c# - Parallel ForEach - 连续调用持续更长时间

Question

我有一个基本上以并行方式迭代以并行化以下过程的函数：

1. 获取网页
2. 解析html代码
3. 返回结果并添加到结果集合

问题是第一次运行大约需要 5 分钟，第二次运行大约需要 40 分钟。输入集合没有改变，因此运行时间应该非常相似。任何想法？

我已经包含了 GetPrices(int)。第二次运行时，它从 netstat 中的 0 个连接开始（因此从第一次开始就没有可用的连接），但它只增长到 5 个连接（而不是第一次运行时的 30 个）。

另外，您认为退货收集是否需要锁定？

    public Dictionary<int, Dictionary<int, double>> GetPrices(List<int> IDs)
    {
        Stopwatch web_time = new Stopwatch(), regex_time = new Stopwatch();
        Dictionary<int, Dictionary<int, double>> ret = new Dictionary<int, Dictionary<int, double>>();
        int aux_bkp = ServicePointManager.DefaultConnectionLimit;
        ParallelOptions pOptions = new ParallelOptions();

        ServicePointManager.DefaultConnectionLimit = 30;

        pOptions.MaxDegreeOfParallelism = 35;
        Parallel.ForEach(IDs, pOptions, ID=>
            {
                Dictionary<int, double> aux = GetPrices(ID);

                lock (ret)
                {
                    ret.Add(ID, new Dictionary<int, double>());
                    foreach (int kID in aux.Keys)
                    {
                        ret[mktID].Add(kID , aux[kID ]);
                    }
                }

            });

        ServicePointManager.DefaultConnectionLimit = aux_bkp;

        return ret;
    }
    public static Dictionary<int, double> GetPrices(int ID)
    {
        Stopwatch web_time = new Stopwatch(), regex_time = new Stopwatch();
        WebClient webclient = new WebClient();
        string resp;
        Dictionary<int, double> ret = new Dictionary<int, double>();
        bool success = false;
        int retries = 0;

        web_time.Start();
        while (!success)
            try
            {
                Debug.WriteLine(string.Format("Get HTML: ({1}) - ({0})", url, ID));

                resp = webclient.DownloadString(url);
                success = true;
                web_time.Stop();

                regex_time.Start();


                regex_time.Start();

                ret = ProcessHTML(resp, ID);
                regex_time.Stop();

            }
            catch (WebException e)
            {

                System.Net.HttpWebResponse aux;

                aux = e.Response as System.Net.HttpWebResponse;
                if (aux == null || aux.StatusCode != HttpStatusCode.NotFound)
                {
                    success = false;
                    retries++;
                    System.Threading.Thread.Sleep(new System.Random().Next(5000));
                    System.Diagnostics.Debug.WriteLine("HTTP Error - " + e.ToString());
                }
                else if (aux != null || aux.StatusCode == HttpStatusCode.NotFound)
                {
                    success = true;
                }


            }

        regex_time.Stop();
        return ret;
        }