Erlang Term Info (Memory Consumption)
=====================================

[![Build Status](https://secure.travis-ci.org/okeuday/erlang_term.png?branch=master)](http://travis-ci.org/okeuday/erlang_term)

Purpose
-------

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
* Small Map (N =< 32): 4 words + 2 words per key/value pair + the size of each pair
* Large Map (N > 32): At least, 2 words + 2 * N words + 2 * log16(N) words + the size of each pair
* 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
-------

BSD

Contact
-------

Michael Truog (mjtruog [at] gmail (dot) com)

Thanks
------

* Sverker Eriksson [`erts_debug:flat_size/1` info](http://erlang.org/pipermail/erlang-bugs/2014-September/004607.html)

