I think the code loading part will be the major difference - more code means more atoms, more binaries (from literals in those modules) and more ets usage (since information which modules are loaded is held in an ets table by the code_server process). Additionally there might be some more system-level services running like SASL or the release handlers, which could account for the extra process size. Have you tried comparing the outputs of Process.list() on both systems? That could shed some more light on it. Similarly for ets tables you could investigate with :ets.info.

Check to see which applications have been loaded, there may be more in the release. Also check which applications have been started. IIRC correctly the release will start all applications at start up time.

Your release is likely starting in embedded mode while mix run is interactive mode. Embedded mode will load all beam modules on start up instead of only when they are used.

See http://erlang.org/doc/system_principles/system_principles.html#code-loading-strategy

passing --preload-modules to mix run seems to do the job, actually it loads 133 modules more than release, while the modules below are loaded by release but not loaded in mix run, even with preload:

[:sasl_report, :systools_rc, :alarm_handler, :release_handler_1,
 :format_lib_supp, :misc_supp, :systools_make, :sasl, :sasl_report_tty_h,
 :sasl_report_file_h, :erlsrv, :systools_lib, :release_handler, :systools, :rb,
 :rb_format_supp, :systools_relup]

It does increase memory significantly, but release still has more (gist with output).

I’ve had similar numbers here, @jola, using your steps.

Following @rvirding suggestion, I’ve ran this code in each:

Application.started_applications() |> Enum.map(&elem(&1, 0)) |> Enum.sort()

# release
# $ _build/prod/rel/hello/bin/hello console
[:artificery, :asn1, :compiler, :cowboy, :cowlib, :crypto, :distillery, :eex,
 :elixir, :gettext, :hello, :iex, :jason, :kernel, :logger, :mime, :mix,
 :phoenix, :phoenix_pubsub, :plug, :plug_cowboy, :plug_crypto, :public_key,
 :ranch, :runtime_tools, :sasl, :ssl, :stdlib]

# mix
# $ MIX_ENV=prod iex -S mix run
[:artificery, :asn1, :compiler, :cowboy, :cowlib, :crypto, :distillery, :eex,
 :elixir, :gettext, :hello, :hex, :iex, :inets, :jason, :kernel, :logger, :mime,
 :mix, :phoenix, :phoenix_pubsub, :plug, :plug_cowboy, :plug_crypto,
 :public_key, :ranch, :runtime_tools, :ssl, :stdlib]

• release had extra: sasl
• mix run had extra: hex, inets

I guess sasl is the likely culprit?

UPDATE: sasl start not relevant to memory. I’ve checked also the loaded applications, and it’s the same as started applications. That’s intriguing

# :erlang.memory in mix option after start
[
  total: 34951112,
  processes: 13154512,
  processes_used: 13153312,
  system: 21796600,
  atom: 512625,
  atom_used: 505434,
  binary: 206016,
  code: 10368934,
  ets: 942080
]

# :erlang.memory in mix option, after starting sasl
[
  total: 35274176,
  processes: 13233528,
  processes_used: 13230984,
  system: 22040648,
  atom: 520817,
  atom_used: 517966,
  binary: 205984,
  code: 10547076,
  ets: 957688
]

# :erlang.memory in release option
[
  total: 60294456,
  processes: 21970552,
  processes_used: 21847048,
  system: 38323904,
  atom: 992433,
  atom_used: 971447,
  binary: 441208,
  code: 21443760,
  ets: 1992328
]