如何：使用过度订阅偏移延迟

项目
06/16/2023

超额订阅可以提高某些包含高延迟任务的应用程序的整体效率。本主题说明如何使用超额订阅来抵消从网络连接读取数据所导致的延迟。

示例

此示例使用异步代理库从 HTTP 服务器下载文件。 http_reader 类从 concurrency::agent 派生，并使用消息传递来异步读取要下载的 URL 名称。

http_reader 类使用 concurrency::task_group 类来同时读取每个文件。每个任务都调用 concurrency::Context::Oversubscribe 方法（其 _BeginOversubscription 参数设置为 true），以便在当前上下文中启用超额订阅。然后，每个任务都使用 Microsoft 基础类 (MFC) CInternetSession 和 CHttpFile 类来下载该文件。最后，每个任务调用 Context::Oversubscribe，其中 _BeginOversubscription 参数设置为 false 以禁用超额订阅。

启用超额订阅后，运行时将创建一个用于运行任务的附加线程。其中每个线程还可以超额订阅当前上下文，从而创建其他线程。 http_reader 类使用 concurrency::unbounded_buffer 对象来限制应用程序所使用的线程数。代理使用固定数量的令牌值初始化缓冲区。对于每个下载操作，代理在操作开始之前从缓冲区读取令牌值，然后在操作完成后将该值写回缓冲区。当缓冲区为空时，代理将等待其中一个下载操作将值写回缓冲区。

以下示例将并发任务数限制为可用硬件线程数的两倍。要体验超额订阅，建议从使用此值开始。可以使用适合特定处理环境的值，也可以动态更改此值以响应实际工作负载。

// 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;
}

本示例在具有四个处理器的计算机上生成以下输出：

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.

启用超额订阅时，该示例可以运行得更快，因为附加任务运行时，其他任务等待潜在操作完成。

编译代码

复制示例代码，并将它粘贴到 Visual Studio 项目中，或粘贴到名为 download-oversubscription.cpp 的文件中，再在 Visual Studio 命令提示符窗口中运行以下命令之一。

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

可靠编程

在不再需要超额订阅后，请始终禁用。考虑不处理由另一个函数引发的异常的函数。如果在函数返回之前未禁用超额订阅，则任何其他并行工作也将超额订阅当前上下文。

可以使用“资源获取即初始化”(RAII) 模式将超额订阅限制到给定范围。在 RAII 模式下，数据结构在堆栈上分配。该数据结构在创建资源时初始化或获取资源，并在销毁数据结构时销毁或释放该资源。 RAII 模式保证在封闭范围退出之前调用析构函数。因此，当引发异常或函数包含多个 return 语句时，可以正确管理资源。

下面的示例定义一个名为 scoped_blocking_signal 的结构。 scoped_blocking_signal 结构的构造函数启用超额订阅，析构函数禁用超额订阅。

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

下面的示例修改 download 方法的主体以使用 RAII，确保在函数返回之前禁用超额订阅。此技术可确保 download 方法是异常安全的。

// 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()));
}

另请参阅

上下文
 Context::Oversubscribe 方法

如何：使用过度订阅偏移延迟

示例

编译代码

可靠编程

另请参阅

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