How do I spawn threads on different CPU cores?

Question

Let\'s say I had a program in C# that did something computationally expensive, like encoding a list of WAV files into MP3s. Ordinarily I would encode the files one at a time

Answer 1

Although I agree with most of the answers here, I think it's worth it to add a new consideration: Speedstep technology.

When running a CPU intensive, single threaded job on a multi-core system, in my case a Xeon E5-2430 with 6 real cores (12 with HT) under windows server 2012, the job got spread out among all the 12 cores, using around 8.33% of each core and never triggering a speed increase. The CPU remained at 1.2 GHz.

When I set the thread affinity to a specific core, it used ~100% of that core, causing the CPU to max out at 2.5 GHz, more than doubling the performance.

This is the program I used, which just loops increasing a variable. When called with -a, it will set the affinity to core 1. The affinity part was based on this post.

using System;
using System.Diagnostics;
using System.Linq;
using System.Runtime.InteropServices;
using System.Threading;

namespace Esquenta
{
    class Program
    {
        private static int numThreads = 1;
        static bool affinity = false;
        static void Main(string[] args)
        {
            if (args.Contains("-a"))
            {
                affinity = true;
            }
            if (args.Length < 1 || !int.TryParse(args[0], out numThreads))
            {
                numThreads = 1;
            }
            Console.WriteLine("numThreads:" + numThreads);
            for (int j = 0; j < numThreads; j++)
            {
                var param = new ParameterizedThreadStart(EsquentaP);
                var thread = new Thread(param);
                thread.Start(j);
            }

        }

        static void EsquentaP(object numero_obj)
        {
            int i = 0;
            DateTime ultimo = DateTime.Now;
            if(affinity)
            {
                Thread.BeginThreadAffinity();
                CurrentThread.ProcessorAffinity = new IntPtr(1);
            }
            try
            {
                while (true)
                {
                    i++;
                    if (i == int.MaxValue)
                    {
                        i = 0;
                        var lps = int.MaxValue / (DateTime.Now - ultimo).TotalSeconds / 1000000;
                        Console.WriteLine("Thread " + numero_obj + " " + lps.ToString("0.000") + " M loops/s");
                        ultimo = DateTime.Now;
                    }
                }
            }
            finally
            {
                Thread.EndThreadAffinity();
            }
        }

        [DllImport("kernel32.dll")]
        public static extern int GetCurrentThreadId();

        [DllImport("kernel32.dll")]
        public static extern int GetCurrentProcessorNumber();
        private static ProcessThread CurrentThread
        {
            get
            {
                int id = GetCurrentThreadId();
                return Process.GetCurrentProcess().Threads.Cast().Single(x => x.Id == id);
            }
        }
    }
}

And the results:

Processor speed, as shown by Task manager, similar to what CPU-Z reports: