How to properly run multiple async tasks in parallel?

Question

What if you need to run multiple asynchronous I/O tasks in parallel but need to make sure that no more than X I/O processes are running at the same time; and pre and post I/O processing tasks shouldn't have such limitation.

Here is a scenario - let's say there are 1000 tasks; each of them accepts a text string as an input parameter; transforms that text (pre I/O processing) then writes that transformed text into a file. The goal is to make pre-processing logic utilize 100% of CPU/Cores and I/O portion of the tasks run with max 10 degree of parallelism (max 10 simultaneously opened for writing files at a time).

Can you provide a sample code how to do it with C# / .NET 4.5?

http://blogs.msdn.com/b/csharpfaq/archive/2012/01/23/using-async-for-file-access-alan-berman.aspx

Answer 1

I think using TPL Dataflow for this would be a good idea: you create pre- and post-process blocks with unbounded parallelism, a file-writing block with limited parallelism and link them together. Something like:

var unboundedParallelismOptions =
    new ExecutionDataflowBlockOptions
    {
        MaxDegreeOfParallelism = DataflowBlockOptions.Unbounded
    };

var preProcessBlock = new TransformBlock<string, string>(
    s => PreProcess(s), unboundedParallelismOptions);

var writeToFileBlock = new TransformBlock<string, string>(
    async s =>
            {
                await WriteToFile(s);
                return s;
            },
    new ExecutionDataflowBlockOptions { MaxDegreeOfParallelism = 10 });

var postProcessBlock = new ActionBlock<string>(
    s => PostProcess(s), unboundedParallelismOptions);

var propagateCompletionOptions =
    new DataflowLinkOptions { PropagateCompletion = true };

preProcessBlock.LinkTo(writeToFileBlock, propagateCompletionOptions);
writeToFileBlock.LinkTo(postProcessBlock, propagateCompletionOptions);

// use something like await preProcessBlock.SendAsync("text") here

preProcessBlock.Complete();
await postProcessBlock.Completion;

Where WriteToFile() could look like this:

private static async Task WriteToFile(string s)
{
    using (var writer = new StreamWriter(GetFileName()))
        await writer.WriteAsync(s);
}