[PLinq] Remove unnecessary iteration of the internal dictionary in OrderedEnumerator and use deep sleeping

mcs/class/System.Core/System.Linq.Parallel/OrderingEnumerator.cs

@@ -44,6 +44,7 @@ namespace System.Linq.Parallel
 			readonly int count;
 			CountdownEvent stagingCount;
 			CountdownEvent participantCount;
+			CancellationTokenSource src = new CancellationTokenSource ();
 
 			public SlotBucket (int count)
 			{
@@ -59,17 +60,25 @@ namespace System.Linq.Parallel
 				long index = value.Key;
 				
 				if (index >= currentIndex && index < currentIndex + count) {
-					stagingArea [index % count] = value;
+					stagingArea[index % count] = value;
 					stagingCount.Signal ();
 				} else {
-					temporaryArea.TryAdd (value.Key, value.Value);
+					temporaryArea.TryAdd (index, value.Value);
+					if (index >= currentIndex && index < currentIndex + count) {
+						T dummy;
+						if (temporaryArea.TryRemove (index, out dummy)) {
+							stagingArea[index % count] = value;
+							stagingCount.Signal ();
+						}
+					}
 				}
 			}
 			
 			// Called by each worker's endAction
 			public void EndParticipation ()
 			{
-				participantCount.Signal ();
+				if (participantCount.Signal ())
+					src.Cancel ();
 			}
 
 			// Called at the end with ContinueAll
@@ -108,21 +117,24 @@ namespace System.Linq.Parallel
 				stagingCount.Reset ();
 				
 				Interlocked.Add (ref currentIndex, count);
-				
-				SpinWait sw = new SpinWait ();
+
+				Skim ();
 
 				while (!stagingCount.IsSet) {
-					if (participantCount.IsSet && temporaryArea.IsEmpty) {
+					if (!participantCount.IsSet)
+						try {
+							stagingCount.Wait (src.Token);
+						} catch {
+							Skim ();
+						}
+
+					if (participantCount.IsSet) {
 						// Totally finished
-						if (stagingCount.CurrentCount == count)
-							return null;
-						else 
+						if (stagingArea[0].HasValue)
 							break;
+						else
+							return null;
 					}
-					Skim ();
-					
-					if (!stagingCount.IsSet)
-						sw.SpinOnce ();
 				}
 				
 				return ((IEnumerable<KeyValuePair<long, T>?>)stagingArea).GetEnumerator ();