpful insights.

r.
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