unbounded

BitFaster.Caching.UnitTests/Lru/ConcurrentLruTests.cs

@@ -348,6 +348,58 @@ public void WhenKeysAreContinuouslyRequestedInTheOrderTheyAreAddedCountIsBounded
                 testOutputHelper.WriteLine($"Total: {lru.Count} Hot: {lru.HotCount} Warm: {lru.WarmCount} Cold: {lru.ColdCount}");
                 lru.Count.Should().BeLessOrEqualTo(capacity + 1);
             }
+        }
+
+        public class KeysInOrderTestDataGenerator : IEnumerable<object[]>
+        {
+            private readonly List<object[]> _data = new List<object[]>
+            {
+                new object[] { new EqualCapacityPartition(hotCap + warmCap + coldCap) },
+                new object[] { new EqualCapacityPartition(128) },
+                new object[] { new EqualCapacityPartition(256) },
+                new object[] { new EqualCapacityPartition(1024) },
+                new object[] { new FavorWarmPartition(128) },
+                new object[] { new FavorWarmPartition(256) },
+                new object[] { new FavorWarmPartition(1024) },
+                new object[] { new FavorWarmPartition(128, 0.6) },
+                new object[] { new FavorWarmPartition(256, 0.6) },
+                new object[] { new FavorWarmPartition(1024, 0.6) },
+                //new object[] { new FavorWarmPartition(10*1024, 0.6) },
+                //new object[] { new FavorWarmPartition(100*1024, 0.6) },
+            };
+
+            public IEnumerator<object[]> GetEnumerator() => _data.GetEnumerator();
+
+            IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
+        }
+
+        [Theory]
+        [ClassData(typeof(KeysInOrderTestDataGenerator))]
+        public void WhenKeysAreContinuouslyRequestedInTheOrderTheyAreAddedCountIsBounded2(ICapacityPartition p)
+        {
+            int capacity = p.Hot + p.Cold + p.Warm;
+            lru = new ConcurrentLru<int, string>(capacity, p, EqualityComparer<int>.Default);
+
+            testOutputHelper.WriteLine($"Capacity: {lru.Capacity} (Hot: {p.Hot} Warm: {p.Warm} Cold: {p.Cold})");
+
+            for (int i = 0; i < capacity + 10; i++)
+            {
+                lru.GetOrAdd(i, valueFactory.Create);
+
+                // Touch all items already cached in hot, warm and cold.
+                // This is worst case scenario, since we touch them in the exact order they
+                // were added.
+                for (int j = 0; j < i; j++)
+                {
+                    lru.GetOrAdd(j, valueFactory.Create);
+                }
+            }
+
+            // For larger cache sizes, I have observed capacity + 5. This is linked to the number of attempts.
+            // This is clearly a bug that needs further investigation, but considered not harmful at this point
+            // since growth is bounded, we just allow 4 more items than we should in the absolute worst case.
+            testOutputHelper.WriteLine($"Total: {lru.Count} Hot: {lru.HotCount} Warm: {lru.WarmCount} Cold: {lru.ColdCount}");
+            lru.Count.Should().BeLessOrEqualTo(capacity + 1);
         }
 
         [Fact]

BitFaster.Caching/Lru/ConcurrentLruCore.cs

@@ -541,13 +541,14 @@ private void Cycle(int hotCount)
                         // Attempt to recover. It is possible that multiple threads read the same queue count here,
                         // so this process has races that could reduce cache size below capacity. This manifests
                         // in 'off by one' which is considered harmless.
-                        (dest, count) = CycleWarm(Volatile.Read(ref counter.warm));
+
+                        (dest, count) = CycleCold(Volatile.Read(ref counter.cold));
                         if (dest != ItemDestination.Remove)
                         {
                             continue;
                         }
 
-                        (dest, count) = CycleCold(Volatile.Read(ref counter.cold));
+                        (dest, count) = CycleWarm(Volatile.Read(ref counter.warm));
                         if (dest != ItemDestination.Remove)
                         {
                             continue;
@@ -637,7 +638,7 @@ private void CycleDuringWarmup(int hotCount)
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private (ItemDestination, int) CycleWarmUnchecked(ItemRemovedReason removedReason)
         {
-            Interlocked.Decrement(ref this.counter.warm);
+            int wc = Interlocked.Decrement(ref this.counter.warm);
 
             if (this.warmQueue.TryDequeue(out var item))
             {
@@ -646,7 +647,7 @@ private void CycleDuringWarmup(int hotCount)
                 // When the warm queue is full, we allow an overflow of 1 item before redirecting warm items to cold.
                 // This only happens when hit rate is high, in which case we can consider all items relatively equal in
                 // terms of which was least recently used.
-                if (where == ItemDestination.Warm && Volatile.Read(ref this.counter.warm) <= this.capacity.Warm)
+                if (where == ItemDestination.Warm && wc <= this.capacity.Warm)
                 {
                     return (ItemDestination.Warm, this.Move(item, where, removedReason));
                 }