mono/netcore/System.Private.CoreLib/shared/System/Threading/SpinWait.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

// =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
//
// Central spin logic used across the entire code-base.
//
// =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-

using System.Diagnostics;

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(); }
    //         }
    //         ...
    //     }

    /// <summary>
    /// Provides support for spin-based waiting.
    /// </summary>
    /// <remarks>
    /// <para>
    /// <see cref="SpinWait"/> encapsulates common spinning logic. On single-processor machines, yields are
    /// always used instead of busy waits, and on computers with Intel(R) processors employing Hyper-Threading
    /// technology, it helps to prevent hardware thread starvation. SpinWait encapsulates a good mixture of
    /// spinning and true yielding.
    /// </para>
    /// <para>
    /// <see cref="SpinWait"/> 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
    /// <see cref="System.Threading.Monitor"/>. For most purposes where spin waiting is required, however,
    /// the <see cref="SpinWait"/> type should be preferred over the <see
    /// cref="System.Threading.Thread.SpinWait"/> method.
    /// </para>
    /// <para>
    /// 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.
    /// </para>
    /// </remarks>
    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 YieldThreshold = 10; // When to switch over to a true yield.
        private const int Sleep0EveryHowManyYields = 5; // After how many yields should we Sleep(0)?
        internal const int DefaultSleep1Threshold = 20; // After how many yields should we Sleep(1) frequently?

        /// <summary>
        /// A suggested number of spin iterations before doing a proper wait, such as waiting on an event that becomes signaled
        /// when the resource becomes available.
        /// </summary>
        /// <remarks>
        /// These numbers were arrived at by experimenting with different numbers in various cases that currently use it. It's
        /// only a suggested value and typically works well when the proper wait is something like an event.
        /// 
        /// Spinning less can lead to early waiting and more context switching, spinning more can decrease latency but may use
        /// up some CPU time unnecessarily. Depends on the situation too, for instance SemaphoreSlim uses more iterations
        /// because the waiting there is currently a lot more expensive (involves more spinning, taking a lock, etc.). It also
        /// depends on the likelihood of the spin being successful and how long the wait would be but those are not accounted
        /// for here.
        /// </remarks>
        internal static readonly int SpinCountforSpinBeforeWait = PlatformHelper.IsSingleProcessor ? 1 : 35;

        // The number of times we've spun already.
        private int _count;

        /// <summary>
        /// Gets the number of times <see cref="SpinOnce()"/> has been called on this instance.
        /// </summary>
        public int Count
        {
            get => _count;
            internal set
            {
                Debug.Assert(value >= 0);
                _count = value;
            }
        }

        /// <summary>
        /// Gets whether the next call to <see cref="SpinOnce()"/> will yield the processor, triggering a
        /// forced context switch.
        /// </summary>
        /// <value>Whether the next call to <see cref="SpinOnce()"/> will yield the processor, triggering a
        /// forced context switch.</value>
        /// <remarks>
        /// On a single-CPU machine, <see cref="SpinOnce()"/> always yields the processor. On machines with
        /// multiple CPUs, <see cref="SpinOnce()"/> may yield after an unspecified number of calls.
        /// </remarks>
        public bool NextSpinWillYield => _count >= YieldThreshold || PlatformHelper.IsSingleProcessor;

        /// <summary>
        /// Performs a single spin.
        /// </summary>
        /// <remarks>
        /// This is typically called in a loop, and may change in behavior based on the number of times a
        /// <see cref="SpinOnce()"/> has been called thus far on this instance.
        /// </remarks>
        public void SpinOnce()
        {
            SpinOnceCore(DefaultSleep1Threshold);
        }

        /// <summary>
        /// Performs a single spin.
        /// </summary>
        /// <param name="sleep1Threshold">
        /// A minimum spin count after which <code>Thread.Sleep(1)</code> may be used. A value of <code>-1</code> may be used to
        /// disable the use of <code>Thread.Sleep(1)</code>.
        /// </param>
        /// <exception cref="ArgumentOutOfRangeException">
        /// <paramref name="sleep1Threshold"/> is less than <code>-1</code>.
        /// </exception>
        /// <remarks>
        /// This is typically called in a loop, and may change in behavior based on the number of times a
        /// <see cref="SpinOnce()"/> has been called thus far on this instance.
        /// </remarks>
        public void SpinOnce(int sleep1Threshold)
        {
            if (sleep1Threshold < -1)
            {
                throw new ArgumentOutOfRangeException(nameof(sleep1Threshold), sleep1Threshold, SR.ArgumentOutOfRange_NeedNonNegOrNegative1);
            }

            if (sleep1Threshold >= 0 && sleep1Threshold < YieldThreshold)
            {
                sleep1Threshold = YieldThreshold;
            }

            SpinOnceCore(sleep1Threshold);
        }

        private void SpinOnceCore(int sleep1Threshold)
        {
            Debug.Assert(sleep1Threshold >= -1);
            Debug.Assert(sleep1Threshold < 0 || sleep1Threshold >= YieldThreshold);

            // (_count - YieldThreshold) % 2 == 0: The purpose of this check is to interleave Thread.Yield/Sleep(0) with
            // Thread.SpinWait. Otherwise, the following issues occur:
            //   - When there are no threads to switch to, Yield and Sleep(0) become no-op and it turns the spin loop into a
            //     busy-spin that may quickly reach the max spin count and cause the thread to enter a wait state, or may
            //     just busy-spin for longer than desired before a Sleep(1). Completing the spin loop too early can cause
            //     excessive context switcing if a wait follows, and entering the Sleep(1) stage too early can cause
            //     excessive delays.
            //   - If there are multiple threads doing Yield and Sleep(0) (typically from the same spin loop due to
            //     contention), they may switch between one another, delaying work that can make progress.
            if ((
                    _count >= YieldThreshold &&
                    ((_count >= sleep1Threshold && sleep1Threshold >= 0) || (_count - YieldThreshold) % 2 == 0)
                ) ||
                PlatformHelper.IsSingleProcessor)
            {
                //
                // 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 occasionally 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 (_count >= sleep1Threshold && sleep1Threshold >= 0)
                {
                    Thread.Sleep(1);
                }
                else
                {
                    int yieldsSoFar = _count >= YieldThreshold ? (_count - YieldThreshold) / 2 : _count;
                    if ((yieldsSoFar % Sleep0EveryHowManyYields) == (Sleep0EveryHowManyYields - 1))
                    {
                        Thread.Sleep(0);
                    }
                    else
                    {
                        Thread.Yield();
                    }
                }
            }
            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.
                //
                // Also, cap the maximum spin count to a value such that many thousands of CPU cycles would not be wasted doing
                // the equivalent of YieldProcessor(), as at that point SwitchToThread/Sleep(0) are more likely to be able to
                // allow other useful work to run. Long YieldProcessor() loops can help to reduce contention, but Sleep(1) is
                // usually better for that.
                //
                // Thread.OptimalMaxSpinWaitsPerSpinIteration:
                //   - See Thread::InitializeYieldProcessorNormalized(), which describes and calculates this value.
                //
                int n = Thread.OptimalMaxSpinWaitsPerSpinIteration;
                if (_count <= 30 && (1 << _count) < n)
                {
                    n = 1 << _count;
                }
                Thread.SpinWait(n);
            }

            // Finally, increment our spin counter.
            _count = (_count == int.MaxValue ? YieldThreshold : _count + 1);
        }

        /// <summary>
        /// Resets the spin counter.
        /// </summary>
        /// <remarks>
        /// This makes <see cref="SpinOnce()"/> and <see cref="NextSpinWillYield"/> behave as though no calls
        /// to <see cref="SpinOnce()"/> had been issued on this instance. If a <see cref="SpinWait"/> instance
        /// is reused many times, it may be useful to reset it to avoid yielding too soon.
        /// </remarks>
        public void Reset()
        {
            _count = 0;
        }

        #region Static Methods
        /// <summary>
        /// Spins until the specified condition is satisfied.
        /// </summary>
        /// <param name="condition">A delegate to be executed over and over until it returns true.</param>
        /// <exception cref="ArgumentNullException">The <paramref name="condition"/> argument is null.</exception>
        public static void SpinUntil(Func<bool> condition)
        {
#if DEBUG
            bool result =
#endif
            SpinUntil(condition, Timeout.Infinite);
#if DEBUG
            Debug.Assert(result);
#endif
        }

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

            // Call wait with the timeout milliseconds
            return SpinUntil(condition, (int)totalMilliseconds);
        }

        /// <summary>
        /// Spins until the specified condition is satisfied or until the specified timeout is expired.
        /// </summary>
        /// <param name="condition">A delegate to be executed over and over until it returns true.</param>
        /// <param name="millisecondsTimeout">The number of milliseconds to wait, or <see
        /// cref="System.Threading.Timeout.Infinite"/> (-1) to wait indefinitely.</param>
        /// <returns>True if the condition is satisfied within the timeout; otherwise, false</returns>
        /// <exception cref="ArgumentNullException">The <paramref name="condition"/> argument is null.</exception>
        /// <exception cref="T:System.ArgumentOutOfRangeException"><paramref name="millisecondsTimeout"/> is a
        /// negative number other than -1, which represents an infinite time-out.</exception>
        public static bool SpinUntil(Func<bool> condition, int millisecondsTimeout)
        {
            if (millisecondsTimeout < Timeout.Infinite)
            {
                throw new ArgumentOutOfRangeException(
                   nameof(millisecondsTimeout), millisecondsTimeout, SR.SpinWait_SpinUntil_TimeoutWrong);
            }
            if (condition == null)
            {
                throw new ArgumentNullException(nameof(condition), SR.SpinWait_SpinUntil_ArgumentNull);
            }
            uint startTime = 0;
            if (millisecondsTimeout != 0 && millisecondsTimeout != Timeout.Infinite)
            {
                startTime = TimeoutHelper.GetTime();
            }
            SpinWait spinner = new SpinWait();
            while (!condition())
            {
                if (millisecondsTimeout == 0)
                {
                    return false;
                }

                spinner.SpinOnce();

                if (millisecondsTimeout != Timeout.Infinite && spinner.NextSpinWillYield)
                {
                    if (millisecondsTimeout <= (TimeoutHelper.GetTime() - startTime))
                    {
                        return false;
                    }
                }
            }
            return true;
        }
        #endregion
    }

    /// <summary>
    /// A helper class to get the number of processors, it updates the numbers of processors every sampling interval.
    /// </summary>
    internal static class PlatformHelper
    {
        private const int PROCESSOR_COUNT_REFRESH_INTERVAL_MS = 30000; // How often to refresh the count, in milliseconds.
        private static volatile int s_processorCount; // The last count seen.
        private static volatile int s_lastProcessorCountRefreshTicks; // The last time we refreshed.

        /// <summary>
        /// Gets the number of available processors
        /// </summary>
        internal static int ProcessorCount
        {
            get
            {
                int now = Environment.TickCount;
                int procCount = s_processorCount;
                if (procCount == 0 || (now - s_lastProcessorCountRefreshTicks) >= PROCESSOR_COUNT_REFRESH_INTERVAL_MS)
                {
                    s_processorCount = procCount = Environment.ProcessorCount;
                    s_lastProcessorCountRefreshTicks = now;
                }

                Debug.Assert(procCount > 0,
                    "Processor count should be greater than 0.");

                return procCount;
            }
        }

        /// <summary>
        /// Gets whether the current machine has only a single processor.
        /// </summary>
        /// <remarks>This typically does not change on a machine, so it's checked only once.</remarks>
        internal static readonly bool IsSingleProcessor = ProcessorCount == 1;
    }
}