feat: Add unbounded parallel processor and increase I/O concurrency limits

- Add UnboundedParallelAsyncProcessor for true unlimited parallelism
- Increase IOBoundParallelAsyncProcessor default concurrency from 100-800 to 1000-4096
- Add ProcessInParallelUnbounded() methods to all builder classes
- Enable scenarios like running 10,000+ unit tests truly in parallel

This addresses bottlenecks reported by TUnit consumers where EnumerableAsyncProcessor
was limiting parallelism and preventing all tests from starting concurrently.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: thomhurst

EnumerableAsyncProcessor/Builders/ActionAsyncProcessorBuilder.cs

@@ -62,6 +62,17 @@ public IAsyncProcessor ProcessOneAtATime()
     {
         return new OneAtATimeAsyncProcessor(_count, _taskSelector, _cancellationTokenSource).StartProcessing();
     }
+    
+    /// <summary>
+    /// Process ALL tasks in parallel without any concurrency limits.
+    /// WARNING: Use with caution - can overwhelm system resources with large task counts.
+    /// Ideal for scenarios requiring maximum parallelism like running thousands of unit tests.
+    /// </summary>
+    /// <returns>An async processor with unbounded parallelism.</returns>
+    public IAsyncProcessor ProcessInParallelUnbounded()
+    {
+        return new UnboundedParallelAsyncProcessor(_count, _taskSelector, _cancellationTokenSource).StartProcessing();
+    }
 
 #if NET6_0_OR_GREATER
     /// <summary>

EnumerableAsyncProcessor/Builders/ActionAsyncProcessorBuilder_1.cs

@@ -62,6 +62,17 @@ public IAsyncProcessor<TOutput> ProcessOneAtATime()
     {
         return new ResultOneAtATimeAsyncProcessor<TOutput>(_count, _taskSelector, _cancellationTokenSource).StartProcessing();
     }
+    
+    /// <summary>
+    /// Process ALL tasks in parallel without any concurrency limits and return results.
+    /// WARNING: Use with caution - can overwhelm system resources with large task counts.
+    /// Ideal for scenarios requiring maximum parallelism like running thousands of unit tests.
+    /// </summary>
+    /// <returns>An async processor with unbounded parallelism that returns results.</returns>
+    public IAsyncProcessor<TOutput> ProcessInParallelUnbounded()
+    {
+        return new ResultUnboundedParallelAsyncProcessor<TOutput>(_count, _taskSelector, _cancellationTokenSource).StartProcessing();
+    }
 
 #if NET6_0_OR_GREATER
     /// <summary>

EnumerableAsyncProcessor/Builders/ItemActionAsyncProcessorBuilder_1.cs

@@ -69,6 +69,18 @@ public IAsyncProcessor ProcessOneAtATime()
         return new OneAtATimeAsyncProcessor<TInput>(_items, _taskSelector, _cancellationTokenSource)
             .StartProcessing();
     }
+    
+    /// <summary>
+    /// Process ALL items in parallel without any concurrency limits.
+    /// WARNING: Use with caution - can overwhelm system resources with large item counts.
+    /// Ideal for scenarios requiring maximum parallelism like running thousands of unit tests.
+    /// </summary>
+    /// <returns>An async processor with unbounded parallelism.</returns>
+    public IAsyncProcessor ProcessInParallelUnbounded()
+    {
+        return new UnboundedParallelAsyncProcessor<TInput>(_items, _taskSelector, _cancellationTokenSource)
+            .StartProcessing();
+    }
 
 #if NET6_0_OR_GREATER
     /// <summary>

EnumerableAsyncProcessor/Builders/ItemActionAsyncProcessorBuilder_2.cs

@@ -62,6 +62,17 @@ public IAsyncProcessor<TOutput> ProcessOneAtATime()
     {
         return new ResultOneAtATimeAsyncProcessor<TInput, TOutput>(_items, _taskSelector, _cancellationTokenSource).StartProcessing();
     }
+    
+    /// <summary>
+    /// Process ALL items in parallel without any concurrency limits and return results.
+    /// WARNING: Use with caution - can overwhelm system resources with large item counts.
+    /// Ideal for scenarios requiring maximum parallelism like running thousands of unit tests.
+    /// </summary>
+    /// <returns>An async processor with unbounded parallelism that returns results.</returns>
+    public IAsyncProcessor<TOutput> ProcessInParallelUnbounded()
+    {
+        return new ResultUnboundedParallelAsyncProcessor<TInput, TOutput>(_items, _taskSelector, _cancellationTokenSource).StartProcessing();
+    }
 
 #if NET6_0_OR_GREATER
     /// <summary>

EnumerableAsyncProcessor/RunnableProcessors/IOBoundParallelAsyncProcessor.cs

@@ -13,8 +13,9 @@ public class IOBoundParallelAsyncProcessor : AbstractAsyncProcessor
 
     internal IOBoundParallelAsyncProcessor(int count, Func<Task> taskSelector, CancellationTokenSource cancellationTokenSource, int? maxConcurrency = null) : base(count, taskSelector, cancellationTokenSource)
     {
-        // For I/O operations, allow much higher concurrency - default to 10x processor count or minimum 100
-        _maxConcurrency = maxConcurrency ?? Math.Max(100, Environment.ProcessorCount * 10);
+        // For I/O operations, allow much higher concurrency - default to 64x processor count or minimum 1000
+        // This enables scenarios like running 10,000 unit tests in parallel
+        _maxConcurrency = maxConcurrency ?? Math.Max(1000, Environment.ProcessorCount * 64);
     }
 
     internal override async Task Process()

EnumerableAsyncProcessor/RunnableProcessors/IOBoundParallelAsyncProcessor_1.cs

@@ -13,8 +13,9 @@ public class IOBoundParallelAsyncProcessor<TInput> : AbstractAsyncProcessor<TInp
 
     internal IOBoundParallelAsyncProcessor(IEnumerable<TInput> items, Func<TInput, Task> taskSelector, CancellationTokenSource cancellationTokenSource, int? maxConcurrency = null) : base(items, taskSelector, cancellationTokenSource)
     {
-        // For I/O operations, allow much higher concurrency - default to 10x processor count or minimum 100
-        _maxConcurrency = maxConcurrency ?? Math.Max(100, Environment.ProcessorCount * 10);
+        // For I/O operations, allow much higher concurrency - default to 64x processor count or minimum 1000
+        // This enables scenarios like running 10,000 unit tests in parallel
+        _maxConcurrency = maxConcurrency ?? Math.Max(1000, Environment.ProcessorCount * 64);
     }
 
     internal override async Task Process()

EnumerableAsyncProcessor/RunnableProcessors/ResultProcessors/ResultIOBoundParallelAsyncProcessor_1.cs

@@ -13,8 +13,9 @@ public class ResultIOBoundParallelAsyncProcessor<TOutput> : ResultAbstractAsyncP
 
     internal ResultIOBoundParallelAsyncProcessor(int count, Func<Task<TOutput>> taskSelector, CancellationTokenSource cancellationTokenSource, int? maxConcurrency = null) : base(count, taskSelector, cancellationTokenSource)
     {
-        // For I/O operations, allow much higher concurrency - default to 10x processor count or minimum 100
-        _maxConcurrency = maxConcurrency ?? Math.Max(100, Environment.ProcessorCount * 10);
+        // For I/O operations, allow much higher concurrency - default to 64x processor count or minimum 1000
+        // This enables scenarios like running 10,000 unit tests in parallel
+        _maxConcurrency = maxConcurrency ?? Math.Max(1000, Environment.ProcessorCount * 64);
     }
 
     internal override async Task Process()

EnumerableAsyncProcessor/RunnableProcessors/ResultProcessors/ResultIOBoundParallelAsyncProcessor_2.cs

@@ -13,8 +13,9 @@ public class ResultIOBoundParallelAsyncProcessor<TInput, TOutput> : ResultAbstra
 
     internal ResultIOBoundParallelAsyncProcessor(IEnumerable<TInput> items, Func<TInput, Task<TOutput>> taskSelector, CancellationTokenSource cancellationTokenSource, int? maxConcurrency = null) : base(items, taskSelector, cancellationTokenSource)
     {
-        // For I/O operations, allow much higher concurrency - default to 10x processor count or minimum 100
-        _maxConcurrency = maxConcurrency ?? Math.Max(100, Environment.ProcessorCount * 10);
+        // For I/O operations, allow much higher concurrency - default to 64x processor count or minimum 1000
+        // This enables scenarios like running 10,000 unit tests in parallel
+        _maxConcurrency = maxConcurrency ?? Math.Max(1000, Environment.ProcessorCount * 64);
     }
 
     internal override async Task Process()

EnumerableAsyncProcessor/RunnableProcessors/ResultProcessors/ResultUnboundedParallelAsyncProcessor_1.cs

@@ -0,0 +1,33 @@
+using EnumerableAsyncProcessor.RunnableProcessors.ResultProcessors.Abstract;
+
+namespace EnumerableAsyncProcessor.RunnableProcessors.ResultProcessors;
+
+/// <summary>
+/// A specialized parallel processor that starts ALL tasks immediately without any concurrency limits and returns results.
+/// WARNING: Use with caution - this can overwhelm system resources with large task counts.
+/// Ideal for scenarios where you need maximum parallelism and have sufficient resources.
+/// </summary>
+public class ResultUnboundedParallelAsyncProcessor<TOutput> : ResultAbstractAsyncProcessor<TOutput>
+{
+    internal ResultUnboundedParallelAsyncProcessor(int count, Func<Task<TOutput>> taskSelector, CancellationTokenSource cancellationTokenSource) : base(count, taskSelector, cancellationTokenSource)
+    {
+    }
+
+    internal override Task Process()
+    {
+        // Start ALL tasks immediately without any throttling
+        // This provides true unbounded parallelism
+        var tasks = TaskWrappers.Select(taskWrapper => 
+        {
+            var task = taskWrapper.Process(CancellationToken);
+            // Fast-path for already completed tasks
+            if (task.IsCompleted)
+            {
+                return task;
+            }
+            return task;
+        });
+
+        return Task.WhenAll(tasks);
+    }
+}

EnumerableAsyncProcessor/RunnableProcessors/ResultProcessors/ResultUnboundedParallelAsyncProcessor_2.cs

@@ -0,0 +1,33 @@
+using EnumerableAsyncProcessor.RunnableProcessors.ResultProcessors.Abstract;
+
+namespace EnumerableAsyncProcessor.RunnableProcessors.ResultProcessors;
+
+/// <summary>
+/// A specialized parallel processor that starts ALL tasks immediately without any concurrency limits and returns results.
+/// WARNING: Use with caution - this can overwhelm system resources with large task counts.
+/// Ideal for scenarios where you need maximum parallelism and have sufficient resources.
+/// </summary>
+public class ResultUnboundedParallelAsyncProcessor<TInput, TOutput> : ResultAbstractAsyncProcessor<TInput, TOutput>
+{
+    internal ResultUnboundedParallelAsyncProcessor(IEnumerable<TInput> items, Func<TInput, Task<TOutput>> taskSelector, CancellationTokenSource cancellationTokenSource) : base(items, taskSelector, cancellationTokenSource)
+    {
+    }
+
+    internal override Task Process()
+    {
+        // Start ALL tasks immediately without any throttling
+        // This provides true unbounded parallelism
+        var tasks = TaskWrappers.Select(taskWrapper => 
+        {
+            var task = taskWrapper.Process(CancellationToken);
+            // Fast-path for already completed tasks
+            if (task.IsCompleted)
+            {
+                return task;
+            }
+            return task;
+        });
+
+        return Task.WhenAll(tasks);
+    }
+}