Add flag-based setting for the distribution buffer busy limit

Id: OTP-8912

This patch creates a new family of flags with the "+z" prefix.  It
further creates a new configuration option called "dbbl" (which is the
first letter of the name dist_buf_busy_limit).  Example usage of this
flag would be "+zdbbl 1048576".

This patch creates an adjustable buffer limit for the amount of data
that may be buffered by the erlang distribution code (in dist.c
specifically). Before this patch, this hard-coded constant was used:

    #define ERTS_DE_BUSY_LIMIT (128*1024)

When large binaries are transmitted between nodes (or simply a lot of
medium-sized binaries), it is very easy to hit the old 128KB limit.
Processes that use the erlang:system_monitor() BIF to monitor system
events can be spammed by {monitor, busy_dist_port, ...} message tuples
at rates of tens to even hundreds of messages/second.

A larger buffer limit will allow processes to buffer more outgoing
messages over the distribution. When the buffer limit has been
reached, sending processes will be suspended until the buffer size has
shrunk. The buffer limit is per distribution channel. A higher limit
will give lower latency and higher throughput at the expense of
higher memory usage.

A variation of this patch has been in commercial production use in at
least two companies that the author is aware of.  Larger buffer values
can reduce the number of {monitor, busy_dist_port, ...} system
messages drastically, lower overall messaging latencies, and prevent
false timeouts and 'nodedown' messages in extremely busy Mnesia systems.

Test suite: there are two tests:

   a. In erlexec_SUITE.erl to test basic set & get of the value
   b. In distribution_SUITE.erl, to verify that setting +zdbbl very
      low will actually change behavior.

1 files changed, 91 insertions, 2 deletions

diff --git a/erts/emulator/test/distribution_SUITE.erl b/erts/emulator/test/distribution_SUITE.erl
index 7c19274696..79252d0593 100644
--- a/erts/emulator/test/distribution_SUITE.erl
+++ b/erts/emulator/test/distribution_SUITE.erl
@@ -27,6 +27,7 @@
 -export([all/1,
 	 ping/1, bulk_send/1, bulk_send_small/1,
 	 bulk_send_big/1,
+	 bulk_send_bigbig/1,
 	 local_send/1, local_send_small/1, local_send_big/1,
 	 local_send_legal/1, link_to_busy/1, exit_to_busy/1,
 	 lost_exit/1, link_to_dead/1, link_to_dead_new_node/1,
@@ -50,7 +51,8 @@
 -export([sender/3, receiver2/2, dummy_waiter/0, dead_process/0,
 	 roundtrip/1, bounce/1, do_dist_auto_connect/1, inet_rpc_server/1,
 	 dist_parallel_sender/3, dist_parallel_receiver/0,
-	 dist_evil_parallel_receiver/0]).
+	 dist_evil_parallel_receiver/0,
+         sendersender/4, sendersender2/4]).
 
 all(suite) -> [
 	       ping, bulk_send, local_send, link_to_busy, exit_to_busy,
@@ -121,7 +123,7 @@ bulk_send(doc) ->
   "the time. This tests that a process that is suspended on a ",
   "busy port will eventually be resumed."];
 bulk_send(suite) ->
-    [bulk_send_small, bulk_send_big].
+    [bulk_send_small, bulk_send_big, bulk_send_bigbig].
 
 bulk_send_small(Config) when is_list(Config) ->
     ?line bulk_send(64, 32).
@@ -129,6 +131,9 @@ bulk_send_small(Config) when is_list(Config) ->
 bulk_send_big(Config) when is_list(Config) ->
     ?line bulk_send(32, 64).
 
+bulk_send_bigbig(Config) when is_list(Config) ->
+    ?line bulk_sendsend(32*5, 4).
+
 bulk_send(Terms, BinSize) ->
     ?line Dog = test_server:timetrap(test_server:seconds(30)),
 
@@ -145,6 +150,53 @@ bulk_send(Terms, BinSize) ->
     ?line test_server:timetrap_cancel(Dog),
     {comment, integer_to_list(trunc(Size/1024/Elapsed+0.5)) ++ " K/s"}.
 
+bulk_sendsend(Terms, BinSize) ->
+    {Rate1, MonitorCount1} = bulk_sendsend2(Terms, BinSize,   5),
+    {Rate2, MonitorCount2} = bulk_sendsend2(Terms, BinSize, 995),
+    Ratio = if MonitorCount2 == 0 -> MonitorCount1 / 1.0;
+               true               -> MonitorCount1 / MonitorCount2
+            end,
+    %% A somewhat arbitrary ratio, but hopefully one that will accomodate
+    %% a wide range of CPU speeds.
+    true = (Ratio > 8.0),
+    {comment,
+     integer_to_list(Rate1) ++ " K/s, " ++
+     integer_to_list(Rate2) ++ " K/s, " ++
+     integer_to_list(MonitorCount1) ++ " monitor msgs, " ++
+     integer_to_list(MonitorCount2) ++ " monitor msgs, " ++
+     float_to_list(Ratio) ++ " monitor ratio"}.    
+
+bulk_sendsend2(Terms, BinSize, BusyBufSize) ->
+    ?line Dog = test_server:timetrap(test_server:seconds(30)),
+
+    ?line io:format("Sending ~w binaries, each of size ~w K",
+		    [Terms, BinSize]),
+    ?line {ok, NodeRecv} = start_node(bulk_receiver),
+    ?line Recv = spawn(NodeRecv, erlang, apply, [fun receiver/2, [0, 0]]),
+    ?line Bin = list_to_binary(lists:duplicate(BinSize*1024, 253)),
+    ?line Size = Terms*size(Bin),
+
+    %% SLF LEFT OFF HERE.
+    %% When the caller uses small hunks, like 4k via
+    %% bulk_sendsend(32*5, 4), then (on my laptop at least), we get
+    %% zero monitor messages.  But if we use "+zdbbl 5", then we
+    %% get a lot of monitor messages.  So, if we can count up the
+    %% total number of monitor messages that we get when running both
+    %% default busy size and "+zdbbl 5", and if the 5 case gets
+    %% "many many more" monitor messages, then we know we're working.
+
+    ?line {ok, NodeSend} = start_node(bulk_sender, "+zdbbl " ++ integer_to_list(BusyBufSize)),
+    ?line _Send = spawn(NodeSend, erlang, apply, [fun sendersender/4, [self(), Recv, Bin, Terms]]),
+    ?line {Elapsed, {TermsN, SizeN}, MonitorCount} =
+        receive {sendersender, BigRes} ->
+                BigRes
+        end,
+    ?line stop_node(NodeRecv),
+    ?line stop_node(NodeSend),
+
+    ?line test_server:timetrap_cancel(Dog),
+    {trunc(SizeN/1024/Elapsed+0.5), MonitorCount}.
+
 sender(To, _Bin, 0) ->
     To ! {done, self()},
     receive
@@ -155,6 +207,43 @@ sender(To, Bin, Left) ->
     To ! {term, Bin},
     sender(To, Bin, Left-1).
 
+%% Sender process to be run on a slave node
+
+sendersender(Parent, To, Bin, Left) ->
+    erlang:system_monitor(self(), [busy_dist_port]),
+    [spawn(fun() -> sendersender2(To, Bin, Left, false) end) ||
+        _ <- lists:seq(1,1)],
+    {USec, {Res, MonitorCount}} =
+        timer:tc(?MODULE, sendersender2, [To, Bin, Left, true]),
+    Parent ! {sendersender, {USec/1000000, Res, MonitorCount}}.
+
+sendersender2(To, Bin, Left, SendDone) ->
+    sendersender3(To, Bin, Left, SendDone, 0).
+
+sendersender3(To, _Bin, 0, SendDone, MonitorCount) ->
+    if SendDone ->
+            To ! {done, self()};
+       true ->
+            ok
+    end,
+    receive
+        {monitor, _Pid, _Type, _Info} = M ->
+            sendersender3(To, _Bin, 0, SendDone, MonitorCount + 1)
+    after 0 ->
+            if SendDone ->
+                    receive
+                        Any when is_tuple(Any), size(Any) == 2 ->
+                            {Any, MonitorCount}
+                    end;
+               true ->
+                    exit(normal)
+            end
+    end;
+sendersender3(To, Bin, Left, SendDone, MonitorCount) ->
+    To ! {term, Bin},
+    %%timer:sleep(50),
+    sendersender3(To, Bin, Left-1, SendDone, MonitorCount).
+
 %% Receiver process to be run on a slave node.
 
 receiver(Terms, Size) ->