c# - 如果用不同数量的线程并行调用，为什么会得到不同的总和？

Question

这只是我提出的以下问题的延伸:

Why can't I reach 100% CPU utilization with my parallel tasks code?

 private static int SumParallel()
        {
            var intList = Enumerable.Range(1, 1000_000_000);
            int count = intList.Count();
            int threads = 6;
            List<Task<int>> l = new List<Task<int>>(threads);
            for(int i = 1; i <= threads; i++)
            {
                int skip = ((i - 1) * count) / threads;
                int take = count / threads;
                l.Add(GetSum(intList, skip, take));
            }
            Task.WaitAll(l.ToArray());
            return l.Sum(t => t.Result);
        }

private static Task<int> GetSum(IEnumerable<int> list, int skip, int take)
        {
            return Task.Run(() =>
             {
                 int temp = 0;
                 foreach(int n in list.Skip(skip).Take(take))
                 {
                     if (n % 2 == 0)
                         temp -= n;
                     else
                     {
                         temp += n;
                     }
                 }

                 Console.WriteLine(temp + " " + Task.CurrentId);
                 return temp;
             });
        }

如果修改并行工作的任务数，则会得到不同的总和。

为什么会这样呢？

Answer 1

因为这条线:

int skip = ((i - 1) * count) / threads;
int take = count / threads;

考虑threads = 3和count = 10它不能涵盖您的所有列表。根据线程数，您在take和skip中会得到不同的舍入，有时它们不能覆盖列表中的所有项。

您也许应该这样更改它:

for(int i = 1; i <= threads; i++)
{
    int skip = ((i - 1) * count) / threads;
    int take = count / threads;

    if(i == threads)
        take = threads - skip;

    l.Add(GetSum(intList, skip, take));
}