Optimise io requests for long message queues

Ensure that the monitor reference is present in all receive clauses, so
that the compiler optimisation kicks in and the run time won't depend on
the length of the message queue of the calling process.

Remove the 'EXIT' clause, as its presence breaks the optimisation, and
that case is handled by the 'DOWN' clause anyway.

The test io_SUITE:io_with_huge_message_queue is an adaptation of
gen_server_SUITE:call_with_huge_message_queue.

1 files changed, 44 insertions, 2 deletions

diff --git a/lib/stdlib/test/io_SUITE.erl b/lib/stdlib/test/io_SUITE.erl
index 3a76275f31..2203dd8f51 100644
--- a/lib/stdlib/test/io_SUITE.erl
+++ b/lib/stdlib/test/io_SUITE.erl
@@ -30,7 +30,8 @@
 	 io_fread_newlines/1, otp_8989/1, io_lib_fread_literal/1,
 	 printable_range/1,
 	 io_lib_print_binary_depth_one/1, otp_10302/1, otp_10755/1,
-         otp_10836/1, io_lib_width_too_small/1]).
+         otp_10836/1, io_lib_width_too_small/1,
+         io_with_huge_message_queue/1]).
 
 -export([pretty/2]).
 
@@ -70,7 +71,7 @@ all() ->
      io_fread_newlines, otp_8989, io_lib_fread_literal,
      printable_range,
      io_lib_print_binary_depth_one, otp_10302, otp_10755, otp_10836,
-     io_lib_width_too_small].
+     io_lib_width_too_small, io_with_huge_message_queue].
 
 groups() -> 
     [].
@@ -2219,3 +2220,44 @@ io_lib_width_too_small(Config) ->
     "**" = lists:flatten(io_lib:format("~2.3w", [3.14])),
     "**" = lists:flatten(io_lib:format("~2.5w", [3.14])),
     ok.
+
+%% Test that the time for a huge message queue is not
+%% significantly slower than with an empty message queue.
+io_with_huge_message_queue(Config) when is_list(Config) ->
+    case test_server:is_native(gen) of
+	true ->
+	    {skip,
+	     "gen is native - huge message queue optimization "
+	     "is not implemented"};
+	false ->
+	    do_io_with_huge_message_queue(Config)
+    end.
+
+do_io_with_huge_message_queue(Config) ->
+    PrivDir = ?privdir(Config),
+    File = filename:join(PrivDir, "slask"),
+    {ok, F1} = file:open(File, [write]),
+
+    {Time,ok} = timer:tc(fun() -> writes(1000, F1) end),
+
+    [self() ! {msg,N} || N <- lists:seq(1, 500000)],
+    erlang:garbage_collect(),
+    {NewTime,ok} = timer:tc(fun() -> writes(1000, F1) end),
+    file:close(F1),
+    io:format("Time for empty message queue: ~p", [Time]),
+    io:format("Time for huge message queue: ~p", [NewTime]),
+
+    IsCover = test_server:is_cover(),
+    case (NewTime+1) / (Time+1) of
+	Q when Q < 10; IsCover ->
+	    ok;
+	Q ->
+	    io:format("Q = ~p", [Q]),
+	    ?t:fail()
+    end,
+    ok.
+
+writes(0, _) -> ok;
+writes(N, F1) ->
+    file:write(F1, "hello\n"),
+    writes(N - 1, F1).