From 933b13f3fafc05db895555fbbf33bab2a12ec74a Mon Sep 17 00:00:00 2001
From: Hampus <hampus@fluxer.app>
Date: Sun, 30 Aug 2026 23:23:26 +0200
Subject: [PATCH] perf(gateway): re-enable generational GC on hot processes
 (#2188)

---
 fluxer_gateway/src/guild/guild.erl            |  4 +-
 .../src/guild/guild_broadcaster.erl           |  2 +-
 fluxer_gateway/src/session/session.erl        |  4 +-
 .../test/gateway_hot_process_gc_tests.erl     | 96 +++++++++++++++++++
 4 files changed, 101 insertions(+), 5 deletions(-)
 create mode 100644 fluxer_gateway/test/gateway_hot_process_gc_tests.erl

diff --git a/fluxer_gateway/src/guild/guild.erl b/fluxer_gateway/src/guild/guild.erl
index 39967a921..fc0ab8c0d 100644
--- a/fluxer_gateway/src/guild/guild.erl
+++ b/fluxer_gateway/src/guild/guild.erl
@@ -30,7 +30,7 @@ update_counts(State) -> guild_maintenance:update_counts(State).
 -spec init(map()) -> {ok, guild_state(), timeout()}.
 init(GuildState) ->
     process_flag(trap_exit, true),
-    erlang:process_flag(fullsweep_after, 0),
+    erlang:process_flag(fullsweep_after, 10),
     State0 = guild_init:init_base_state(GuildState),
     State1 = guild_init:init_member_list(State0),
     State2 = guild_init:init_counts(State1),
@@ -431,7 +431,7 @@ terminate(Reason, State) ->
 
 -spec code_change(term(), guild_state(), term()) -> {ok, guild_state()}.
 code_change(_OldVsn, State, _Extra) ->
-    erlang:process_flag(fullsweep_after, 0),
+    erlang:process_flag(fullsweep_after, 10),
     erlang:garbage_collect(),
     {ok, State}.
 
diff --git a/fluxer_gateway/src/guild/guild_broadcaster.erl b/fluxer_gateway/src/guild/guild_broadcaster.erl
index 129fb5c34..50f5ddeb7 100644
--- a/fluxer_gateway/src/guild/guild_broadcaster.erl
+++ b/fluxer_gateway/src/guild/guild_broadcaster.erl
@@ -71,7 +71,7 @@ maybe_cast(_BroadcasterPid, _Msg) ->
 
 -spec init([integer() | pid()]) -> {ok, map()}.
 init([GuildId, GuildPid]) when is_integer(GuildId), is_pid(GuildPid) ->
-    erlang:process_flag(fullsweep_after, 0),
+    erlang:process_flag(fullsweep_after, 10),
     _ = erlang:monitor(process, GuildPid),
     {ok, #{guild_id => GuildId, guild_pid => GuildPid}}.
 
diff --git a/fluxer_gateway/src/session/session.erl b/fluxer_gateway/src/session/session.erl
index 0b813608b..a70acc367 100644
--- a/fluxer_gateway/src/session/session.erl
+++ b/fluxer_gateway/src/session/session.erl
@@ -80,7 +80,7 @@ start_link(SessionData) ->
 -spec init(map()) -> {ok, session_state()}.
 init(SessionData) ->
     process_flag(trap_exit, true),
-    erlang:process_flag(fullsweep_after, 0),
+    erlang:process_flag(fullsweep_after, 10),
     State0 = session_init:build_state(SessionData),
     ScheduleStartedAt = gateway_timings:start(),
     session_init:schedule_timers(State0),
@@ -318,7 +318,7 @@ terminate(Reason, State) ->
 
 -spec code_change(term(), session_state(), term()) -> {ok, session_state()}.
 code_change(_OldVsn, State, _Extra) ->
-    erlang:process_flag(fullsweep_after, 0),
+    erlang:process_flag(fullsweep_after, 10),
     erlang:garbage_collect(),
     {ok, State}.
 
diff --git a/fluxer_gateway/test/gateway_hot_process_gc_tests.erl b/fluxer_gateway/test/gateway_hot_process_gc_tests.erl
new file mode 100644
index 000000000..a659341c5
--- /dev/null
+++ b/fluxer_gateway/test/gateway_hot_process_gc_tests.erl
@@ -0,0 +1,96 @@
+%% SPDX-License-Identifier: AGPL-3.0-or-later
+
+-module(gateway_hot_process_gc_tests).
+-typing([eqwalizer]).
+
+-include_lib("eunit/include/eunit.hrl").
+
+-define(GENERATIONAL_FULLSWEEP_AFTER, 10).
+
+session_init_keeps_generational_gc_test() ->
+    ?assertEqual(
+        ?GENERATIONAL_FULLSWEEP_AFTER,
+        fullsweep_after_in(fun() -> session:init(session_data()) end)
+    ).
+
+session_code_change_keeps_generational_gc_test() ->
+    ?assertEqual(
+        ?GENERATIONAL_FULLSWEEP_AFTER,
+        fullsweep_after_in(fun() -> session:code_change(0, #{}, []) end)
+    ).
+
+guild_init_keeps_generational_gc_test() ->
+    ?assertEqual(
+        ?GENERATIONAL_FULLSWEEP_AFTER,
+        fullsweep_after_in(fun() -> guild:init(guild_data()) end)
+    ).
+
+guild_code_change_keeps_generational_gc_test() ->
+    ?assertEqual(
+        ?GENERATIONAL_FULLSWEEP_AFTER,
+        fullsweep_after_in(fun() -> guild:code_change(0, #{}, []) end)
+    ).
+
+guild_broadcaster_init_keeps_generational_gc_test() ->
+    Parent = self(),
+    ?assertEqual(
+        ?GENERATIONAL_FULLSWEEP_AFTER,
+        fullsweep_after_in(fun() -> guild_broadcaster:init([900301, Parent]) end)
+    ).
+
+session_data() ->
+    #{
+        id => <<"gc-session">>,
+        user_id => 900401,
+        user_data => #{<<"username">> => <<"gc">>},
+        version => 9,
+        token_hash => <<"token_hash">>,
+        auth_session_id_hash => <<"auth_hash">>,
+        properties => #{},
+        status => online,
+        ready => undefined,
+        socket_pid => self(),
+        guilds => []
+    }.
+
+guild_data() ->
+    #{
+        id => 900300,
+        member_count => 0,
+        sessions => #{},
+        data => #{
+            <<"guild">> => #{
+                <<"id">> => 900300,
+                <<"owner_id">> => 900401,
+                <<"features">> => [],
+                <<"member_count">> => 0
+            },
+            <<"roles">> => [],
+            <<"channels">> => [],
+            <<"members">> => []
+        }
+    }.
+
+fullsweep_after_in(Fun) ->
+    Ref = make_ref(),
+    Parent = self(),
+    {Pid, MonRef} = spawn_monitor(fun() ->
+        erlang:process_flag(fullsweep_after, 0),
+        _ = Fun(),
+        Parent ! {Ref, fullsweep_after(self())}
+    end),
+    Result =
+        receive
+            {Ref, Value} -> Value;
+            {'DOWN', MonRef, process, Pid, Reason} -> {crashed, Reason}
+        after 10000 -> timeout
+        end,
+    erlang:demonitor(MonRef, [flush]),
+    exit(Pid, kill),
+    Result.
+
+fullsweep_after(Pid) ->
+    case erlang:process_info(Pid, garbage_collection) of
+        {garbage_collection, Info} -> proplists:get_value(fullsweep_after, Info);
+        _ -> undefined
+    end.