Thread.Sleep的精确替代

Question

I have a method Limit() which counts a bandwidth passed thought some channel in certain time and limits by using Thread.Sleep() it (if bandwidth limit is reached). Method itself produces proper ( in my opinion results ) but Thread.Sleep doesn't ( due to multithreaded CPU usage ) because i have proper "millisecondsToWait" but speed check afterwards is far from limitation i've passed.

Is there a way to make limitation more precise ?

Limiter Class

    private readonly int m_maxSpeedInKbps;
    public Limiter(int maxSpeedInKbps)
    {
        m_maxSpeedInKbps = maxSpeedInKbps;
    }

    public int Limit(DateTime startOfCycleDateTime, long writtenInBytes)
    {
        if (m_maxSpeedInKbps > 0)
        {
            double totalMilliseconds = DateTime.Now.Subtract(startOfCycleDateTime).TotalMilliseconds;
            int currentSpeedInKbps = (int)((writtenInBytes / totalMilliseconds));
            if (currentSpeedInKbps - m_maxSpeedInKbps > 0)
            {
                double delta = (double)currentSpeedInKbps / m_maxSpeedInKbps;
                int millisecondsToWait = (int)((totalMilliseconds * delta) - totalMilliseconds);
                if (millisecondsToWait > 0)
                {
                    Thread.Sleep(millisecondsToWait);
                    return millisecondsToWait;
                }
            }
        }

        return 0;
    }

Test Class which always fails in large delta

[TestMethod]
public void ATest()
{
    List<File> files = new List<File>();
    for (int i = 0; i < 1; i++)
    {
        files.Add(new File(i + 1, 100));
    }

    const int maxSpeedInKbps = 1024; // 1MBps
    Limiter limiter = new Limiter(maxSpeedInKbps);

    DateTime startDateTime = DateTime.Now;
    Parallel.ForEach(files, new ParallelOptions {MaxDegreeOfParallelism = 5}, file =>
    {
        DateTime currentFileStartTime = DateTime.Now;
        Thread.Sleep(5);
        limiter.Limit(currentFileStartTime, file.Blocks * Block.Size);
    });

    long roundOfWriteInKB = (files.Sum(i => i.Blocks.Count) * Block.Size) / 1024;
    int currentSpeedInKbps = (int) (roundOfWriteInKB/DateTime.Now.Subtract(startDateTime).TotalMilliseconds*1000);

    Assert.AreEqual(maxSpeedInKbps, currentSpeedInKbps, string.Format("maxSpeedInKbps {0} currentSpeedInKbps {1}", maxSpeedInKbps, currentSpeedInKbps));
} 

Answer 1

I used to use Thread.Sleep a lot until I discovered waithandles . Using waithandles you can suspend threads, which will come alive again when the waithandle is triggered from elsewhere, or when a time threshold is reached. Perhaps it's possible to re-engineer your limit methodology to use waithandles in some way, because in a lot of situations they are indeed much more precise than Thread.Sleep ?

Answer 2

You can do it fairly accurately using a busy wait, but I wouldn't recommend it. You should use one of the multimedia timers to wait instead.

However, this method will wait fairly accurately:

void accurateWait(int millisecs)
{
    var sw = Stopwatch.StartNew();

    if (millisecs >= 100)
        Thread.Sleep(millisecs - 50);

    while (sw.ElapsedMilliseconds < millisecs)
        ;
}

But it is a busy wait and will consume CPU cycles terribly. Also it could be affected by garbage collections or task rescheduling.

Here's the test program:

using System;
using System.Diagnostics;
using System.Collections.Generic;
using System.Threading;

namespace Demo
{
    class Program
    {
        void run()
        {
            for (int i = 1; i < 10; ++i)
                test(i);

            for (int i = 10; i < 100; i += 5)
                test(i);

            for (int i = 100; i < 200; i += 10)
                test(i);

            for (int i = 200; i < 500; i += 20)
                test(i);
        }

        void test(int millisecs)
        {
            var sw = Stopwatch.StartNew();
            accurateWait(millisecs);
            Console.WriteLine("Requested wait = " + millisecs + ", actual wait = " + sw.ElapsedMilliseconds);
        }

        void accurateWait(int millisecs)
        {
            var sw = Stopwatch.StartNew();

            if (millisecs >= 100)
                Thread.Sleep(millisecs - 50);

            while (sw.ElapsedMilliseconds < millisecs)
                ;
        }

        static void Main()
        {
            new Program().run();
        }
    }
}