ThreadPool.SparseArray improvement for DequeueSteal

src/mscorlib/src/System/Threading/ThreadPool.cs

@@ -71,42 +71,80 @@ internal sealed class ThreadPoolWorkQueue
         // Simple sparsely populated array to allow lock-free reading.
         internal class SparseArray<T> where T : class
         {
-            private volatile T[] m_array;
+            private Snapshot m_current;
+
+            public class Snapshot
+            {
+                private T[] m_array;
+                private int m_length;
+                private int m_mask;
+
+                internal Snapshot(int initialSize, int initalLength = 0)
+                {
+                    if ((initialSize >> 1) << 1 != initialSize)
+                    {
+                        throw new ArgumentOutOfRangeException(nameof(initialSize), "initialSize must be a power of 2");
+                    }
+
+                    m_array = new T[initialSize];
+                    m_length = initalLength;
+                    m_mask = initialSize - 1;
+                }
+
+                internal T[] Data => m_array;
+                internal int Mask => m_mask;
+                internal int Length => m_length;
+
+                internal void IncrementLength()
+                {
+                    m_length++;
+                }
+            }
 
             internal SparseArray(int initialSize)
             {
-                m_array = new T[initialSize];
+                m_current = new Snapshot(initialSize);  
             }
 
-            internal T[] Current
+            internal Snapshot Current
             {
-                get { return m_array; }
+                get { return m_current; }
             }
 
             internal int Add(T e)
             {
                 while (true)
                 {
-                    T[] array = m_array;
-                    lock (array)
+                    var current = m_current;
+                    lock (current)
                     {
+                        if (current != m_current)
+                        {
+                            // If there was a race condition, we start over again.
+                            continue;
+                        }
+
+                        var array = current.Data;
+
                         for (int i = 0; i < array.Length; i++)
                         {
                             if (array[i] == null)
                             {
+                                if (i > current.Length)
+                                {
+                                    current.IncrementLength();
+                                }
                                 Volatile.Write(ref array[i], e);
                                 return i;
                             }
                             else if (i == array.Length - 1)
                             {
-                                // Must resize. If there was a race condition, we start over again.
-                                if (array != m_array)
-                                    continue;
+                                var newSnapshot = new Snapshot(array.Length * 2, array.Length + 1);
+                                T[] newArray = newSnapshot.Data;
 
-                                T[] newArray = new T[array.Length * 2];
                                 Array.Copy(array, newArray, i + 1);
                                 newArray[i + 1] = e;
-                                m_array = newArray;
+                                m_current = newSnapshot;
                                 return i + 1;
                             }
                         }
@@ -116,16 +154,29 @@ internal int Add(T e)
 
             internal void Remove(T e)
             {
-                T[] array = m_array;
-                lock (array)
+                while (true)
                 {
-                    for (int i = 0; i < m_array.Length; i++)
+                    var current = m_current;
+                    lock (current)
                     {
-                        if (m_array[i] == e)
+                        if (current != m_current)
                         {
-                            Volatile.Write(ref m_array[i], null);
-                            break;
+                            // If there was a race condition, we start over again.
+                            continue;
+                        }
+
+                        var array = current.Data;
+                        var length = current.Length;
+
+                        for (int i = 0; i < length; i++)
+                        {
+                            if (array[i] == e)
+                            {
+                                Volatile.Write(ref array[i], null);
+                                break;
+                            }
                         }
+                        break;
                     }
                 }
             }
@@ -741,12 +792,14 @@ private void DequeueSeek(ThreadPoolWorkQueueThreadLocals tl, ref IThreadPoolWork
         private static void DequeueSteal(ThreadPoolWorkQueueThreadLocals tl, ref IThreadPoolWorkItem callback, ref bool missedSteal)
         {
             WorkStealingQueue wsq = tl.workStealingQueue;
-            WorkStealingQueue[] otherQueues = allThreadQueues.Current;
-            int i = tl.random.Next(otherQueues.Length);
-            int c = otherQueues.Length;
-            while (c > 0)
+            var otherQueues = allThreadQueues.Current;
+            var remaining = otherQueues.Length;
+            var i = tl.random.Next(remaining);
+            var data = otherQueues.Data;
+            var mask = otherQueues.Mask;
+            while (remaining > 0)
             {
-                WorkStealingQueue otherQueue = Volatile.Read(ref otherQueues[i % otherQueues.Length]);
+                WorkStealingQueue otherQueue = Volatile.Read(ref data[i & mask]);
                 if (otherQueue != null &&
                     otherQueue != wsq &&
                     otherQueue.TrySteal(out callback, ref missedSteal))
@@ -755,7 +808,7 @@ private static void DequeueSteal(ThreadPoolWorkQueueThreadLocals tl, ref IThread
                     break;
                 }
                 i++;
-                c--;
+                remaining--;
             }
         }
 
@@ -1766,7 +1819,7 @@ internal static bool TryPopCustomWorkItem(IThreadPoolWorkItem workItem)
         [SecurityCritical]
         internal static IEnumerable<IThreadPoolWorkItem> GetQueuedWorkItems()
         {
-            return EnumerateQueuedWorkItems(ThreadPoolWorkQueue.allThreadQueues.Current, ThreadPoolGlobals.workQueue.queueTail);
+            return EnumerateQueuedWorkItems(ThreadPoolWorkQueue.allThreadQueues.Current.Data, ThreadPoolGlobals.workQueue.queueTail);
         }
 
         internal static IEnumerable<IThreadPoolWorkItem> EnumerateQueuedWorkItems(ThreadPoolWorkQueue.WorkStealingQueue[] wsQueues, ThreadPoolWorkQueue.QueueSegment globalQueueTail)