Why I am getting a different sum if invoked with different number of threads in parallel?

问题

This is just an extension of the following question I asked:

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;
             });
        }

If I modify the number of tasks working in parallel, I get a different sum.

Why is this so?

回答1:

It's because of this line:

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

consider threads = 3 and count = 10 It can't cover all of your list. Based on thread count you get different roundings in take and skip and sometimes they can't cover all items in the list.

You maybe should change it this way:

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));
}

来源：https://stackoverflow.com/questions/62100278/why-i-am-getting-a-different-sum-if-invoked-with-different-number-of-threads-in

标签

multithreading