BufferBlock and ActionBlock with BoundedCapacity does not use max DOP

Question

I have this code:

var data = new BufferBlock<int>(new DataflowBlockOptions { BoundedCapacity = 1 });

var action = new ActionBlock<int>(async id =>
{
    Console.WriteLine("[{0:T}] #{1}: Start", DateTime.Now, id);

    await Task.Delay(1000);

    Console.WriteLine("[{0:T}] #{1}: End", DateTime.Now, id);
}, new ExecutionDataflowBlockOptions
{
    BoundedCapacity = 1,
    MaxDegreeOfParallelism = -1
});

data.LinkTo(action, new DataflowLinkOptions { PropagateCompletion = true });

for (var id = 1; id <= 3; id++)
{
    Console.WriteLine("[{0:T}] Sending {1}", DateTime.Now, id);
    data.SendAsync(id).Wait();
    Console.WriteLine("[{0:T}] Sending {1} complete", DateTime.Now, id);
}

data.Complete();

Task.WhenAll(data.Completion, action.Completion).Wait();

And this code gets me this output:

[22:31:22] Sending 1
[22:31:22] Sending 1 complete
[22:31:22] Sending 2
[22:31:22] #1: Start
[22:31:22] Sending 2 complete
[22:31:22] Sending 3
[22:31:23] #1: End
[22:31:23] #2: Start
[22:31:23] Sending 3 complete
[22:31:24] #2: End
[22:31:24] #3: Start
[22:31:25] #3: End

Why isn't ActionBlock working in parallel even though it has an unbounded DOP?

Answer 1

The reason your ActionBlock seems to be have a limited degree of parallelism is because it has a BoundedCapacity of 1. BoundedCapacity (unlike InputCount) includes the item being processed at the moment. This can be easily demonstrated:

var block = new ActionBlock<int>(_ => Task.Delay(-1), new ExecutionDataflowBlockOptions
{
    BoundedCapacity = 1,
    MaxDegreeOfParallelism = DataflowBlockOptions.Unbounded
});

await block.SendAsync(4); // Adds a new item
await block.SendAsync(4); // Blocks forever

That means that while you set MaxDegreeOfParallelism = DataflowBlockOptions.Unbounded the block can't accept more than a single item at the time and so practically limiting your degree of parallelism.

You can fix that by setting a larger BoundedCapacity:

var action = new ActionBlock<int>(async id =>
{
    Console.WriteLine("[{0:T}] #{1}: Start", DateTime.Now, id);
    await Task.Delay(1000);
    Console.WriteLine("[{0:T}] #{1}: End", DateTime.Now, id);
}, new ExecutionDataflowBlockOptions
{
    BoundedCapacity = 10,
    MaxDegreeOfParallelism = DataflowBlockOptions.Unbounded
});