Every enqueued request now includes a hint as to how much work is pending in

the lower C-code.  We use that to scale the reduction count penalty so that we
can (hopefully) signal to the Erlang scheduler enough information for it to
properly throttle work.  'eagain' should only happen when queues are full, we
have no choice but to keep this calling proc busy in a recursive loop trying
the request over and over if we're going to preserve request ordering.

src/async_nif.hrl

@@ -22,20 +22,28 @@
 %% -------------------------------------------------------------------
 
 -define(ASYNC_NIF_CALL(Fun, Args),
-	R = erlang:make_ref(),
-	case erlang:apply(Fun, [R|Args]) of
-	    {ok, {enqueued, PercentFull}} ->
-		erlang:bump_reductions(erlang:trunc(2000 * PercentFull)),
-		receive
-		    {R, {error, shutdown}=Error} ->
-			%% Work unit was queued, but not executed.
-			Error;
-		    {R, {error, _Reason}=Error} ->
-			%% Work unit returned an error.
-			Error;
-		    {R, Reply} ->
-			Reply
-		end;
-	    Other ->
-		Other
-	end).
+	F = fun(F, A) ->
+		    R = erlang:make_ref(),
+		    case erlang:apply(F, [R|A]) of
+			{ok, {enqueued, PctBusy}} ->
+			    case PctBusy of
+				0.0 -> ok;
+				_   -> erlang:bump_reductions(erlang:trunc(2000 * PctBusy))
+			    end,
+			    receive
+				{R, {error, shutdown}=Error} ->
+				    %% Work unit was queued, but not executed.
+				    Error;
+				{R, {error, _Reason}=Error} ->
+				    %% Work unit returned an error.
+				    Error;
+				{R, Reply} ->
+				    Reply
+			    end;
+			{error, eagain} ->
+			    F(F, A);
+			Other ->
+			    Other
+		    end
+	    end,
+	F(Fun, Args)).