具有 BoundedCapacity 的 BufferBlock 和 ActionBlock 不使用最大 DOP

Question

我有这个代码：

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();

这段代码让我得到这个输出：

[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

为什么ActionBlock 没有并行工作，即使它具有无限的 DOP？

Answer 1

您的ActionBlock 似乎具有有限的并行度的原因是它的BoundedCapacity 为1。BoundedCapacity（与InputCount 不同）包括目前正在处理的项目。这很容易证明：

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

这意味着当您设置 MaxDegreeOfParallelism = DataflowBlockOptions.Unbounded 时，该块同时不能接受多个项目，因此实际上限制了您的并行度。

你可以通过设置一个更大的BoundedCapacity来解决这个问题：

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
});