CODEWITHSUNDEEP
c#.netmultithreadingperformance
Cronan
Cronan

Reputation: 193

Poor multithreading performance in .Net

Why would I be getting such poor performance from the code below?

The following command line uses 16 threads, with a load of 60. On my machine this takes approximately 31 seconds to finish (with some slight variations if you rerun)

testapp.exe 16 60

Using a load of 60, on Microsoft Windows Server 2008 R2 Enterprise SP1, running on 16 Intel Xeon E5-2670 @ 2.6 GHz CPUs I get the following performance:

1 cpu - 305 seconds

2 cpus - 155 seconds

4 cpus - 80 seconds

8 cpus - 45 seconds

10 cpus - 41 seconds

12 cpus - 37 seconds

14 cpus - 34 seconds

16 cpus - 31 seconds

18 cpus - 27 seconds

20 cpus - 24 seconds

22 cpus - 23 seconds

24 cpus - 21 seconds

26 cpus - 20 seconds

28 cpus - 19 seconds

After this it flat-lines ...

I get approximately the same performance using .Net 3.5, 4, 4.5 or 4.5.1.

I understand the drop-off in performance after 22 cpus, as I only have 16 on the box. What I don't understand is the poor performance after 8 cpus. Can anyone explain? Is this normal?

private static void Main(string[] args)
{
    int threadCount;
    if (args == null || args.Length < 1 || !int.TryParse(args[0], out threadCount))
        threadCount = Environment.ProcessorCount;

    int load;
    if (args == null || args.Length < 2 || !int.TryParse(args[1], out load))
        load = 1;

    Console.WriteLine("ThreadCount:{0} Load:{1}", threadCount, load);

    List<Thread> threads = new List<Thread>();

    for (int i = 0; i < threadCount; i++)
    {
        int i1 = i;
        threads.Add(new Thread(() => DoWork(i1, threadCount, load)));
    }

    Stopwatch timer = Stopwatch.StartNew();

    foreach (var thread in threads)
    {
        thread.Start();
    }

    foreach (var thread in threads)
    {
        thread.Join();
    }

    timer.Stop();

    Console.WriteLine("Time:{0} seconds", timer.ElapsedMilliseconds/1000.0);
}

static void DoWork(int seed, int threadCount, int load)
{
    double[,] mtx = new double[3,3];

    for (int i = 0; i < ((100000 * load)/threadCount); i++)
    {
        for (int j = 0; j < 100; j++)
        {
            mtx = new double[3,3];

            for (int k = 0; k < 3; k++)
            {
                for (int l = 0; l < 3; l++)
                {
                    mtx[k, l] = Math.Sin(j + (k*3) + l + seed);
                }
            }
        }
    }
}

Upvotes: 0

Views: 1012

Answers (2)

Yazan Ati
Yazan Ati

Reputation: 60

It is not that the threads that causes the performance to go down. But it is the "creation" of the thread itself.

Instead of creating a brand new thread, you need to borrow an already created thread form the OS thread pool. Use ThreadPool class instead of using new Thread()

Upvotes: 1

Luis Ramirez
Luis Ramirez

Reputation: 424

Please refer to the Intel ARK for the XEON E5-2670

This particular processor has 8 physical cores which are hyper-threaded. This is why you see a performance drop after 8 threads. Calling Environment.ProcessorCount gets 16 logical cores (2 logical cores per physical core because they are hyperthreaded).

A similar question has been answered on SuperUser.

You can try to set the affinity of the threads see if it makes a difference, but the scheduler usually does a good job of allocating resource.

Hope this helps.

Upvotes: 1

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