并行比较图像的问题 [英] problem with comparing images in parallel

问题描述

我使用.NET 4.0在c＃中构建应用程序，以使用并行库比较两个图像。我有一台带有两个核心的PC，但是当我在一个核心中运行应用程序时，我的运行速度比我在两个核心运行时快。



这是比较图像的方法。

I have build an application in c# with .NET 4.0 to compare two images using parallel library. I have a PC with two cores, but when i run the app in one core i runs faster than when i run it in two.

this is the method that compares images.

private unsafe Bitmap krahas(Bitmap nje, Bitmap dy)
        {
            Bitmap a = new Bitmap(nje);
            Bitmap b = new Bitmap(dy);
            int gjersia = Math.Min(a.Width, b.Width);
            int lartesia = Math.Min(a.Height, b.Height);
            Bitmap rez = new Bitmap(gjersia, lartesia);
            int numri_i_procesorve = Numri_i_berthamave.Value;
            //Bëjmë lock bitat e fotove
            Rectangle per_a = new Rectangle(0, 0, a.Width, a.Height);
            Rectangle per_b = new Rectangle(0, 0, b.Width, b.Height);
            Rectangle per_rez = new Rectangle(0, 0, gjersia, lartesia);
            BitmapData t_per_a = a.LockBits(per_a, ImageLockMode.ReadWrite, a.PixelFormat);
            BitmapData t_per_b = b.LockBits(per_b, ImageLockMode.ReadWrite, b.PixelFormat);
            BitmapData t_per_rez = rez.LockBits(per_rez, ImageLockMode.ReadWrite, PixelFormat.Format32bppArgb);

            byte* adresa_per_a = (byte*)t_per_a.Scan0;
            byte* adresa_per_b = (byte*)t_per_b.Scan0;
            byte* rezult = (byte*)t_per_rez.Scan0;

            Color cl = Color.Red;
            byte[] ngj = new byte[4];
            ngj[3] = cl.A;
            ngj[2] = cl.R;
            ngj[1] = cl.G;
            ngj[0] = cl.B;

            int rowpadding = t_per_a.Stride - (a.Width * 4);
            ParallelOptions po = new ParallelOptions();
            po.MaxDegreeOfParallelism = numri_i_procesorve;
            koha_e_kaluar.Reset();
            koha_e_kaluar.Start();
            object lok = new object();
            Parallel.For(0, lartesia, po, i =>
        {
            lock (lok)
            {
                for (int j = 0; j < gjersia; j++)
                {
                    int same = 0;
                    byte[] tmp = new byte[4];
                    for (int x = 0; x < 4; x++)
                    {
                        tmp[x] = adresa_per_b[0];
                        if (adresa_per_a[0] == adresa_per_b[0])
                        {
                            same++;
                        }
                        adresa_per_a++;
                        adresa_per_b++;
                    }
                    for (int x = 0; x < 4; x++)
                    {
                        rezult[0] = (same != 4) ? ngj[x] : tmp[x];
                        rezult++;
                    }
                }
                if (rowpadding > 0)
                {
                    adresa_per_a += rowpadding;
                    adresa_per_b += rowpadding;
                    rezult += rowpadding;
                }
            }
        });
            koha_e_kaluar.Stop();
            a.UnlockBits(t_per_a);
            b.UnlockBits(t_per_b);
            rez.UnlockBits(t_per_rez);
            return rez;
        }

我现在不知道问题出在哪里。谁能帮我！请。

I don''t now where is the problem. Can anyone help me! Please.

推荐答案

嗯，我不是并行编码的专家，但我在自己的代码中遇到了同样的问题（两者都使用了并行编码）编码设施和原始线程）。我想在你的情况下（就像在我的情况下）循环中的操作非常简单，以至于并行执行开销占据执行时间。

Well, I''m not an expert on parallel coding but I have experienced the same ''problem'' in my own code (both using parallel coding facilities and raw threads). I suppose that in your case (like in mine) the operations inside the loop are so simple that parallel execution overhead dominates the execution time.

我猜这个问题是 lock（lok）。

在一个核心中，没有争用锁定。

有多个核心，有可能会强制进行上下文切换等。

我认为你应该考虑如何对你的算法进行分区，这样你就不需要锁定了。

I''d guess that the problem is the lock(lok).
In a single core there will be no contention for the lock.
With multiple cores, there can be, which will force context switching, etc.
I think you should think about how to partition your algorithm such that you don''t need locking.

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