SpinWait.cs source code in C# .NET

                        Code:
                         / 4.0 / 4.0 / untmp / DEVDIV_TFS / Dev10 / Releases / RTMRel / ndp / clr / src / BCL / System / Threading / SpinWait.cs / 1305376 / SpinWait.cs
                        
                        
                            // ==++== 
//
//   Copyright (c) Microsoft Corporation.  All rights reserved.
//
// ==--== 
// =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
// 
// SpinWait.cs 
//
// [....] 
//
// Central spin logic used across the entire code-base.
//
// =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- 

using System; 
using System.Runtime.ConstrainedExecution; 
using System.Security.Permissions;
using System.Threading; 
using System.Diagnostics.Contracts;

namespace System.Threading
{ 
    // SpinWait is just a little value type that encapsulates some common spinning
    // logic. It ensures we always yield on single-proc machines (instead of using busy 
    // waits), and that we work well on HT. It encapsulates a good mixture of spinning 
    // and real yielding. It's a value type so that various areas of the engine can use
    // one by allocating it on the stack w/out unnecessary GC allocation overhead, e.g.: 
    //
    //     void f() {
    //         SpinWait wait = new SpinWait();
    //         while (!p) { wait.SpinOnce(); } 
    //         ...
    //     } 
    // 
    // Internally it just maintains a counter that is used to decide when to yield, etc.
    // 
    // A common usage is to spin before blocking. In those cases, the NextSpinWillYield
    // property allows a user to decide to fall back to waiting once it returns true:
    //
    //     void f() { 
    //         SpinWait wait = new SpinWait();
    //         while (!p) { 
    //             if (wait.NextSpinWillYield) { /* block! */ } 
    //             else { wait.SpinOnce(); }
    //         } 
    //         ...
    //     }

    ///  
    /// Provides support for spin-based waiting.
    ///  
    ///  
    /// 
    ///  encapsulates common spinning logic. On single-processor machines, yields are 
    /// always used instead of busy waits, and on computers with Intelï¿½ processors employing Hyper-Threadingï¿½
    /// technology, it helps to prevent hardware thread starvation. SpinWait encapsulates a good mixture of
    /// spinning and true yielding.
    ///  
    /// 
    ///  is a value type, which means that low-level code can utilize SpinWait without 
    /// fear of unnecessary allocation overheads. SpinWait is not generally useful for ordinary applications. 
    /// In most cases, you should use the synchronization classes provided by the .NET Framework, such as
    /// . For most purposes where spin waiting is required, however, 
    /// the  type should be preferred over the  method.
    /// 
    ///  
    /// While SpinWait is designed to be used in concurrent applications, it is not designed to be
    /// used from multiple threads concurrently.  SpinWait's members are not thread-safe.  If multiple 
    /// threads must spin, each should use its own instance of SpinWait. 
    /// 
    ///  
    [HostProtection(Synchronization = true, ExternalThreading = true)]
    public struct SpinWait
    {
 
        // These constants determine the frequency of yields versus spinning. The
        // numbers may seem fairly arbitrary, but were derived with at least some 
        // thought in the design document.  I fully expect they will need to change 
        // over time as we gain more experience with performance.
        internal const int YIELD_THRESHOLD = 10; // When to switch over to a true yield. 
        internal const int SLEEP_0_EVERY_HOW_MANY_TIMES = 5; // After how many yields should we Sleep(0)?
        internal const int SLEEP_1_EVERY_HOW_MANY_TIMES = 20; // After how many yields should we Sleep(1)?

        // The number of times we've spun already. 
        private int m_count;
 
        ///  
        /// Gets the number of times  has been called on this instance.
        ///  
        public int Count
        {
            get { return m_count; }
        } 

        ///  
        /// Gets whether the next call to  will yield the processor, triggering a 
        /// forced context switch.
        ///  
        /// Whether the next call to  will yield the processor, triggering a
        /// forced context switch.
        /// 
        /// On a single-CPU machine,  always yields the processor. On machines with 
        /// multiple CPUs,  may yield after an unspecified number of calls.
        ///  
        public bool NextSpinWillYield 
        {
            get { return m_count > YIELD_THRESHOLD || PlatformHelper.IsSingleProcessor; } 
        }

        /// 
        /// Performs a single spin. 
        /// 
        ///  
        /// This is typically called in a loop, and may change in behavior based on the number of times a 
        ///  has been called thus far on this instance.
        ///  
        public void SpinOnce()
        {
            if (NextSpinWillYield)
            { 
                //
                // We must yield. 
                // 
                // We prefer to call Thread.Yield first, triggering a SwitchToThread. This
                // unfortunately doesn't consider all runnable threads on all OS SKUs. In 
                // some cases, it may only consult the runnable threads whose ideal processor
                // is the one currently executing code. Thus we oc----ionally issue a call to
                // Sleep(0), which considers all runnable threads at equal priority. Even this
                // is insufficient since we may be spin waiting for lower priority threads to 
                // execute; we therefore must call Sleep(1) once in a while too, which considers
                // all runnable threads, regardless of ideal processor and priority, but may 
                // remove the thread from the scheduler's queue for 10+ms, if the system is 
                // configured to use the (default) coarse-grained system timer.
                // 

#if !FEATURE_PAL && !FEATURE_CORECLR   // PAL doesn't support  eventing, and we don't compile CDS providers for Coreclr
                CdsSyncEtwBCLProvider.Log.SpinWait_NextSpinWillYield();
#endif 
                int yieldsSoFar = (m_count >= YIELD_THRESHOLD ? m_count - YIELD_THRESHOLD : m_count);
 
                if ((yieldsSoFar % SLEEP_1_EVERY_HOW_MANY_TIMES) == (SLEEP_1_EVERY_HOW_MANY_TIMES - 1)) 
                {
                    Thread.Sleep(1); 
                }
                else if ((yieldsSoFar % SLEEP_0_EVERY_HOW_MANY_TIMES) == (SLEEP_0_EVERY_HOW_MANY_TIMES - 1))
                {
                    Thread.Sleep(0); 
                }
                else 
                { 
#if PFX_LEGACY_3_5
                    Platform.Yield(); 
#else
                    Thread.Yield();
#endif
                } 
            }
            else 
            { 
                //
                // Otherwise, we will spin. 
                //
                // We do this using the CLR's SpinWait API, which is just a busy loop that
                // issues YIELD/PAUSE instructions to ensure multi-threaded CPUs can react
                // intelligently to avoid starving. (These are NOOPs on other CPUs.) We 
                // choose a number for the loop iteration count such that each successive
                // call spins for longer, to reduce cache contention.  We cap the total 
                // number of spins we are willing to tolerate to reduce delay to the caller, 
                // since we expect most callers will eventually block anyway.
                // 
                Thread.SpinWait(4 << m_count);
            }

            // Finally, increment our spin counter. 
            m_count = (m_count == int.MaxValue ? YIELD_THRESHOLD : m_count + 1);
        } 
 
        /// 
        /// Resets the spin counter. 
        /// 
        /// 
        /// This makes  and  behave as though no calls
        /// to  had been issued on this instance. If a  instance 
        /// is reused many times, it may be useful to reset it to avoid yielding too soon.
        ///  
        public void Reset() 
        {
            m_count = 0; 
        }

        #region Static Methods
#if !FEATURE_CORECLR 
        /// 
        /// Spins until the specified condition is satisfied. 
        ///  
        /// A delegate to be executed over and over until it returns true.
        /// The  argument is null. 
        public static void SpinUntil(Func condition)
        {
#if DEBUG
            bool result = 
#endif
            SpinUntil(condition, Timeout.Infinite); 
#if DEBUG 
            Contract.Assert(result);
#endif 
        }

        /// 
        /// Spins until the specified condition is satisfied or until the specified timeout is expired. 
        /// 
        /// A delegate to be executed over and over until it returns true. 
        ///  
        /// A  that represents the number of milliseconds to wait,
        /// or a TimeSpan that represents -1 milliseconds to wait indefinitely. 
        /// True if the condition is satisfied within the timeout; otherwise, false
        /// The  argument is null.
        ///  is a negative number
        /// other than -1 milliseconds, which represents an infinite time-out -or- timeout is greater than 
        /// .
        public static bool SpinUntil(Func condition, TimeSpan timeout) 
        { 
            // Validate the timeout
            Int64 totalMilliseconds = (Int64)timeout.TotalMilliseconds; 
            if (totalMilliseconds < -1 || totalMilliseconds > Int32.MaxValue)
            {
                throw new System.ArgumentOutOfRangeException(
                    "timeout", timeout, Environment.GetResourceString("SpinWait_SpinUntil_TimeoutWrong")); 
            }
 
            // Call wait with the timeout milliseconds 
            return SpinUntil(condition, (int)timeout.TotalMilliseconds);
        } 

        /// 
        /// Spins until the specified condition is satisfied or until the specified timeout is expired.
        ///  
        /// A delegate to be executed over and over until it returns true.
        /// The number of milliseconds to wait, or  (-1) to wait indefinitely. 
        /// True if the condition is satisfied within the timeout; otherwise, false
        /// The  argument is null. 
        ///  is a
        /// negative number other than -1, which represents an infinite time-out.
        public static bool SpinUntil(Func condition, int millisecondsTimeout)
        { 
            if (millisecondsTimeout < Timeout.Infinite)
            { 
                throw new ArgumentOutOfRangeException( 
                   "millisecondsTimeout", millisecondsTimeout, Environment.GetResourceString("SpinWait_SpinUntil_TimeoutWrong"));
            } 
            if (condition == null)
            {
                throw new ArgumentNullException("condition", Environment.GetResourceString("SpinWait_SpinUntil_ArgumentNull"));
            } 
            long startTicks = 0; ;
            if (millisecondsTimeout != 0 && millisecondsTimeout != Timeout.Infinite) 
            { 
                startTicks = DateTime.UtcNow.Ticks;
            } 
            SpinWait spinner = new SpinWait();
            while (!condition())
            {
                if (millisecondsTimeout == 0) 
                {
                    return false; 
                } 

                spinner.SpinOnce(); 

                if (millisecondsTimeout != Timeout.Infinite && spinner.NextSpinWillYield)
                {
                    if (millisecondsTimeout <= (DateTime.UtcNow.Ticks - startTicks) / TimeSpan.TicksPerMillisecond) 
                    {
                        return false; 
                    } 
                }
            } 
            return true;

        }
#endif //!FEATURE_CORECLR 
        #endregion
 
    } 

 
    /// 
    /// A helper class to get the number of preocessors, it updates the numbers of processors every sampling interval
    /// 
    internal static class PlatformHelper 
    {
        private const int PROCESSOR_COUNT_REFRESH_INTERVAL_MS = 30000; // How often to refresh the count, in milliseconds. 
        private static int s_processorCount = -1; // The last count seen. 
        private static DateTime s_nextProcessorCountRefreshTime = DateTime.MinValue; // The next time we'll refresh.
 
        /// 
        /// Gets the number of available processors
        /// 
        internal static int ProcessorCount 
        {
            get 
            { 
                if (DateTime.UtcNow.CompareTo(s_nextProcessorCountRefreshTime) >= 0)
                { 
                    s_processorCount = Environment.ProcessorCount;
                    s_nextProcessorCountRefreshTime = DateTime.UtcNow.AddMilliseconds(PROCESSOR_COUNT_REFRESH_INTERVAL_MS);
                }
 
                Contract.Assert(s_processorCount > 0 && s_processorCount <= 64,
                    "Processor count not within the expected range (1 - 64)."); 
 
                return s_processorCount;
            } 
        }

        /// 
        /// Gets whether the current machine has only a single processor. 
        /// 
        internal static bool IsSingleProcessor 
        { 
            get { return ProcessorCount == 1; }
        } 
    }

}

// File provided for Reference Use Only by Microsoft Corporation (c) 2007.
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SpinWait.cs source code in C# .NET

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