39 lines
1.8 KiB
Markdown
39 lines
1.8 KiB
Markdown
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# Memory Management
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In order to serialize any non-primitive type, off-heap memory needs to be
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allocated. When memory is allocated inside the JVM, the memory is associated
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with an arena. If none is provided, the arena is an implicit arena, and the
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memory will be freed when the serialized object is garbage collected.
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In many cases this is not desirable, because the memory is not freed in a
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deterministic manner, causing garbage collection pauses to become longer, as
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well as changing allocation performance. Instead of an implicit arena, there
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are other kinds of arenas as well. A `confined-arena` is a thread-local arena.
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Confined arenas are `Closeable`, which means they should usually be used in a
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`with-open` form. When a `confined-arena` is closed, it immediately frees all
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the memory associated with it. The previous example, `out-int`, can be
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implemented with a confined arena.
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```clojure
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(defcfn out-int
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"out_int" [::mem/pointer] ::mem/void
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native-fn
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[i]
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(with-open [arena (mem/confined-arena)]
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(let [int-ptr (mem/serialize i [::mem/pointer ::mem/int] arena)]
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(native-fn int-ptr)
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(mem/deserialize int-ptr [::mem/pointer ::mem/int]))))
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```
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This will free the pointer immediately upon leaving the function.
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When memory needs to be accessible from multiple threads, there's
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`shared-arena`. When a `shared-arena` is `.close`d, it will release all its
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associated memory immediately, and so this should only be done once all other
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threads are done accessing memory associated with it.
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In addition, two non-`Closeable` arenas are `global-arena`, which never frees
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the resources associated with it, and `auto-arena`, which is an arena that frees
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its resources once all of them are unreachable during a garbage collection
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cycle, like an implicit arena, but potentially for multiple allocations rather
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than just one.
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