基于 CAS 无锁实现的 Disruptor.NET 居然慢于 BlockingCollection，是真的吗？

StackOverflow 有人说自己的 Disruptor.NET 代码比 BlockingCollection 还有慢 2 倍，并且把完整代码贴出，楼下几个老外也的回复说了一堆，但是没研究出个所以然来，讨论到最后甚至说可能你的场景不适合 Disruptor，我对此表示怀疑，BlockingCollection 内部实现比较简单粗暴，必要时就加锁，取数据时用信号量等待添加操作完成，而 Disruptor 是专门针对 CPU 缓存的特性优化过的，内部没有锁只有 CAS 原子操作，而且还考虑到了 false sharing，因此理论上 Disruptor 不会比 BlockingCollection 慢。

可是既然实际应用上出现问题，那就要分析下原因了。

把他的代码弄下来看了一下，问题多多啊。

在 Disruptor EventHandler 里面不定时调用 Console.WriteLine ，但是在 BlockingCollection 的 Handler 里面却只是记录了数据， Console.WriteLine 内部可是有锁的，调用的开销很大，如何能取得公平的结果呢？

另外 RingBuffer 的 Size 太小只有 64，严重影响 Disruptor 的表现，实际测试对比下来，应该 1024 或更大。

还有 BlockingCollection 里面的 while (!dataItems.IsCompleted) 写的也有问题，即使 BlockingCollection Producer 在循环中一直做添加操作，BlockingCollection 内部状态也并不是一直在添加状态中，这样导致添加循环还没做完，可是计时器的循环已经提前结束，导致 BlockingCollection 测得时间少于实际时间。

Task.Factory.StartNew(() => {
    while (!dataItems.IsCompleted)
    {

        ValueEntry ve = null;
        try
        {
    ve = dataItems.Take();
    long microseconds = sw[ve.Value].ElapsedTicks / (Stopwatch.Frequency / (1000L * 1000L));
    results[ve.Value] = microseconds;

    //Console.WriteLine("elapsed microseconds = " + microseconds);
    //Console.WriteLine("Event handled: Value = {0} (processed event {1}", ve.Value, ve.Value);
        }
        catch (InvalidOperationException) { }
    }
}, TaskCreationOptions.LongRunning);


for (int i = 0; i < length; i++)
{
    var valueToSet = i;

    ValueEntry entry = new ValueEntry();
    entry.Value = valueToSet;

    sw[i].Restart();
    dataItems.Add(entry);

    //Console.WriteLine("Published entry {0}, value {1}", valueToSet, entry.Value);
    //Thread.Sleep(1000);
}

然后重新修改了他的代码，实测 Disruptor 10 倍速度于 BlockingCollection （这里插一句题外话，Disruptor .NET 版本的速度全面快于 Java 版本，不少场景下的速度比 Java 版本要快 10 倍，.NET 版是从 Java 移植过来的实现也和 Java 保持一直，是哪些语言特性导致性能差异这么大呢？）。

然后我拿着实测的结果和修改后的代码，在 Stackoverflow 上这个问题下面贴上了我的回答：

I read the BlockingCollecton code, You add many Console.WriteLine in Disruptor but no one in BlockingCollection, Console.WriteLine is slow, it have a lock inside.

Your RingBufferSize is too small, this effects performance, shoule be 1024 or larger.

and while (!dataItems.IsCompleted) may have some problem, BlockCollection isn't always in adding state, it will cause thread ends early.

I have rewrite you code, Disruptor is 10x faster than BlockingCollection with multi producer (10 parallel producet), 2x faster than BlockingCollection with Single producer:

using System;
using System.Collections.Concurrent;
using System.Diagnostics;
using System.Threading;
using System.Threading.Tasks;
using Disruptor;
using Disruptor.Dsl;
using NUnit.Framework;

namespace DisruptorTest.Ds
{
    public sealed class ValueEntry
    {
        internal int Id { get; set; }
    }

    class MyHandler : IEventHandler<ValueEntry>
    {
        public void OnEvent(ValueEntry data, long sequence, bool endOfBatch)
        {
        }
    }

    [TestFixture]
    public class DisruptorPerformanceTest
    {
        private volatile bool collectionAddEnded;

        private int producerCount = 10;
        private int runCount = 1000000;
        private int RingBufferAndCapacitySize = 1024;

        [TestCase()]
        public async Task TestBoth()
        {
            for (int i = 0; i < 1; i++)
            {
                foreach (var rs in new int[] {64, 512, 1024, 2048 /*,4096,4096*2*/})
                {
                    Console.WriteLine($"RingBufferAndCapacitySize:{rs}, producerCount:{producerCount}, runCount:{runCount} of {i}");
                    RingBufferAndCapacitySize = rs;
                    await DisruptorTest();
                    await BlockingCollectionTest();
                }
            }
        }

        [TestCase()]
        public async Task BlockingCollectionTest()
        {
            var sw = new Stopwatch();
            BlockingCollection<ValueEntry> dataItems = new BlockingCollection<ValueEntry>(RingBufferAndCapacitySize);

            sw.Start();

            collectionAddEnded = false;

            // A simple blocking consumer with no cancellation.
            var task = Task.Factory.StartNew(() =>
            {
                while (!collectionAddEnded && !dataItems.IsCompleted)
                {
                    //if (!dataItems.IsCompleted && dataItems.TryTake(out var ve))
                    if (dataItems.TryTake(out var ve))
                    {
                    }
                }
            }, TaskCreationOptions.LongRunning);


            var tasks = new Task[producerCount];
            for (int t = 0; t < producerCount; t++)
            {
                tasks[t] = Task.Run(() =>
                {
                    for (int i = 0; i < runCount; i++)
                    {
                        ValueEntry entry = new ValueEntry();
                        entry.Id = i;
                        dataItems.Add(entry);
                    }
                });
            }

            await Task.WhenAll(tasks);

            collectionAddEnded = true;
            await task;

            sw.Stop();

            Console.WriteLine($"BlockingCollectionTest Time:{sw.ElapsedMilliseconds/1000d}");
        }


        [TestCase()]
        public async Task DisruptorTest()
        {
            var disruptor =
                new Disruptor.Dsl.Disruptor<ValueEntry>(() => new ValueEntry(), RingBufferAndCapacitySize, TaskScheduler.Default,
                    producerCount > 1 ? ProducerType.Multi : ProducerType.Single, new BlockingWaitStrategy());
            disruptor.HandleEventsWith(new MyHandler());

            var _ringBuffer = disruptor.Start();

            Stopwatch sw = Stopwatch.StartNew();

            sw.Start();


            var tasks = new Task[producerCount];
            for (int t = 0; t < producerCount; t++)
            {
                tasks[t] = Task.Run(() =>
                {
                    for (int i = 0; i < runCount; i++)
                    {
                        long sequenceNo = _ringBuffer.Next();
                        _ringBuffer[sequenceNo].Id = 0;
                        _ringBuffer.Publish(sequenceNo);
                    }
                });
            }


            await Task.WhenAll(tasks);


            disruptor.Shutdown();

            sw.Stop();
            Console.WriteLine($"DisruptorTest Time:{sw.ElapsedMilliseconds/1000d}s");
        }
    }
}

BlockingCollectionTest with a shared ValueEntry instance (no new ValueEntry() in for loop)

• RingBufferAndCapacitySize:64, producerCount:10, runCount:1000000 of 0

  DisruptorTest Time:16.962s

  BlockingCollectionTest Time:18.399

• RingBufferAndCapacitySize:512, producerCount:10, runCount:1000000 of 0 DisruptorTest Time:6.101s

  BlockingCollectionTest Time:19.526

• RingBufferAndCapacitySize:1024, producerCount:10, runCount:1000000 of 0

  DisruptorTest Time:2.928s

  BlockingCollectionTest Time:20.25

• RingBufferAndCapacitySize:2048, producerCount:10, runCount:1000000 of 0

  DisruptorTest Time:2.448s

  BlockingCollectionTest Time:20.649

BlockingCollectionTest create a new ValueEntry() in for loop

• RingBufferAndCapacitySize:64, producerCount:10, runCount:1000000 of 0

  DisruptorTest Time:27.374s

  BlockingCollectionTest Time:21.955

• RingBufferAndCapacitySize:512, producerCount:10, runCount:1000000 of 0

  DisruptorTest Time:5.011s

  BlockingCollectionTest Time:20.127

• RingBufferAndCapacitySize:1024, producerCount:10, runCount:1000000 of 0

  DisruptorTest Time:2.877s

  BlockingCollectionTest Time:22.656

• RingBufferAndCapacitySize:2048, producerCount:10, runCount:1000000 of 0

  DisruptorTest Time:2.384s

  BlockingCollectionTest Time:23.567