Serialize GPU prime sieve per accelerator

EvenPerfectBitScanner.Tests/ProgramTests.cs

@@ -360,6 +360,48 @@ public class ProgramTests
         }
     }
 
+    [Fact]
+    [Trait("Category", "Fast")]
+    public void Residue_mode_accepts_known_primes_in_parallel()
+    {
+        var useDivisorField = typeof(Program).GetField("_useDivisor", BindingFlags.NonPublic | BindingFlags.Static)!;
+        var mersenneField = typeof(Program).GetField("MersenneTesters", BindingFlags.NonPublic | BindingFlags.Static)!;
+        var primeField = typeof(Program).GetField("PrimeTesters", BindingFlags.NonPublic | BindingFlags.Static)!;
+        var residueField = typeof(Program).GetField("PResidue", BindingFlags.NonPublic | BindingFlags.Static)!;
+        var gcdFilterField = typeof(Program).GetField("_useGcdFilter", BindingFlags.NonPublic | BindingFlags.Static)!;
+        var forceCpuProp = typeof(GpuContextPool).GetProperty("ForceCpu");
+
+        useDivisorField.SetValue(null, false);
+        gcdFilterField.SetValue(null, false);
+        mersenneField.SetValue(null, new ThreadLocal<MersenneNumberTester>(() => new MersenneNumberTester(
+            useIncremental: true,
+            useResidue: true,
+            maxK: 5_000_000UL,
+            residueDivisorSets: PerfectNumberConstants.ExtraDivisorCycleSearchLimit), trackAllValues: true));
+        primeField.SetValue(null, new ThreadLocal<PrimeTester>(() => new PrimeTester(), trackAllValues: true));
+        residueField.SetValue(null, new ThreadLocal<ModResidueTracker>(() => new ModResidueTracker(ResidueModel.Identity, 2UL, true), trackAllValues: true));
+        forceCpuProp!.SetValue(null, false);
+
+        try
+        {
+            ulong[] primes = [107UL, 127UL, 521UL, 607UL];
+            Parallel.ForEach(primes, prime =>
+            {
+                Program.IsEvenPerfectCandidate(prime, 0UL, out bool searched, out bool detailed).Should().BeTrue();
+                searched.Should().BeTrue();
+                detailed.Should().BeTrue();
+            });
+        }
+        finally
+        {
+            useDivisorField.SetValue(null, false);
+            mersenneField.SetValue(null, null);
+            primeField.SetValue(null, null);
+            residueField.SetValue(null, null);
+            forceCpuProp!.SetValue(null, false);
+        }
+    }
+
     [Fact]
     public void Residue_mode_with_default_limits_accepts_large_known_primes()
     {

PerfectNumbers.Core.Tests/PrimeTester/PrimeTesterIsPrimeTests.cs

@@ -36,5 +36,33 @@ public class PrimeTesterIsPrimeTests
 
         GpuContextPool.ForceCpu = false;
     }
+
+    [Fact]
+    [Trait("Category", "Fast")]
+    public void IsPrimeGpu_handles_parallel_calls_for_known_primes()
+    {
+        GpuContextPool.ForceCpu = false;
+        GpuPrimeWorkLimiter.SetLimit(64);
+        PrimeTester.GpuBatchSize = 256_000;
+
+        ulong[] primes = [107UL, 127UL, 521UL, 607UL];
+        var results = new bool[primes.Length];
+
+        var testers = new ThreadLocal<PrimeTester>(() => new PrimeTester(), trackAllValues: true);
+        Parallel.For(0, primes.Length, i =>
+        {
+            results[i] = testers.Value!.IsPrimeGpu(primes[i], CancellationToken.None);
+        });
+
+        results.Should().AllBeEquivalentTo(true);
+
+        Parallel.For(0, Environment.ProcessorCount * 16, _ =>
+        {
+            foreach (ulong prime in primes)
+            {
+                testers.Value!.IsPrimeGpu(prime, CancellationToken.None).Should().BeTrue();
+            }
+        });
+    }
 }
 

PerfectNumbers.Core/PrimeTester.cs

@@ -83,34 +83,31 @@ public sealed class PrimeTester(bool useInternal = false)
         // Limit concurrency and declare variables outside loops for performance and reuse
         var limiter = GpuPrimeWorkLimiter.Acquire();
         var gpu = GpuContextPool.Rent();
+
+        try
+        {
             var accelerator = gpu.Accelerator;
 
             // Get per-accelerator cached kernel and device primes buffer.
             var state = GpuKernelState.GetOrCreate(accelerator);
+            int totalLength = values.Length;
+            int batchSize = Math.Max(1, GpuBatchSize);
 
-        MemoryBuffer1D<ulong, Stride1D.Dense> input;
+            lock (gpu.ExecutionLock)
-        MemoryBuffer1D<byte, Stride1D.Dense> output;
+            {
-        int totalLength, batchSize, pos, remaining, count;
-        ulong[] temp;
-
-        totalLength = values.Length;
-        batchSize = Math.Max(1, GpuBatchSize);
-
-        // Rent per-call scratch buffers sized to current batch (thread-safe)
                 var scratch = state.RentScratch(batchSize, accelerator);
-        input = scratch.Input;
+                var input = scratch.Input;
-        output = scratch.Output;
+                var output = scratch.Output;
+                ulong[] temp = ArrayPool<ulong>.Shared.Rent(batchSize);
 
-        // Reusable host buffer per call
+                try
-        temp = ArrayPool<ulong>.Shared.Rent(batchSize);
+                {
-
+                    int pos = 0;
-        pos = 0;
                     while (pos < totalLength)
                     {
-            remaining = totalLength - pos;
+                        int remaining = totalLength - pos;
-            count = remaining > batchSize ? batchSize : remaining;
+                        int count = remaining > batchSize ? batchSize : remaining;
 
-            // Copy chunk to host temp then to device
                         values.Slice(pos, count).CopyTo(temp);
                         input.View.CopyFromCPU(ref temp[0], count);
 
@@ -120,12 +117,20 @@ public sealed class PrimeTester(bool useInternal = false)
 
                         pos += count;
                     }
-
+                }
+                finally
+                {
                     ArrayPool<ulong>.Shared.Return(temp);
                     state.ReturnScratch(scratch);
+                }
+            }
+        }
+        finally
+        {
             gpu.Dispose();
             limiter.Dispose();
         }
+    }
 
     // Per-accelerator GPU state for prime sieve (kernel + uploaded primes).
     private sealed class KernelState