Merge pull request #10478 from rabbitmq/mergify/bp/v3.12.x/pr-10476

Rework consumer sample deletion when a channel is closed (backport #10476)

Author: michaelklishin

deps/rabbit/src/rabbit_channel.erl

@@ -832,7 +832,8 @@ terminate(_Reason,
           end, CM),
     rabbit_core_metrics:channel_closed(self()),
     rabbit_event:notify(channel_closed, [{pid, self()},
-                                         {user_who_performed_action, Username}]),
+                                         {user_who_performed_action, Username},
+                                         {consumer_count, maps:size(CM)}]),
     case rabbit_confirms:size(State#ch.unconfirmed) of
         0 -> ok;
         NumConfirms ->

deps/rabbitmq_management_agent/src/rabbit_mgmt_metrics_gc.erl

@@ -19,6 +19,8 @@
 -export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2,
          code_change/3]).
 
+-define(LARGE_CONSUMER_COUNT, 1000).
+
 name(EventType) ->
     list_to_atom((atom_to_list(EventType) ++ "_metrics_gc")).
 
@@ -42,7 +44,8 @@ handle_cast({event, #event{type  = connection_closed, props = Props}},
 handle_cast({event, #event{type  = channel_closed, props = Props}},
             State = #state{basic_i = BIntervals}) ->
     Pid = pget(pid, Props),
-    remove_channel(Pid, BIntervals),
+    ConsumerCount = pget(consumer_count, Props),
+    remove_channel(Pid, ConsumerCount, BIntervals),
     {noreply, State};
 handle_cast({event, #event{type  = consumer_deleted, props = Props}}, State) ->
     remove_consumer(Props),
@@ -82,13 +85,13 @@ remove_connection(Id, BIntervals) ->
     delete_samples(connection_stats_coarse_conn_stats, Id, BIntervals),
     ok.
 
-remove_channel(Id, BIntervals) ->
+remove_channel(Id, ConsumerCount, BIntervals) ->
     ets:delete(channel_created_stats, Id),
     ets:delete(channel_stats, Id),
     delete_samples(channel_process_stats, Id, BIntervals),
     delete_samples(channel_stats_fine_stats, Id, BIntervals),
     delete_samples(channel_stats_deliver_stats, Id, BIntervals),
-    index_delete(consumer_stats, channel, Id),
+    index_delete(consumer_stats, {channel, ConsumerCount}, Id),
     index_delete(channel_exchange_stats_fine_stats, channel, Id),
     index_delete(channel_queue_stats_deliver_stats, channel, Id),
     ok.
@@ -136,18 +139,32 @@ delete_samples(Table, Id, Intervals) ->
     [ets:delete(Table, {Id, I}) || I <- Intervals],
     ok.
 
-index_delete(consumer_stats = Table, channel = Type, Id) ->
-    IndexTable = rabbit_mgmt_metrics_collector:index_table(Table, Type),
-    MatchPattern = {'_', Id, '_'},
-    %% Delete consumer_stats_queue_index
-    ets:match_delete(consumer_stats_queue_index,
-                     {'_', MatchPattern}),
-    %% Delete consumer_stats
-    ets:match_delete(consumer_stats,
-                     {MatchPattern,'_'}),
-    %% Delete consumer_stats_channel_index
-    ets:delete(IndexTable, Id),
-    ok;
+index_delete(consumer_stats = Table, {channel = Type, ConsumerCount}, Id) ->
+    %% This uses two different deletion strategies depending on how many
+    %% consumers a channel had. Most of the time there are many channels
+    %% with a few (or even just one) consumers. For this common case, `ets:delete/2` is optimal
+    %% since it avoids table scans.
+    %%
+    %% In the rather extreme scenario where only a handful of channels have a very large
+    %% (e.g. tens of thousands) of consumers, `ets:match_delete/2` becomes a more efficient option.
+    %%
+    %% See rabbitmq-server/rabbitmq#10451, rabbitmq-server/rabbitmq#9356.
+    case ConsumerCount > ?LARGE_CONSUMER_COUNT of
+        true ->
+            IndexTable = rabbit_mgmt_metrics_collector:index_table(Table, Type),
+            MatchPattern = {'_', Id, '_'},
+            %% Delete consumer_stats_queue_index
+            ets:match_delete(consumer_stats_queue_index,
+                             {'_', MatchPattern}),
+            %% Delete consumer_stats
+            ets:match_delete(consumer_stats,
+                             {MatchPattern,'_'}),
+            %% Delete consumer_stats_channel_index
+            ets:delete(IndexTable, Id),
+            ok;
+        false ->
+            index_delete(Table, Type, Id)
+    end;
 index_delete(Table, Type, Id) ->
     IndexTable = rabbit_mgmt_metrics_collector:index_table(Table, Type),
     Keys = ets:lookup(IndexTable, Id),