diff --git a/PerfectNumbers.Core/PrimeOrderCalculator.Cpu.cs b/PerfectNumbers.Core/PrimeOrderCalculator.Cpu.cs
index 237aac1..bf04f37 100644
--- a/PerfectNumbers.Core/PrimeOrderCalculator.Cpu.cs
+++ b/PerfectNumbers.Core/PrimeOrderCalculator.Cpu.cs
@@ -3,6 +3,7 @@ using System.Diagnostics;
 using System.Numerics;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
+using System.Threading;
 using PerfectNumbers.Core.Gpu;
 
 namespace PerfectNumbers.Core;
@@ -661,8 +662,41 @@ internal static partial class PrimeOrderCalculator
 		return exponent.Pow2MontgomeryModWindowedCpu(divisorData, keepMontgomery: false) == 1UL;
 	}
 
-	[ThreadStatic]
-	private static Dictionary<ulong, bool>? _primalityCache;
+	// [ThreadStatic]
+	private static readonly Dictionary<ulong, bool> _primalityCache = [];
+	private static readonly ReaderWriterLockSlim _primalityCacheLock = new(LockRecursionPolicy.NoRecursion);
+
+	// [ThreadStatic]
+	// private static UInt128 _isPrimeHits;
+
+	[MethodImpl(MethodImplOptions.AggressiveInlining)]
+	private static bool TryGetCachedPrimality(ulong value, out bool isPrime)
+	{
+		_primalityCacheLock.EnterReadLock();
+		bool result = _primalityCache.TryGetValue(value, out isPrime);
+		_primalityCacheLock.ExitReadLock();
+		return result;
+	}
+
+	[MethodImpl(MethodImplOptions.AggressiveInlining)]
+	private static void CachePrimalityResult(ulong value, bool isPrime)
+	{
+		_primalityCacheLock.EnterWriteLock();
+		_primalityCache[value] = isPrime;
+		_primalityCacheLock.ExitWriteLock();
+	}
+
+	private static bool GetOrComputePrimality(ulong value)
+	{
+		if (TryGetCachedPrimality(value, out bool isPrime))
+		{
+			return isPrime;
+		}
+
+		isPrime = Open.Numeric.Primes.Prime.Numbers.IsPrime(value);
+		CachePrimalityResult(value, isPrime);
+		return isPrime;
+	}
 
 	private static PartialFactorResult PartialFactor(ulong value, in PrimeOrderSearchConfig config)
 	{
@@ -700,7 +734,6 @@ internal static partial class PrimeOrderCalculator
 		List<ulong> pending = ThreadStaticPools.RentUlongList(2);
 		Stack<ulong>? compositeStack = null;
 		PartialFactorResult result;
-		var cache = _primalityCache ??= [];
 
 		if (!gpuFactored)
 		{
@@ -739,11 +772,7 @@ internal static partial class PrimeOrderCalculator
 					// 	continue;
 					// }
 
-					if (!cache.TryGetValue(composite, out bool isPrime))
-					{
-						isPrime = Open.Numeric.Primes.Prime.Numbers.IsPrime(composite);
-						cache.Add(composite, isPrime);
-					}
+					bool isPrime = GetOrComputePrimality(composite);
 
 					if (isPrime)
 					{
@@ -779,11 +808,7 @@ internal static partial class PrimeOrderCalculator
 		for (int i = 0; i < pendingCount; i++)
 		{
 			ulong composite = pending[i];
-			if (!cache.TryGetValue(composite, out bool isPrime))
-			{
-				isPrime = Open.Numeric.Primes.Prime.Numbers.IsPrime(composite);
-				cache.Add(composite, isPrime);
-			}
+			bool isPrime = GetOrComputePrimality(composite);
 
 			if (isPrime)
 			{