From 13e2a811ec2eb0ee1c8d08e022686b264c308012 Mon Sep 17 00:00:00 2001
From: Oneric <oneric@oneric.stub>
Date: Thu, 4 Apr 2024 17:19:58 +0200
Subject: [PATCH] Avoid accumulation of stale data in websockets
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We’ve received reports of some specific instances slowly accumulating
more and more binary data over time up to OOMs and globally setting
ERL_FULLSWEEP_AFTER=0 has proven to be an effective countermeasure.
However, this incurs increased cpu perf costs everywhere and is
thus not suitable to apply out of the box.

Apparently long-lived Phoenix websocket processes are known to
often cause exactly this by getting into a state unfavourable
for the garbage collector.
Therefore it seems likely affected instances are using timeline
streaming and do so in just the right way to trigger this. We
can tune the garbage collector just for websocket processes
and use a more lenient value of 20 to keep the added perf cost
in check.

Testing on one affected instance appears to confirm this theory

Ref.:
  https://www.erlang.org/doc/man/erlang#ghlink-process_flag-2-idp226
  https://blog.guzman.codes/using-phoenix-channels-high-memory-usage-save-money-with-erlfullsweepafter
  https://git.pleroma.social/pleroma/pleroma/-/merge_requests/4060

Tested-by: bjo
---
 lib/pleroma/web/mastodon_api/websocket_handler.ex | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/lib/pleroma/web/mastodon_api/websocket_handler.ex b/lib/pleroma/web/mastodon_api/websocket_handler.ex
index bd7c56243..9dae7bf93 100644
--- a/lib/pleroma/web/mastodon_api/websocket_handler.ex
+++ b/lib/pleroma/web/mastodon_api/websocket_handler.ex
@@ -18,6 +18,8 @@ defmodule Pleroma.Web.MastodonAPI.WebsocketHandler do
   @timeout :timer.seconds(60)
   # Hibernate every X messages
   @hibernate_every 100
+  # Tune garabge collect for long-lived websocket process
+  @fullsweep_after 20
 
   def init(%{qs: qs} = req, state) do
     with params <- Enum.into(:cow_qs.parse_qs(qs), %{}),
@@ -59,6 +61,10 @@ defmodule Pleroma.Web.MastodonAPI.WebsocketHandler do
       "#{__MODULE__} accepted websocket connection for user #{(state.user || %{id: "anonymous"}).id}, topic #{state.topic}"
     )
 
+    # process is long-lived and can sometimes accumulate stale data in such a way it's
+    # not freed by young garbage cycles, thus make full collection sweeps more frequent
+    :erlang.process_flag(:fullsweep_after, @fullsweep_after)
+
     Streamer.add_socket(state.topic, state.oauth_token)
     {:ok, %{state | timer: timer()}}
   end