c# - C# - ConcurrentBag 与 List 的性能比较

Question

前言：我之所以问这个只是因为我没有一个环境（足够大的数据集+计算能力）来以可靠的方式对其进行测试。

问题：给定一个Concurrent Bag，加载了数十亿个项目，由单个线程访问/使用，它的性能是否类似于List？换句话说，对 a 的枚举是否比对 a 的枚举Concurrent Bag更多或更少List<T>？

Answer 1

ConcurrentBag<T>将不可避免地低于List<T>. 尽管您只能从单个线程访问它，但该结构仍然需要有适当的机制来防止出现并发访问时出现竞争风险的可能性。

如果您将在开始枚举之前从单个线程加载集合，则可以通过使用构造函数来避免性能开销，而不是通过其方法ConcurrentBag(IEnumerable<T>)单独添加每个项目。Add

ConcurrentBag<T>为枚举提供“即时快照”语义；GetEnumerator其方法见备注。当您ConcurrentBag<T>从foreach循环访问时，它会首先将其全部内容复制到一个 plainList<T>中，然后对其进行枚举。每次在循环中使用它时，都会产生大量的性能开销（计算和内存方面）。

如果您的情况是您的列表将由多个线程填充，但随后只能由一个线程读取，那么您应该List<T>在编写器完成后立即将其转换为 a。

Answer 2

数十亿的物品和列表或并发包？那是“不行”。

就性能而言，试试这个来测试添加：（随意修改它来测试其他操作）

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace ConcurrentBagTest
{
    // You must compile this for x64 or you will get OutOfMemory exception
    class Program
    {
        static void Main(string[] args)
        {
            ListTest(10000000);
            ListTest(100000000);
            ListTest(1000000000);
            ConcurrentBagTest(10000000);
            ConcurrentBagTest(100000000);

            Console.ReadKey();

        }


        static void ConcurrentBagTest(long count)
        {
            try
            {
                var bag = new ConcurrentBag<long>();
                Console.WriteLine($"--- ConcurrentBagTest count = {count}");
                Console.WriteLine($"I will use {(count * sizeof(long)) / Math.Pow(1024, 2)} MiB of RAM");
                Stopwatch stopwatch = new Stopwatch();
                stopwatch.Start();
                for (long i = 0; i < count; i++)
                {
                    bag.Add(i);
                }
                stopwatch.Stop();
                Console.WriteLine($"Inserted {bag.LongCount()} items in {stopwatch.Elapsed.TotalSeconds} s");
                Console.WriteLine();
                Console.WriteLine();
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.ToString());
            }
            GC.Collect();
            GC.WaitForPendingFinalizers();
        }

        static void ListTest(long count)
        {
            try
            {
                var list = new List<long>();
                Console.WriteLine($"--- ListTest count = {count}");
                Console.WriteLine($"I will use {(count * sizeof(long)) / Math.Pow(1024, 2)} MiB of RAM");
                Stopwatch stopwatch = new Stopwatch();
                stopwatch.Start();
                for (long i = 0; i < count; i++)
                {
                    list.Add(i);
                }
                stopwatch.Stop();
                Console.WriteLine($"Inserted {list.LongCount()} items in {stopwatch.Elapsed.TotalSeconds} s");
                Console.WriteLine();
                Console.WriteLine();
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.ToString());
            }
            GC.Collect();
            GC.WaitForPendingFinalizers();
        }
    }
}

我的输出：

--- ListTest count = 10000000
I will use 76,2939453125 MiB of RAM
Inserted 10000000 items in 0,0807315 s


--- ListTest count = 100000000
I will use 762,939453125 MiB of RAM
Inserted 100000000 items in 0,7741546 s


--- ListTest count = 1000000000
I will use 7629,39453125 MiB of RAM
System.OutOfMemoryException: Array dimensions exceeded supported range.

--- ConcurrentBagTest count = 10000000
I will use 76,2939453125 MiB of RAM
Inserted 10000000 items in 1,0744069 s


--- ConcurrentBagTest count = 100000000
I will use 762,939453125 MiB of RAM
Inserted 100000000 items in 11,3976436 s

使用 CPU：Intel Core i7-2600 @ 3.4 GHz，

使用内存：16 GB

另请查看此答案以了解限制。

Answer 3

但是，如果您需要删除项目，ConcurrentBag 比 List 快得多

void Main()
{
    ConcurrentBag<int> bag = new ConcurrentBag<int>();
    ConcurrentStack<int> stack = new ConcurrentStack<int>();
    ConcurrentQueue<int> q = new ConcurrentQueue<int>();
    List<int> list = new List<int>();

    Stopwatch sw = new Stopwatch();
    int count = 100000;
    sw.Start();
    for (int i = 0; i < count; i++)
    {
        bag.Add(i);
    }
    for (int i = 0; i< count; i++)
    {
        bag.TryTake(out _);
    }
    sw.Elapsed.Dump("BAG");
    sw.Start();
    for (int i = 0; i < count; i++)
    {
        stack.Push(i);
    }
    for (int i = 0; i < count; i++)
    {
        stack.TryPop(out _);
    }
    sw.Elapsed.Dump("Stack");
    sw.Start();
    for (int i = 0; i < count; i++)
    {
        q.Enqueue(i);
    }
    for (int i = 0; i < count; i++)
    {
        q.TryDequeue(out _);
    }
    sw.Elapsed.Dump("Q");
    sw.Start();
    for (int i = 0; i < count; i++)
    {
        list.Add(i);
    }
    for (int i = 0; i < count; i++)
    {
        list.RemoveAt(0);
    }
    sw.Elapsed.Dump("list remove at 0");
    sw.Start();
    for (int i = 0; i < count; i++)
    {
        list.Add(i);
    }
    for (int i = 0; i < count; i++)
    {
        list.RemoveAt(list.Count -1);
    }
    sw.Elapsed.Dump("list remove at end");
}

结果：

袋 00:00:00.0144421

堆栈 00:00:00.0341379

问 00:00:00.0400114

在 0 00:00:00.6188329删除列表

列表在末尾删除 00:00:00.6202170