Batching and running tasks in parallel

I’ve had a few projects where I’ve had a large number of asyncronous tasks to execute but haven’t wanted to execute them all at once as this would create too many threads and would result in system degredation.

Initially I used something similar to the below to execute all tasks at once.

// Run all tasks
await Task.WhenAll(sources.Select(i => serviceProvider.GetService<App>().Run(i)).ToArray());

This starts a service for all items in a list which works fine when there’s only a small number of items that need processing.

I then split the list into chunks and processed each chunk seperly in a foreach loop using the above code, this again works fine but it can be inefficent if 9 out of 10 tasks started complet quickly as we will have to wait for the final task to complete before we can start the next batch.

Happily Microsoft have something called the Task Parallel Library (TPL) which is designed for just such workloads. I based the below code an an example I found here.

This works by creating action blocks which are then processed in a dataflow with the specified options, in this case the number of tasks executing in parallel is controlled by the MaxDegreeOfParallelism setting.

static async Task MainAsync()
	// Create service collection
	ServiceCollection serviceCollection = new ServiceCollection();

	// Create service provider
	IServiceProvider serviceProvider = serviceCollection.BuildServiceProvider();

	// Get backup sources for client
	List<String> sources = configuration.GetSection("Backup:Sources").GetChildren().Select(x => x.Value).ToList();

	// Create a block with an asynchronous action
	var block = new ActionBlock<string>(
		async x => await serviceProvider.GetService<App>().Run(x),
		new ExecutionDataflowBlockOptions
			MaxDegreeOfParallelism = int.Parse(configuration["Backup:MaxDegreeOfParallelism"])
			//MaxDegreeOfParallelism = Environment.ProcessorCount, // Parallelize on all cores

	// Add items to the block and asynchronously wait if BoundedCapacity is reached
	foreach (string source in sources)
		await block.SendAsync(source);

	await block.Completion;

A full working example can be found in this GitHub project.

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