define conc level by CPU count

BitFaster.Caching.UnitTests/Lfu/ConcurrentLfuTests.cs

@@ -6,6 +6,7 @@
 using System.Threading.Tasks;
 using BitFaster.Caching.Buffers;
 using BitFaster.Caching.Lfu;
+using BitFaster.Caching.Lru;
 using BitFaster.Caching.Scheduler;
 using BitFaster.Caching.UnitTests.Lru;
 using FluentAssertions;
@@ -56,13 +57,23 @@ public void WhenCapacityIsValidCacheIsCreated()
         }
 
         [Fact]
-        public void WhenConcurrencyIsLessThan1CtorThrows()
+        public void WhenConcurrencyIsZeroCtorThrows()
         {
             Action constructor = () => { var x = new ConcurrentLfu<int, string>(0, 20, new ForegroundScheduler(), EqualityComparer<int>.Default); };
 
             constructor.Should().Throw<ArgumentOutOfRangeException>();
         }
 
+#if NET8_0_OR_GREATER
+        // -1 is default conc level on .NET 8+. This can cause invalid striped buffer size without explicit handling.
+        [Fact]
+        public void WhenConcurrencyIsNegativeOneReadBufferIsCorrectSize()
+        {
+            var x = new ConcurrentLfu<int, string>(-1, 20, new ForegroundScheduler(), EqualityComparer<int>.Default);
+            x.Core.readBuffer.Capacity.Should().Be(BitOps.CeilingPowerOfTwo(Defaults.ConcurrencyLevel) * 128);
+        }
+#endif
+
         [Fact]
         public void DefaultSchedulerIsThreadPool()
         {

BitFaster.Caching/Lfu/ConcurrentLfuCore.cs

@@ -11,6 +11,8 @@
 using BitFaster.Caching.Buffers;
 using BitFaster.Caching.Counters;
 using BitFaster.Caching.Scheduler;
+using BitFaster.Caching.Lru;
+
 
 #if DEBUG
 using System.Text;
@@ -96,6 +98,14 @@ public ConcurrentLfuCore(int concurrencyLevel, int capacity, IScheduler schedule
             int dictionaryCapacity = ConcurrentDictionarySize.Estimate(capacity);
             this.dictionary = new(concurrencyLevel, dictionaryCapacity, comparer);
 
+            // On .NET 8+, -1 can be used for default conc level. concurrencyLevel is guarded by the dictionary ctor that will not throw in this case.
+#if NET8_0_OR_GREATER
+            if (concurrencyLevel == -1)
+            { 
+                concurrencyLevel = Defaults.ConcurrencyLevel;
+            }
+#endif
+
             // cap concurrency at proc count * 2
             int readStripes = Math.Min(BitOps.CeilingPowerOfTwo(concurrencyLevel), BitOps.CeilingPowerOfTwo(Environment.ProcessorCount * 2));
             this.readBuffer = new(readStripes, DefaultBufferSize);

BitFaster.Caching/Lru/Defaults.cs

@@ -4,11 +4,6 @@ namespace BitFaster.Caching.Lru
 {
     internal static class Defaults
     {
-#if NET8_0_OR_GREATER
-        // Note that on .net8+, -1 indicates the default concurrency level
-        public static int ConcurrencyLevel => -1;
-#else
         public static int ConcurrencyLevel => Environment.ProcessorCount;
-#endif
     }
 }