如何：使用過度訂閱使延遲產生位移

文章
10/12/2023

過度訂閱可以改善某些應用程式的整體效率，這些應用程式包含具有高延遲度的工作。本主題說明如何使用過度訂閱來抵消從網路連線讀取資料所造成的延遲。

範例

此範例會使用非同步代理程式程式庫，從 HTTP 伺服器下載檔案。類別 http_reader 衍生自並行：：agent ，並使用傳遞訊息以非同步方式讀取要下載的 URL 名稱。

類別 http_reader 會使用並行：：task_group 類別同時讀取每個檔案。每個工作都會呼叫 concurrency：：CoNtext：：Oversubscribe 方法 _BeginOversubscription ，並將參數設定為， true 以在目前內容中啟用過度訂閱。接著，每項工作都會使用 Microsoft Foundation Classes （MFC） CInternetSession 和 CHttpFile 類別來下載檔案。最後，每個工作都會呼叫 Context::Oversubscribe ， _BeginOversubscription 並將參數設定為來 false 停用超訂閱。

啟用過度訂閱時，執行時間會建立一個額外的執行緒，在其中執行工作。每個執行緒也可以過度訂閱目前的內容，藉此建立其他執行緒。類別 http_reader 會使用並行：：unbounded_buffer 物件來限制應用程式使用的執行緒數目。代理程式會使用固定數目的權杖值來初始化緩衝區。針對每個下載作業，代理程式會在作業開始之前從緩衝區讀取權杖值，然後在作業完成之後將該值寫回緩衝區。當緩衝區是空的時，代理程式會等候其中一個下載作業將值寫回緩衝區。

下列範例會將同時工作的數目限制為可用硬體執行緒數目的兩倍。當您實驗過度訂閱時，這個值是很好的起點。您可以使用符合特定處理環境的值，或動態變更此值以回應實際工作負載。

// download-oversubscription.cpp
// compile with: /EHsc /MD /D "_AFXDLL"
#define _WIN32_WINNT 0x0501
#include <afxinet.h>
#include <concrtrm.h>
#include <agents.h>
#include <ppl.h>
#include <sstream>
#include <iostream>
#include <array>

using namespace concurrency;
using namespace std;

// Calls the provided work function and returns the number of milliseconds 
// that it takes to call that function.
template <class Function>
__int64 time_call(Function&& f)
{
   __int64 begin = GetTickCount();
   f();
   return GetTickCount() - begin;
}

// Downloads the file at the given URL.
CString GetHttpFile(CInternetSession& session, const CString& strUrl);

// Reads files from HTTP servers.
class http_reader : public agent
{
public:
   explicit http_reader(CInternetSession& session,      
      ISource<string>& source,
      unsigned int& total_bytes,
      unsigned int max_concurrent_reads)
      : _session(session)
      , _source(source)
      , _total_bytes(total_bytes)
   {
      // Add one token to the available tasks buffer for each 
      // possible concurrent read operation. The value of each token 
      // is not important, but can be useful for debugging.
      for (unsigned int i = 0; i < max_concurrent_reads; ++i)
         send(_available_tasks, i);
   }

   // Signals to the agent that there are no more items to download.
   static const string input_sentinel;
 
protected:
   void run()
   {
      // A task group. Each task in the group downloads one file.
      task_group tasks;

      // Holds the total number of bytes downloaded.
      combinable<unsigned int> total_bytes;

      // Read from the source buffer until the application 
      // sends the sentinel value.
      string url;
      while ((url = receive(_source)) != input_sentinel)
      {
         // Wait for a task to release an available slot.
         unsigned int token = receive(_available_tasks);

         // Create a task to download the file.
         tasks.run([&, token, url] {

            // Print a message.
            wstringstream ss;
            ss << L"Downloading " << url.c_str() << L"..." << endl;
            wcout << ss.str();

            // Download the file.
            string content = download(url);

            // Update the total number of bytes downloaded.
            total_bytes.local() += content.size();

            // Release the slot for another task.
            send(_available_tasks, token);
         });
      }

      // Wait for all tasks to finish.
      tasks.wait();
      
      // Compute the total number of bytes download on all threads.
      _total_bytes = total_bytes.combine(plus<unsigned int>());

      // Set the status of the agent to agent_done.
      done();
   }

   // Downloads the file at the given URL.
   string download(const string& url)
   {
      // Enable oversubscription.
      Context::Oversubscribe(true);

      // Download the file.
      string content = GetHttpFile(_session, url.c_str());
      
      // Disable oversubscription.
      Context::Oversubscribe(false);

      return content;
   }

private:
   // Manages the network connection.
   CInternetSession& _session;
   // A message buffer that holds the URL names to download.
   ISource<string>& _source;
   // The total number of bytes downloaded
   unsigned int& _total_bytes;
   // Limits the agent to a given number of simultaneous tasks.
   unbounded_buffer<unsigned int> _available_tasks;
};
const string http_reader::input_sentinel("");

int wmain()
{
   // Create an array of URL names to download.
   // A real-world application might read the names from user input.
   array<string, 21> urls = {
      "http://www.adatum.com/",
      "http://www.adventure-works.com/", 
      "http://www.alpineskihouse.com/",
      "http://www.cpandl.com/", 
      "http://www.cohovineyard.com/",
      "http://www.cohowinery.com/",
      "http://www.cohovineyardandwinery.com/", 
      "http://www.contoso.com/",
      "http://www.consolidatedmessenger.com/",
      "http://www.fabrikam.com/", 
      "http://www.fourthcoffee.com/",
      "http://www.graphicdesigninstitute.com/",
      "http://www.humongousinsurance.com/",
      "http://www.litwareinc.com/",
      "http://www.lucernepublishing.com/",
      "http://www.margiestravel.com/",
      "http://www.northwindtraders.com/",
      "http://www.proseware.com/", 
      "http://www.fineartschool.net",
      "http://www.tailspintoys.com/",
      http_reader::input_sentinel,
   };
      
   // Manages the network connection.
   CInternetSession session("Microsoft Internet Browser");

   // A message buffer that enables the application to send URL names to the 
   // agent.
   unbounded_buffer<string> source_urls;

   // The total number of bytes that the agent has downloaded.
   unsigned int total_bytes = 0u;

   // Create an http_reader object that can oversubscribe each processor by one.
   http_reader reader(session, source_urls, total_bytes, 2*GetProcessorCount());

   // Compute the amount of time that it takes for the agent to download all files.
   __int64 elapsed = time_call([&] {
      
      // Start the agent.
      reader.start();
      
      // Use the message buffer to send each URL name to the agent.
      for_each(begin(urls), end(urls), [&](const string& url) {
         send(source_urls, url);
      });

      // Wait for the agent to finish downloading.
      agent::wait(&reader);      
   });

   // Print the results.
   wcout << L"Downloaded " << total_bytes
         << L" bytes in " << elapsed << " ms." << endl;
}

// Downloads the file at the given URL and returns the size of that file.
CString GetHttpFile(CInternetSession& session, const CString& strUrl)
{
   CString strResult;

   // Reads data from an HTTP server.
   CHttpFile* pHttpFile = NULL;

   try
   {
      // Open URL.
      pHttpFile = (CHttpFile*)session.OpenURL(strUrl, 1, 
         INTERNET_FLAG_TRANSFER_ASCII | 
         INTERNET_FLAG_RELOAD | INTERNET_FLAG_DONT_CACHE);

      // Read the file.
      if(pHttpFile != NULL)
      {           
         UINT uiBytesRead;
         do
         {
            char chBuffer[10000];
            uiBytesRead = pHttpFile->Read(chBuffer, sizeof(chBuffer));
            strResult += chBuffer;
         }
         while (uiBytesRead > 0);
      }
    }
   catch (CInternetException)
   {
      // TODO: Handle exception
   }

   // Clean up and return.
   delete pHttpFile;

   return strResult;
}

此範例會在具有四個處理器的電腦上產生下列輸出：

Downloading http://www.adatum.com/...
Downloading http://www.adventure-works.com/...
Downloading http://www.alpineskihouse.com/...
Downloading http://www.cpandl.com/...
Downloading http://www.cohovineyard.com/...
Downloading http://www.cohowinery.com/...
Downloading http://www.cohovineyardandwinery.com/...
Downloading http://www.contoso.com/...
Downloading http://www.consolidatedmessenger.com/...
Downloading http://www.fabrikam.com/...
Downloading http://www.fourthcoffee.com/...
Downloading http://www.graphicdesigninstitute.com/...
Downloading http://www.humongousinsurance.com/...
Downloading http://www.litwareinc.com/...
Downloading http://www.lucernepublishing.com/...
Downloading http://www.margiestravel.com/...
Downloading http://www.northwindtraders.com/...
Downloading http://www.proseware.com/...
Downloading http://www.fineartschool.net...
Downloading http://www.tailspintoys.com/...
Downloaded 1801040 bytes in 3276 ms.

當啟用超訂閱時，此範例可以更快執行，因為其他工作會執行，而其他工作則等候潛在作業完成。

編譯程式碼

複製範例程式碼，並將其貼到 Visual Studio 專案中，或貼到名為 download-oversubscription.cpp 的檔案中，然後在 Visual Studio 命令提示 字元視窗中執行下列其中一個 命令。

cl.exe /EHsc /MD /D "_AFXDLL" download-oversubscription.cpp
cl.exe /EHsc /MT download-oversubscription.cpp

穩固程式設計

在您不再需要訂閱之後，請一律停用超額訂閱。請考慮未處理另一個函式擲回例外狀況的函式。如果您未在函式傳回之前停用超訂閱，則任何其他平行工作也會過度訂閱目前的內容。

您可以使用資源擷取為初始化（RAII）模式，將超額訂閱限制為指定的範圍。在 RAII 模式下，會在堆疊上配置資料結構。該資料結構會在建立資源時初始化或取得資源，並在資料結構終結時終結或釋放該資源。 RAII 模式保證解構函式會在封入範圍結束之前呼叫。因此，當擲回例外狀況或函式包含多個 return 語句時，資源就會正確管理。

下列範例會定義名為 scoped_blocking_signal 的結構。結構的建構 scoped_blocking_signal 函式會啟用 oversubscription，而解構函式會停用 oversubscription。

struct scoped_blocking_signal
{
    scoped_blocking_signal()
    {
        concurrency::Context::Oversubscribe(true);
    }
    ~scoped_blocking_signal()
    {
        concurrency::Context::Oversubscribe(false);
    }
};

下列範例會修改方法的 download 主體，以使用 RAII，以確保在函式傳回之前停用過度訂閱。這項技術可確保 download 方法是安全的例外狀況。

// Downloads the file at the given URL.
string download(const string& url)
{
   scoped_blocking_signal signal;

   // Download the file.
   return string(GetHttpFile(_session, url.c_str()));
}

另請參閱

內容
 CoNtext：：Oversubscribe 方法

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如何：使用過度訂閱使延遲產生位移

範例

編譯程式碼

穩固程式設計

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意見反映

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