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    <p>Erlang has a few properties that make GC actually pretty easy.</p> <p>1 - Every variable is immutable, so a variable can never point to a value that was created after it.</p> <p>2 - Values are copied between Erlang processes, so the memory referenced in a process is almost always completely isolated.</p> <p>Both of these (especially the latter) significantly limit the amount of the heap that the GC has to scan during a collection.</p> <p>Erlang uses a copying GC. During a GC, the process is stopped then the live pointers are copied from the from-space to the to-space. I forget the exact percentages, but the heap will be increased if something like only 25% of the heap can be collected during a collection, and it will be decreased if 75% of the process heap can be collected. A collection is triggered when a process's heap becomes full.</p> <p>The only exception is when it comes to large values that are sent to another process. These will be copied into a shared space and are reference counted. When a reference to a shared object is collected the count is decreased, when that count is 0 the object is freed. No attempts are made to handle fragmentation in the shared heap.</p> <p>One interesting consequence of this is, for a shared object, the size of the shared object does not contribute to the calculated size of a process's heap, only the size of the reference does. That means, if you have a lot of large shared objects, your VM could run out of memory before a GC is triggered.</p> <p>Most if this is taken from the talk Jesper Wilhelmsson gave at EUC2012.</p>
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