From 1bf66ae960b65a91983070faf25bdfe34f7d8fc1 Mon Sep 17 00:00:00 2001
From: Gregory Burd <greg@basho.com>
Date: Wed, 21 Aug 2013 12:15:34 -0400
Subject: [PATCH] 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 | 42 +++++++++++++++++++++++++-----------------
 1 file changed, 25 insertions(+), 17 deletions(-)

diff --git a/src/async_nif.hrl b/src/async_nif.hrl
index c225f16..e410def 100644
--- a/src/async_nif.hrl
+++ b/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)).