diff --git a/Implab/Parallels/ArrayTraits.cs b/Implab/Parallels/ArrayTraits.cs
--- a/Implab/Parallels/ArrayTraits.cs
+++ b/Implab/Parallels/ArrayTraits.cs
@@ -163,16 +163,18 @@ namespace Implab.Parallels {
                         break; // stop processing in case of error or cancellation
                     var idx = i;
 
-                    lock(locker) {
-                        while(slots == 0)
-                            Monitor.Wait(locker);
-                        slots--;
+                    if (Interlocked.Decrement(ref slots) < 0) {
+                        lock(locker) {
+                            while(slots < 0)
+                                Monitor.Wait(locker);
+                        }
                     }
+
                     try {
                         transform(source[i])
                             .Anyway(() => {
-                                lock(locker) {
-                                    slots ++;
+                                Interlocked.Increment(ref slots);
+                                lock (locker) {
                                     Monitor.Pulse(locker);
                                 }
                             })
@@ -183,7 +185,7 @@ namespace Implab.Parallels {
                                     if (left == 0)
                                         promise.Resolve(res);
                                 },
-                                e => promise.Reject(e)
+                                promise.Reject
                             );
 
                     } catch (Exception e) {
diff --git a/Implab/Parallels/DispatchPool.cs b/Implab/Parallels/DispatchPool.cs
--- a/Implab/Parallels/DispatchPool.cs
+++ b/Implab/Parallels/DispatchPool.cs
@@ -69,7 +69,7 @@ namespace Implab.Parallels {
 
         protected abstract bool TryDequeue(out TUnit unit);
 
-        private bool Dequeue(out TUnit unit, int timeout) {
+        bool Dequeue(out TUnit unit, int timeout) {
             int ts = Environment.TickCount;
             if (TryDequeue(out unit))
                 return true;