Set the default vm_memory_high_watermark to 0.6 (#12161)

The default of 0.4 was very conservative even when it was
set years ago. Since then:
- we moved to CQv2, which have much more predictable memory usage than (non-lazy) CQv1 used to
- we removed CQ mirroring which caused large sudden memory spikes in some situations
- we removed the option to store message payload in memory in quorum queues

For the past two years or so, we've been running all our internal tests and benchmarks
using the value of 0.8 with no OOMkills at all (note: we do this on
Kubernetes where the Cluster Operators overrides the available memory
levaing some additional headroom, but effectively we are still using  more than
0.6 of memory).

deps/rabbit/BUILD.bazel

@@ -34,7 +34,7 @@ _APP_ENV = """[
 	    {ssl_listeners, []},
 	    {num_ssl_acceptors, 10},
 	    {ssl_options, []},
-	    {vm_memory_high_watermark, 0.4},
+	    {vm_memory_high_watermark, 0.6},
 	    {vm_memory_calculation_strategy, rss},
 	    {disk_free_limit, 50000000}, %% 50MB
 	    {backing_queue_module, rabbit_variable_queue},

deps/rabbit/Makefile

@@ -14,7 +14,7 @@ define PROJECT_ENV
 	    {ssl_listeners, []},
 	    {num_ssl_acceptors, 10},
 	    {ssl_options, []},
-	    {vm_memory_high_watermark, 0.4},
+	    {vm_memory_high_watermark, 0.6},
 	    {vm_memory_calculation_strategy, rss},
 	    {disk_free_limit, 50000000}, %% 50MB
 	    {backing_queue_module, rabbit_variable_queue},

deps/rabbit/docs/rabbitmq.conf.example

@@ -382,7 +382,7 @@
 
 ## Memory-based Flow Control threshold.
 ##
-# vm_memory_high_watermark.relative = 0.4
+# vm_memory_high_watermark.relative = 0.6
 
 ## Alternatively, we can set a limit (in bytes) of RAM used by the node.
 ##

deps/rabbit/priv/schema/rabbit.schema

@@ -1103,7 +1103,7 @@ end}.
 
 %% Memory-based Flow Control threshold.
 %%
-%% {vm_memory_high_watermark, 0.4},
+%% {vm_memory_high_watermark, 0.6},
 
 %% Alternatively, we can set a limit (in bytes) of RAM used by the node.
 %%

deps/rabbit/test/unit_vm_memory_monitor_SUITE.erl

@@ -118,4 +118,4 @@ set_and_verify_vm_memory_high_watermark_absolute(MemLimit0) ->
         _           ->
             ct:fail("Expected memory high watermark to be ~tp but it was ~tp", [Interpreted, MemLimit])
     end,
-    vm_memory_monitor:set_vm_memory_high_watermark(0.4).
+    vm_memory_monitor:set_vm_memory_high_watermark(0.6).

deps/rabbitmq_mqtt/test/shared_SUITE.erl

@@ -1417,7 +1417,7 @@ block(Config) ->
     puback_timeout = publish_qos1_timeout(C, Topic, <<"Still blocked">>, 1000),
 
     %% Unblock
-    rpc(Config, vm_memory_monitor, set_vm_memory_high_watermark, [0.4]),
+    rpc(Config, vm_memory_monitor, set_vm_memory_high_watermark, [0.6]),
     ok = expect_publishes(C, Topic, [<<"Not blocked yet">>,
                                      <<"Now blocked">>,
                                      <<"Still blocked">>]),
@@ -1458,7 +1458,7 @@ block_only_publisher(Config) ->
     ?assertEqual(puback_timeout, publish_qos1_timeout(Con, Topic, <<"from Con 2">>, 500)),
     ?assertEqual(pong, emqtt:ping(Sub)),
 
-    rpc(Config, vm_memory_monitor, set_vm_memory_high_watermark, [0.4]),
+    rpc(Config, vm_memory_monitor, set_vm_memory_high_watermark, [0.6]),
     %% Let it unblock
     timer:sleep(100),