Revert "Remove consumer bias & allow queues under max load to drain quickly"

This reverts commit 155eb6b.

Author: Daniil Fedotov

src/rabbit_amqqueue_process.erl

@@ -22,6 +22,7 @@
 
 -define(SYNC_INTERVAL,                 200). %% milliseconds
 -define(RAM_DURATION_UPDATE_INTERVAL, 5000).
+-define(CONSUMER_BIAS_RATIO,           1.1). %% i.e. consume 10% faster
 
 -export([info_keys/0]).
 
@@ -968,26 +969,26 @@ emit_consumer_deleted(ChPid, ConsumerTag, QName) ->
 
 %%----------------------------------------------------------------------------
 
-prioritise_call(Msg, _From, _Len, _State) ->
+prioritise_call(Msg, _From, _Len, State) ->
     case Msg of
         info                                       -> 9;
         {info, _Items}                             -> 9;
         consumers                                  -> 9;
         stat                                       -> 7;
-        {basic_consume, _, _, _, _, _, _, _, _, _} -> 1;
-        {basic_cancel, _, _, _}                    -> 1;
+        {basic_consume, _, _, _, _, _, _, _, _, _} -> consumer_bias(State, 0, 2);
+        {basic_cancel, _, _, _}                    -> consumer_bias(State, 0, 2);
         _                                          -> 0
     end.
 
-prioritise_cast(Msg, _Len, _State) ->
+prioritise_cast(Msg, _Len, State) ->
     case Msg of
         delete_immediately                   -> 8;
         {set_ram_duration_target, _Duration} -> 8;
         {set_maximum_since_use, _Age}        -> 8;
         {run_backing_queue, _Mod, _Fun}      -> 6;
         {ack, _AckTags, _ChPid}              -> 4; %% [1]
         {resume, _ChPid}                     -> 3;
-        {notify_sent, _ChPid, _Credit}       -> 2;
+        {notify_sent, _ChPid, _Credit}       -> consumer_bias(State, 0, 2);
         _                                    -> 0
     end.
 
@@ -1003,6 +1004,13 @@ prioritise_cast(Msg, _Len, _State) ->
 %% credit to self is hard to reason about. Consumers can continue while
 %% reduce_memory_use is in progress.
 
+consumer_bias(#q{backing_queue = BQ, backing_queue_state = BQS}, Low, High) ->
+    case BQ:msg_rates(BQS) of
+        {0.0,          _} -> Low;
+        {Ingress, Egress} when Egress / Ingress < ?CONSUMER_BIAS_RATIO -> High;
+        {_,            _} -> Low
+    end.
+
 prioritise_info(Msg, _Len, #q{q = #amqqueue{exclusive_owner = DownPid}}) ->
     case Msg of
         {'DOWN', _, process, DownPid, _}     -> 8;