README.markdown
Erlang Term Info (Memory Consumption)
=====================================
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Purpose
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To provide the in-memory size of Erlang terms, ignoring where the Erlang terms
are stored.
Size information should match the [Erlang Efficiency Guide memory information](http://www.erlang.org/doc/efficiency_guide/advanced.html#id68923):
* Small integer: 1 word
* On 32-bit architectures: -134217729 < i < 134217728 (28 bits)
* On 64-bit architectures: -576460752303423489 < i < 576460752303423488 (60 bits)
* Big integer: 3..N words
* Atom: 1 word
* Float:
* On 32-bit architectures: 4 words
* On 64-bit architectures: 3 words
* Binary: 3..6 + data
* List: 1 word + 1 word per element + the size of each element
* Tuple: 2 words + the size of each element
* Pid:
* From local node: 1 word
* From remote node: 5 words
* Port:
* From local node: 1 word
* From remote node: 5 words
* Reference:
* On 32-bit architectures:
* From local node: 5 words
* From remote node: 7 words
* On 64-bit architectures:
* From local node: 4 words
* From remote node: 6 words
* Fun: 9..13 words + size of environment
Maps use a "flatmap" (a pair of tuples) before switching to a
Hash Array Mapped Trie (HAMT) implementation after the size becomes larger than
`MAP_SMALL_MAP_LIMIT` (32). The upper-limit of a HAMT map is used in ERTS
(`HASHMAP_ESTIMATED_HEAP_SIZE(SIZE) == (SIZE*3 + (2*SIZE/5)*2)`)
but the result severely exaggerates the size of a map. Due to the difficulty
anticipating the in-memory size of a map, the size is taken directly by using
`erts_debug:flat_size/1`. The C macros above may change in the future
(the macros above are from `erts/emulator/beam/erl_map.h` in Erlang 18.1).
License
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BSD
Contact
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Michael Truog (mjtruog [at] gmail (dot) com)
Thanks
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* Sverker Eriksson [`erts_debug:flat_size/1` info](http://erlang.org/pipermail/erlang-bugs/2014-September/004607.html)