SpinWait.cs

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// SpinWait.cs
//
// Copyright (c) 2008 Jérémie "Garuma" Laval
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
//
//

#if NET_4_0
using System;

namespace System.Threading
{
    public struct SpinWait
    {
        // The number of step until SpinOnce yield on multicore machine
        const           int  step = 10;
        const           int  maxTime = 200;
        static readonly bool isSingleCpu = (Environment.ProcessorCount == 1);

        int ntime;

        public void SpinOnce ()
        {
            ntime += 1;

            if (isSingleCpu) {
                // On a single-CPU system, spinning does no good
                Yield ();
            } else {
                if (ntime % step == 0)
                    Yield ();
                else
                    // Multi-CPU system might be hyper-threaded, let other thread run
                    Thread.SpinWait (Math.Min (ntime, maxTime) << 1);
            }
        }
        
        // Spicy Pixel: Thread.Yield () is not supported in .NET < 4
        const int s1step = 20;
        int s1time;
        void Yield ()
        {
            // Replace sched_yield by Thread.Sleep(0) which does almost the same thing
            // (going back in kernel mode and yielding) but avoid the branching and unmanaged bridge
            //Thread.Sleep (0); // Thread.Yield() is better

            /*
                http://stackoverflow.com/questions/1413630/switchtothread-thread-yield-vs-thread-sleep0-vs-thead-sleep1
                
                SwitchToThread [win32] / Thread.Yield [.NET 4 Beta 1]: yields to any thread on same processor
                Advantage: about twice as fast as Thread.Sleep(0)
                Disadvantage: yields only to threads on same processor
                
                Thread.Sleep(0): yields to any thread of same or higher priority on any processor
                Advantage: faster than Thread.Sleep(1)
                Disadvantage: yields only to threads of same or higher priority
                
                Thread.Sleep(1): yields to any thread on any processor
                Advantage: yields to any thread on any processor
                Disadvantage: slowest option (Thread.Sleep(1) will usually suspend the thread by about 15ms if timeBeginPeriod/timeEndPeriod [win32] are not used)
            */

            // Because Sleep(1) pulls us out of the queue for 10-15ms or so, 
            // don't do it that often.
            s1time += 1;

            if(s1time % s1step == 0)
                Thread.Sleep(1);
            else
                Thread.Sleep(0);
        }

        public static void SpinUntil (Func<bool> condition)
        {
            SpinWait sw = new SpinWait ();
            while (!condition ())
                sw.SpinOnce ();
        }

        public static bool SpinUntil (Func<bool> condition, TimeSpan timeout)
        {
            return SpinUntil (condition, (int)timeout.TotalMilliseconds);
        }

        public static bool SpinUntil (Func<bool> condition, int millisecondsTimeout)
        {
            SpinWait sw = new SpinWait ();
            Watch watch = Watch.StartNew ();

            while (!condition ()) {
                if (watch.ElapsedMilliseconds > millisecondsTimeout)
                    return false;
                sw.SpinOnce ();
            }

            return true;
        }

        public void Reset ()
        {
            ntime = 0;
            s1time = 0;
        }

        public bool NextSpinWillYield {
            get {
                return isSingleCpu ? true : ntime % step == 0;
            }
        }

        public int Count {
            get {
                return ntime;
            }
        }
    }
}
#endif