3 files changed, 41 insertions, 3 deletions

diff --git a/README.md b/README.md
index 0b86613..293e678 100644
--- a/README.md
+++ b/README.md
@@ -1256,7 +1256,7 @@ cargo.push({name: 'baz'}, function (err) {
 ---------------------------------------
 
 <a name="auto" />
-### auto(tasks, [callback])
+### auto(tasks, [callback], [concurrency])
 
 Determines the best order for running the functions in `tasks`, based on their requirements. Each function can optionally depend on other functions being completed first, and each function is run as soon as its requirements are satisfied.
 
@@ -1307,6 +1307,8 @@ __Arguments__
   pass an error to their callback. Results are always returned; however, if
   an error occurs, no further `tasks` will be performed, and the results
   object will only contain partial results.
+* `concurrency` - An `integer` for determining the maximum number of tasks that
+  can be run in parallel. By default, as many as possible.
 
 
 __Example__
diff --git a/lib/async.js b/lib/async.js
index 6b14514..644dccb 100644
--- a/lib/async.js
+++ b/lib/async.js
@@ -509,15 +509,19 @@
         }
     };
 
-    async.auto = function (tasks, callback) {
+    async.auto = function (tasks, callback, concurrency) {
         callback = _once(callback || noop);
         var keys = _keys(tasks);
         var remainingTasks = keys.length;
         if (!remainingTasks) {
             return callback(null);
         }
+        if (!concurrency) {
+            concurrency = remainingTasks;
+        }
 
         var results = {};
+        var runningTasks = 0;
 
         var listeners = [];
         function addListener(fn) {
@@ -543,6 +547,7 @@
         _arrayEach(keys, function (k) {
             var task = _isArray(tasks[k]) ? tasks[k]: [tasks[k]];
             var taskCallback = _restParam(function(err, args) {
+                runningTasks--;
                 if (args.length <= 1) {
                     args = args[0];
                 }
@@ -572,11 +577,12 @@
                 }
             }
             function ready() {
-                return _reduce(requires, function (a, x) {
+                return runningTasks < concurrency && _reduce(requires, function (a, x) {
                     return (a && results.hasOwnProperty(x));
                 }, true) && !results.hasOwnProperty(k);
             }
             if (ready()) {
+                runningTasks++;
                 task[task.length - 1](taskCallback, results);
             }
             else {
@@ -584,6 +590,7 @@
             }
             function listener() {
                 if (ready()) {
+                    runningTasks++;
                     removeListener(listener);
                     task[task.length - 1](taskCallback, results);
                 }
diff --git a/test/test-async.js b/test/test-async.js
index e98eaf4..34fabaf 100755
--- a/test/test-async.js
+++ b/test/test-async.js
@@ -331,6 +331,35 @@ exports['auto'] = function(test){
     });
 };
 
+exports['auto concurrency'] = function (test) {
+    var concurrency = 2;
+    var runningTasks = [];
+    var makeCallback = function(taskName) {
+        return function(callback) {
+            runningTasks.push(taskName);
+            setTimeout(function(){
+                // Each task returns the array of running tasks as results.
+                var result = runningTasks.slice(0);
+                runningTasks.splice(runningTasks.indexOf(taskName), 1);
+                callback(null, result);
+            });
+        };
+    };
+    async.auto({
+        task1: ['task2', makeCallback('task1')],
+        task2: makeCallback('task2'),
+        task3: ['task2', makeCallback('task3')],
+        task4: ['task1', 'task2', makeCallback('task4')],
+        task5: ['task2', makeCallback('task5')],
+        task6: ['task2', makeCallback('task6')]
+    }, function(err, results){
+        Object.keys(results).forEach(function(taskName) {
+            test.ok(results[taskName].length <= concurrency);
+        });
+        test.done();
+    }, concurrency);
+};
+
 exports['auto petrify'] = function (test) {
     var callOrder = [];
     async.auto({