Question

我有一个访问数据库和下载图像的程序。我为此目的使用BlockingCollection。但是，要访问某些UI元素，我决定使用Backgroundworker和BlockingCollection的组合。与仅使用Blockingcollection时的速度相比，它大大降低了处理速度。可能是什么原因？或者，因为我现在正在访问UI元素，速度会降低吗？

以下是我正在处理的代码：

 private void button_Start_Click(object sender, System.EventArgs e)
    {
        BackgroundWorker bgWorker = new BackgroundWorker();
        bgWorker.DoWork += bw_DoWork;
        bgWorker.RunWorkerCompleted += bw_RunWorkerCompleted;
        bgWorker.ProgressChanged += bw_ProgressChanged;

        bgWorker.WorkerSupportsCancellation = true;
        bgWorker.WorkerReportsProgress = true;

        Button btnSender = (Button)sender;
        btnSender.Enabled = false;

        bgWorker.RunWorkerAsync();
    }

和Do_Work()如下：

{
        HttpWebRequest request = null;
        using (BlockingCollection<ImageFileName> bc = new BlockingCollection<ImageFileName>(30))
        {
            using (Task task1 = Task.Factory.StartNew(() =>
            {

                foreach (var fileName in fileNames)
                {

                        string baseUrl = "http://some url";
                        string url = string.Format(baseUrl, fileName);
                        request = (HttpWebRequest)WebRequest.Create(url);
                        request.Method = "GET";
                        request.ContentType = "application/x-www-form-urlencoded";
                        var response = (HttpWebResponse)request.GetResponse();
                        Stream stream = response.GetResponseStream();
                        img = Image.FromStream(stream);
                        FileNameImage = new ImageFileName(fileName.ToString(), img);
                        bc.Add(FileNameImage);
                        Thread.Sleep(100);
                        Console.WriteLine("Size of BlockingCollection: {0}", bc.Count);
                    }



            }))
            {
                using (Task task2 = Task.Factory.StartNew(() =>
                {


                    foreach (ImageFileName imgfilename2 in bc.GetConsumingEnumerable())
                    {
                        if (bw.CancellationPending == true)
                        {
                            e.Cancel = true;
                            break;
                        }
                        else
                        {
                            int numIterations = 4;
                            Image img2 = imgfilename2.Image;
                            for (int i = 0; i < numIterations; i++)
                            {
                                img2.Save("C:\\path" + imgfilename2.ImageName);
                                ZoomThumbnail = img2;
                                ZoomSmall = img2;
                                ZoomLarge = img2;
                                ZoomThumbnail = GenerateThumbnail(ZoomThumbnail, 86, false);
                                ZoomThumbnail.Save("C:\\path" + imgfilename2.ImageName + "_Thumb.jpg");
                                ZoomThumbnail.Dispose();
                                ZoomSmall = GenerateThumbnail(ZoomSmall, 400, false);
                                ZoomSmall.Save("C:\\path" + imgfilename2.ImageName + "_Small.jpg");
                                ZoomSmall.Dispose();
                                ZoomLarge = GenerateThumbnail(ZoomLarge, 1200, false);
                                ZoomLarge.Save("C:\\path" + imgfilename2.ImageName + "_Large.jpg");
                                ZoomLarge.Dispose();

                                //  progressBar1.BeginInvoke(ProgressBarChange);
                                int percentComplete = (int)(((i + 1.0) / (double)numIterations) * 100.0);
                                //if (progressBar1.InvokeRequired)
                                //{
                                //    BeginInvoke(new MethodInvoker(delegate{bw.ReportProgress(percentComplete)};))
                                //}
                            }
                            Console.WriteLine("This is Take part and size is: {0}", bc.Count);
                        }
                    }


                }))
                    Task.WaitAll(task1, task2);


            }

        }

    }

Answer 1

更好的选择可能是检索数据并将其写入磁盘同步运行，而是使用Parallel.ForEach()允许多个请求同时在飞行中。这应该可以减少几个地方的等待量：

在发出后续请求之前，无需等待一个HTTP请求完成。
无需阻止该BlockingCollection
在关闭下一个磁盘写入之前，无需等待一个磁盘写入完成。

所以也许更像这样的东西：

Parallel.ForEach(fileNames, 
    (name) => 
    {
        string baseUrl = "http://some url";
        string url = string.Format(baseUrl, fileName);
        var request = (HttpWebRequest)WebRequest.Create(url);
        request.Method = "GET";
        request.ContentType = "application/x-www-form-urlencoded";
        var response = (HttpWebResponse)request.GetResponse();
        Stream stream = response.GetResponseStream();
        var img = Image.FromStream(stream);

        // Cutting out a lot of steps from the 2nd Task to simplify the example
        img.Save(Path.Combine("C:\\path", fileName.ToString()));  
    });

使用这种方法可能遇到的一个问题是，它会立即开始生成太多请求。这可能会导致资源争用问题，或者网络服务器可能会将其解释为恶意行为并停止响应您。您可以通过设置MaxDegreeOfParallelism来限制同时发生的请求数。以下示例显示了如何限制操作同时处理不超过4个文件。

var options = new ParallelOptions { MaxDegreeOfParallelism = 4 };
Parallel.ForEach(fileNames, (name) => { /* do stuff */ }, options);

为什么Backgroundeworker + BlockingCollection组合更慢？

1 个答案: