读 dotnet 源代码 为何 Thread.Sleep 半毫秒和一毫秒等待时间差距如此之大

using System.Diagnostics;

var stopwatch = Stopwatch.StartNew();

for (int i = 0; i < 1000; i++)
{
    Thread.Sleep(TimeSpan.FromMilliseconds(0.99));
}

stopwatch.Stop();

Console.WriteLine($"耗时：{stopwatch.ElapsedMilliseconds}ms");

stopwatch.Restart();
for (int i = 0; i < 1000; i++)
{
    Thread.Sleep(TimeSpan.FromMilliseconds(1));
}
Console.WriteLine($"耗时：{stopwatch.ElapsedMilliseconds}ms");

耗时：0ms
耗时：15665ms

namespace System.Threading
{
    public sealed partial class Thread
    {
        ... // 忽略其他代码
        public static void Sleep(TimeSpan timeout) => Sleep(WaitHandle.ToTimeoutMilliseconds(timeout));
    }
}

namespace System.Threading
{
    public abstract partial class WaitHandle : MarshalByRefObject, IDisposable
    {
        internal static int ToTimeoutMilliseconds(TimeSpan timeout)
        {
            long timeoutMilliseconds = (long)timeout.TotalMilliseconds;
             ... // 忽略其他代码
            return (int)timeoutMilliseconds;
        }
        ... // 忽略其他代码
    }
}

        public void SpinOnce(int sleep1Threshold)
        {
            ArgumentOutOfRangeException.ThrowIfLessThan(sleep1Threshold, -1);

            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)
                ) ||
                Environment.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.
                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);
        }

namespace System.Threading
{
    public sealed partial class Thread
    {
        internal static void UninterruptibleSleep0() => Interop.Kernel32.Sleep(0);

#if !CORECLR
        private static void SleepInternal(int millisecondsTimeout)
        {
            Debug.Assert(millisecondsTimeout >= -1);
            Interop.Kernel32.Sleep((uint)millisecondsTimeout);
        }
#endif

        ... // 忽略其他代码
    }
}