Optimize ConcurrentLru read throughput (#645)

* rem volatile

* ckd

---------

Author: bitfaster

BitFaster.Caching/Lru/AfterAccessPolicy.cs

@@ -40,7 +40,7 @@ public LongTickCountLruItem<K, V> CreateItem(K key, V value)
         public void Touch(LongTickCountLruItem<K, V> item)
         {
             item.TickCount = this.time.Last;
-            item.WasAccessed = true;
+            item.MarkAccessed();
         }
 
         ///<inheritdoc/>

BitFaster.Caching/Lru/DiscretePolicy.cs

@@ -35,7 +35,7 @@ public void Touch(LongTickCountLruItem<K, V> item)
             var currentExpiry = new Duration(item.TickCount - this.time.Last);
             var newExpiry = expiry.GetExpireAfterRead(item.Key, item.Value, currentExpiry);
             item.TickCount = this.time.Last + newExpiry.raw;
-            item.WasAccessed = true;
+            item.MarkAccessed();
         }
 
         ///<inheritdoc/>

BitFaster.Caching/Lru/LruItem.cs

@@ -14,8 +14,8 @@ public class LruItem<K, V>
     {
         private V data;
 
-        private volatile bool wasAccessed;
-        private volatile bool wasRemoved;
+        private bool wasAccessed;
+        private bool wasRemoved;
 
         // only used when V is a non-atomic value type to prevent torn reads
         private int sequence;
@@ -85,6 +85,17 @@ public bool WasRemoved
             set => this.wasRemoved = value;
         }
 
+        /// <summary>
+        /// Marks the item as accessed, if it was not already accessed.
+        /// </summary>
+        public void MarkAccessed()
+        { 
+            if (!this.wasAccessed)
+            { 
+                this.wasAccessed = true;    
+            }
+        }
+
         internal V SeqLockRead()
         { 
             var spin = new SpinWait();

BitFaster.Caching/Lru/LruPolicy.cs

@@ -23,7 +23,7 @@ public LruItem<K, V> CreateItem(K key, V value)
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public void Touch(LruItem<K, V> item)
         {
-            item.WasAccessed = true;
+            item.MarkAccessed();
         }
 
         ///<inheritdoc/>

BitFaster.Caching/Lru/TLruLongTicksPolicy.cs

@@ -38,7 +38,7 @@ public LongTickCountLruItem<K, V> CreateItem(K key, V value)
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public void Touch(LongTickCountLruItem<K, V> item)
         {
-            item.WasAccessed = true;
+            item.MarkAccessed();
         }
 
         ///<inheritdoc/>

BitFaster.Caching/Lru/TlruDateTimePolicy.cs

@@ -35,7 +35,7 @@ public TimeStampedLruItem<K, V> CreateItem(K key, V value)
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public void Touch(TimeStampedLruItem<K, V> item)
         {
-            item.WasAccessed = true;
+            item.MarkAccessed();
         }
 
         ///<inheritdoc/>

BitFaster.Caching/Lru/TlruTicksPolicy.cs

@@ -40,7 +40,7 @@ public TickCountLruItem<K, V> CreateItem(K key, V value)
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public void Touch(TickCountLruItem<K, V> item)
         {
-            item.WasAccessed = true;
+            item.MarkAccessed();
         }
 
         ///<inheritdoc/>