A Phoenix app I am working on is experiencing binaries related memory leak during Quantum triggered processing. This is the gist of it:

• a long running process is started with the app, given it does the work using a pool of workers
• Quantum triggers the processing via GenServer.cast(:start_processing)
• workers fetch data from external service, via JSON API
• then parse data and store result in DB

The result is pretty much in line with findings in Memory leak with quantum and processing binary files. I tried the strings: :copy idea, but it did not work. Also tried hibernating related process (including poolboy workers), as suggested in Memory leaking with long running processes - #9 by dom, but binaries are still leaking.

Things tried without result:

• pass strings: :copy to Jason, via plug(Tesla.Middleware.JSON, engine_opts: [strings: :copy]) (we use Tesla for the HTTP client)
• do :hibernate in related processes (and workers), although this was added only to final handle_* callbacks, e.g. only those callbacks where we actually know the work is completed (not all)

Then I did some investigation to see which processes were actually hanging on to binaries after all this, and found them all to be telemetry related. Here’s the findings:

processes_with_large_binaries =
  Process.list()
  |> Enum.map(fn pid -> {pid, Process.info(pid, :binary)} end)
  |> Enum.reject(fn {pid, binaries} -> match?({:binary, []}, binaries) end)
  |> Enum.map(fn {pid, {:binary, binaries}} -> {pid, Enum.map(binaries, fn {_, size, _} -> size end)
  |> Enum.sum()} end)
  |> Enum.sort_by(fn {_, s} -> s end, &>=/2)

binary_largest = pids_with_details |> Enum.sort_by(& &1.binaries_size, :desc) |> Enum.take(10)

All seem to be telemetry related:

Enum.map(binary_largest, & &1.process_info[:dictionary])
[
  [
    "$initial_call": {:h2_stream, :init, 1},
    "$ancestors": [#PID<0.5412.0>, #PID<0.4567.0>, #PID<0.4550.0>,
     :otel_batch_processor, :opentelemetry_sup, #PID<0.4474.0>]
  ],
  [
    "$initial_call": {:h2_stream, :init, 1},
    "$ancestors": [#PID<0.5412.0>, #PID<0.4567.0>, #PID<0.4550.0>,
     :otel_batch_processor, :opentelemetry_sup, #PID<0.4474.0>]
  ],
  [
    "$initial_call": {:h2_stream, :init, 1},
    "$ancestors": [#PID<0.5412.0>, #PID<0.4567.0>, #PID<0.4550.0>,
     :otel_batch_processor, :opentelemetry_sup, #PID<0.4474.0>]
  ],
  [
    "$initial_call": {:h2_stream, :init, 1},
    "$ancestors": [#PID<0.5412.0>, #PID<0.4567.0>, #PID<0.4550.0>,
     :otel_batch_processor, :opentelemetry_sup, #PID<0.4474.0>]
  ],
  [
    "$initial_call": {:h2_stream, :init, 1},
    "$ancestors": [#PID<0.5412.0>, #PID<0.4567.0>, #PID<0.4550.0>,
     :otel_batch_processor, :opentelemetry_sup, #PID<0.4474.0>]
  ],
  [
    "$initial_call": {:h2_stream, :init, 1},
    "$ancestors": [#PID<0.5412.0>, #PID<0.4567.0>, #PID<0.4550.0>,
     :otel_batch_processor, :opentelemetry_sup, #PID<0.4474.0>]
  ],
  [
    "$initial_call": {:h2_stream, :init, 1},
    "$ancestors": [#PID<0.5412.0>, #PID<0.4567.0>, #PID<0.4550.0>,
     :otel_batch_processor, :opentelemetry_sup, #PID<0.4474.0>]
  ],
  [
    "$initial_call": {:h2_stream, :init, 1},
    "$ancestors": [#PID<0.5412.0>, #PID<0.4567.0>, #PID<0.4550.0>,
     :otel_batch_processor, :opentelemetry_sup, #PID<0.4474.0>]
  ],
  [
    "$initial_call": {:h2_stream, :init, 1},
    "$ancestors": [#PID<0.5412.0>, #PID<0.4567.0>, #PID<0.4550.0>,
     :otel_batch_processor, :opentelemetry_sup, #PID<0.4474.0>]
  ],
  [
    "$initial_call": {:h2_stream, :init, 1},
    "$ancestors": [#PID<0.5412.0>, #PID<0.4567.0>, #PID<0.4550.0>,
     :otel_batch_processor, :opentelemetry_sup, #PID<0.4474.0>]
  ]
]

A sample:

pid_with_details = Enum.random(binary_largest)
%{
  binaries_size: 977410683,
  memory: 196896,
  pid: #PID<0.7970.48>,
  process_info: [
    current_function: {:gen_statem, :loop_receive, 3},
    initial_call: {:proc_lib, :init_p, 5},
    status: :waiting,
    message_queue_len: 0,
    links: [#PID<0.5412.0>],
    dictionary: [
      "$initial_call": {:h2_stream, :init, 1},
      "$ancestors": [#PID<0.5412.0>, #PID<0.4567.0>, #PID<0.4550.0>,
       :otel_batch_processor, :opentelemetry_sup, #PID<0.4474.0>]
    ],
    trap_exit: true,
    error_handler: :error_handler,
    priority: :normal,
    group_leader: #PID<0.4473.0>,
    total_heap_size: 24503,
    heap_size: 6772,
    stack_size: 11,
    reductions: 93231,
    garbage_collection: [
      max_heap_size: %{error_logger: true, kill: true, size: 0},
      min_bin_vheap_size: 46422,
      min_heap_size: 233,
      fullsweep_after: 65535,
      minor_gcs: 10
    ],
    suspending: []
  ]
}

Before garbage collection:

Process.info(pid_with_details.pid, :binary)
{:binary,
 [
   {139802410680360, 6147237, 160},
   {139802410680360, 6147237, 160},
   {139802410680360, 6147237, 160},
   {139802410680360, 6147237, 160},
   {139802410680360, 6147237, 160},
   {139802410680360, 6147237, ...},
   {139802410680360, ...},
   {...},
   ...
 ]}

After garbage collecting:

:erlang.garbage_collect(pid_with_details.pid)
true

Process.info(pid_with_details.pid, :binary)
{:binary, []}

So even though the processes responsible for actual work don’t hold on to those binaries (at least I could not detect this), telemetry processes do hold on for some reason.

I am thinking of trying:

• doing the :erlang.garbage_collect() in related processes and workers; but not sure if it makes sense to garbage collect them since binaries are held by telemetry processes
• do a periodic scan for processes with large binaries, like above, and garbage collect them, maybe even trigger that from actual processes that do the work once they know they are done
• make use of dynamic supervisor; but not sure how to build such a tree and still use pool of workers and likely this would not help given binaries are held in telemetry processes

But, given strings: :copy was used for Jason and those worker processes don’t seem to hold on to binaries, don’t think any solution to improve workers related code will help. And I would like to avoid garbage collection, seems like a sledge hammer solution to the problem.

Thoughts/ideas welcome!

Versions of libs:

• telemetry 1.1.0
• telemetry_metrics 0.6.0
• telemetry_poller 0.5.1
• opentelemetry 1.0.5
• opentelemetry_api 1.0.3
• opentelemetry_exporter 1.0.4
• opentelemetry_telemetry 1.0.0
• phoenix 1.5.8
• jason 1.3.0