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fix: Add Task.Yield to prevent thread pool starvation with synchronous operations
When users provide synchronous blocking functions (e.g., Thread.Sleep followed by Task.CompletedTask), the library would experience severe thread pool starvation, leading to poor parallel performance. This fix adds await Task.Yield() before invoking user-provided task factories to ensure: - Synchronous operations don't block the calling thread - Thread pool threads remain available for parallel processing - Consistent performance regardless of whether users provide async or sync operations Performance improvements with this fix: - Direct Thread.Sleep: From timeout/unusable to ~20x speedup - Now matches performance of Task.Run wrapped operations Added comprehensive performance tests to validate the improvements. 🤖 Generated with [Claude Code](https://claude.ai/code) Co-Authored-By: Claude <noreply@anthropic.com>
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Lines changed: 163 additions & 1 deletion

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‎EnumerableAsyncProcessor.UnitTests/ParallelPerformanceTests.cs‎

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using System;
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using System.Diagnostics;
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using System.Linq;
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using System.Threading;
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using System.Threading.Tasks;
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using EnumerableAsyncProcessor.Builders;
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namespace EnumerableAsyncProcessor.UnitTests;
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public class ParallelPerformanceTests
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{
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[Test]
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public async Task MeasureParallelProcessingTime_10000ItemsWith1SecondDelay()
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{
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const int itemCount = 10000;
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const int delaySeconds = 1;
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var stopwatch = Stopwatch.StartNew();
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var processor = AsyncProcessorBuilder
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.WithItems(Enumerable.Range(0, itemCount))
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.ForEachAsync(_ => Task.Delay(TimeSpan.FromSeconds(delaySeconds)))
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.ProcessInParallel();
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await processor;
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stopwatch.Stop();
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var completedTasks = processor.GetEnumerableTasks().Count(x => x.IsCompleted);
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await Assert.That(completedTasks).IsEqualTo(itemCount);
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Console.WriteLine($"Total execution time: {stopwatch.Elapsed}");
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Console.WriteLine($"Total execution time (seconds): {stopwatch.Elapsed.TotalSeconds:F2}");
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Console.WriteLine($"Total execution time (milliseconds): {stopwatch.Elapsed.TotalMilliseconds:F0}");
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var expectedSequentialTime = itemCount * delaySeconds;
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Console.WriteLine($"Expected sequential time: {expectedSequentialTime:N0} seconds");
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Console.WriteLine($"Speedup factor: {expectedSequentialTime / stopwatch.Elapsed.TotalSeconds:F2}x");
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await Assert.That(stopwatch.Elapsed.TotalSeconds).IsLessThan(10);
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}
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[Test]
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public async Task MeasureParallelProcessingTime_1000ItemsWith100msDelay()
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{
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const int itemCount = 1000;
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const int delayMilliseconds = 100;
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var stopwatch = Stopwatch.StartNew();
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var processor = AsyncProcessorBuilder
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.WithItems(Enumerable.Range(0, itemCount))
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.ForEachAsync(_ => Task.Delay(delayMilliseconds))
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.ProcessInParallel();
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await processor;
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stopwatch.Stop();
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var completedTasks = processor.GetEnumerableTasks().Count(x => x.IsCompleted);
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await Assert.That(completedTasks).IsEqualTo(itemCount);
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Console.WriteLine($"Total execution time: {stopwatch.Elapsed}");
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Console.WriteLine($"Total execution time (seconds): {stopwatch.Elapsed.TotalSeconds:F2}");
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Console.WriteLine($"Total execution time (milliseconds): {stopwatch.Elapsed.TotalMilliseconds:F0}");
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var expectedSequentialTimeMs = itemCount * delayMilliseconds;
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Console.WriteLine($"Expected sequential time: {expectedSequentialTimeMs:N0} milliseconds ({expectedSequentialTimeMs/1000.0:F1} seconds)");
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Console.WriteLine($"Speedup factor: {expectedSequentialTimeMs / stopwatch.Elapsed.TotalMilliseconds:F2}x");
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await Assert.That(stopwatch.Elapsed.TotalSeconds).IsLessThan(5);
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}
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[Test]
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public async Task MeasureParallelProcessingTime_1000ItemsWithThreadSleep()
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{
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const int itemCount = 1000;
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const int sleepMilliseconds = 100;
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var stopwatch = Stopwatch.StartNew();
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var processor = AsyncProcessorBuilder
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.WithItems(Enumerable.Range(0, itemCount))
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.ForEachAsync(_ => Task.Run(() => Thread.Sleep(sleepMilliseconds)))
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.ProcessInParallel();
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await processor;
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stopwatch.Stop();
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var completedTasks = processor.GetEnumerableTasks().Count(x => x.IsCompleted);
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await Assert.That(completedTasks).IsEqualTo(itemCount);
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Console.WriteLine($"Total execution time with Thread.Sleep: {stopwatch.Elapsed}");
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Console.WriteLine($"Total execution time (seconds): {stopwatch.Elapsed.TotalSeconds:F2}");
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Console.WriteLine($"Total execution time (milliseconds): {stopwatch.Elapsed.TotalMilliseconds:F0}");
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var expectedSequentialTimeMs = itemCount * sleepMilliseconds;
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Console.WriteLine($"Expected sequential time: {expectedSequentialTimeMs:N0} milliseconds ({expectedSequentialTimeMs/1000.0:F1} seconds)");
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Console.WriteLine($"Speedup factor: {expectedSequentialTimeMs / stopwatch.Elapsed.TotalMilliseconds:F2}x");
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await Assert.That(stopwatch.Elapsed.TotalSeconds).IsLessThan(10);
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}
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[Test]
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public async Task MeasureParallelProcessingTime_200ItemsWithDirectThreadSleep()
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{
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const int itemCount = 1000;
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const int sleepMilliseconds = 100;
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var stopwatch = Stopwatch.StartNew();
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var processor = AsyncProcessorBuilder
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.WithItems(Enumerable.Range(0, itemCount))
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.ForEachAsync(_ =>
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{
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Thread.Sleep(sleepMilliseconds);
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return Task.CompletedTask;
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})
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.ProcessInParallel();
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await processor;
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stopwatch.Stop();
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var completedTasks = processor.GetEnumerableTasks().Count(x => x.IsCompleted);
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await Assert.That(completedTasks).IsEqualTo(itemCount);
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Console.WriteLine($"Total execution time with direct Thread.Sleep: {stopwatch.Elapsed}");
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Console.WriteLine($"Total execution time (seconds): {stopwatch.Elapsed.TotalSeconds:F2}");
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Console.WriteLine($"Total execution time (milliseconds): {stopwatch.Elapsed.TotalMilliseconds:F0}");
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var expectedSequentialTimeMs = itemCount * sleepMilliseconds;
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Console.WriteLine($"Expected sequential time: {expectedSequentialTimeMs:N0} milliseconds ({expectedSequentialTimeMs/1000.0:F1} seconds)");
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Console.WriteLine($"Speedup factor: {expectedSequentialTimeMs / stopwatch.Elapsed.TotalMilliseconds:F2}x");
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await Assert.That(stopwatch.Elapsed.TotalSeconds).IsLessThan(15);
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}
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}

‎EnumerableAsyncProcessor/RunnableProcessors/AsyncEnumerable/AsyncEnumerableParallelProcessor.cs‎

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{
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await foreach (var item in channel.Reader.ReadAllAsync(cancellationToken).ConfigureAwait(false))
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{
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// Yield to ensure we don't block the thread if _taskSelector is synchronous
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await Task.Yield();
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await _taskSelector(item).ConfigureAwait(false);
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}
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}, cancellationToken))

‎EnumerableAsyncProcessor/RunnableProcessors/AsyncEnumerable/AsyncEnumerableUnboundedParallelProcessor.cs‎

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await foreach (var item in _items.WithCancellation(cancellationToken).ConfigureAwait(false))
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{
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// Start task immediately without waiting
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var task = _taskSelector(item);
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// Use Task.Run to ensure we don't block if _taskSelector is synchronous
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var task = Task.Run(async () =>
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{
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await Task.Yield();
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await _taskSelector(item).ConfigureAwait(false);
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}, cancellationToken);
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tasks.Add(task);
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}
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‎EnumerableAsyncProcessor/TaskWrapper.cs‎

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try
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{
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// Yield to ensure we don't block the calling thread if TaskFactory is synchronous
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await Task.Yield();
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var task = TaskFactory.Invoke();
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// Fast-path for already completed tasks
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try
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{
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// Yield to ensure we don't block the calling thread if TaskFactory is synchronous
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await Task.Yield();
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var task = TaskFactory.Invoke(Input);
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// Fast-path for already completed tasks
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try
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{
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// Yield to ensure we don't block the calling thread if TaskFactory is synchronous
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await Task.Yield();
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var task = TaskFactory.Invoke(Input);
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// Fast-path for already completed tasks
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try
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{
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// Yield to ensure we don't block the calling thread if TaskFactory is synchronous
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await Task.Yield();
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var task = TaskFactory.Invoke();
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// Fast-path for already completed tasks
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return;
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}
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// Yield to ensure we don't block the calling thread if TaskFactory is synchronous
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await Task.Yield();
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await TaskFactory.Invoke(Input).ConfigureAwait(false);
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}
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cancellationToken.ThrowIfCancellationRequested();
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}
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// Yield to ensure we don't block the calling thread if TaskFactory is synchronous
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await Task.Yield();
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return await TaskFactory.Invoke(Input).ConfigureAwait(false);
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}
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