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(librep.info)Garbage Collection

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Garbage Collection
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   In Lisp, data objects are used very freely; a side effect of this is
that it is not possible to (easily) know when an object is "stale",
that is, no references to it exist and it can therefore be reused.

   The "garbage collector" is used to overcome this problem; whenever
enough memory has been allocated to make it worthwhile, evaluation
stops and the garbage collector works its way through memory deciding
which objects may still be referenced, and which are stale. The stale
objects are then recorded as being available for reuse and evaluation
continues. (But Note: Guardians)

 - Function: garbage-collect
     Runs the garbage collector, usually this function doesn't need to
     be called manually.

 - Variable: garbage-threshold
     The number of bytes of data that must have been allocated since the
     last garbage collection before evaluation pauses and the garbage
     collector is invoked. Its default value is about 100K.

 - Variable: idle-garbage-threshold
     When the input loop is idle (due to a lack of input), this is the
     number of bytes of data that must have been allocated since the
     garbage collection, for another collection to be triggered.

     This is usually set to a lot less than `garbage-threshold' since
     the small delay caused by garbage collection is unnoticeable if the
     system is already idle.

 - Variable: after-gc-hook
     A hook (Note: Normal Hooks) called immediately after each
     invocation of the garbage collector.