如何：使用过度订阅抵消延迟

项目
05/01/2013

对于一些所含任务存在大量延迟的应用程序，过度订阅可以提高这些应用程序的整体效率。本主题阐释如何使用过度订阅来抵消通过网络连接读取数据导致的延迟。

示例

此示例使用异步代理库从 HTTP 服务器下载文件。http_reader从派生类 concurrency::agent 和使用消息传递到异步读取要下载的 URL 名称。

http_reader类使用 concurrency::task_group 类可以同时读取每个文件。每个任务调用 concurrency::Context::Oversubscribe 方法使用_BeginOversubscription参数设置为true启用超额预定在当前上下文中的。然后，每个任务使用 Microsoft 基础类 (MFC) CInternetSession 和 CHttpFile 类来下载文件。最后，每个任务在 _BeginOversubscription 参数设置为 false 的情况下调用 Context::Oversubscribe 以禁用过度订阅。

启用过度订阅后，运行时会创建一个在其中运行任务的附加线程。此外，每个线程都可以过度订阅当前上下文，从而创建附加线程。http_reader类使用 concurrency::unbounded_buffer 对象的应用程序使用的线程数的限制。代理将使用固定数目的标记值来初始化缓冲区。对于每个下载操作，代理会在操作开始前读取缓冲区中的一个标记值，然后在操作完成后将该值写回到缓冲区中。当缓冲区为空时，代理会等待其中的某个下载操作将值写回到缓冲区。

下面的示例将同步任务数限制为可用硬件线程数的两倍。在试验过度订阅时，这个值很适合于初次使用。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/",
      "https://www.adventure-works.com/", 
      "http://www.alpineskihouse.com/",
      "http://www.cpandl.com/", 
      "http://www.cohovineyard.com/",
      "http://www.cohowinery.com/",
      "http://www.cohovineyardandwinery.com/", 
      "https://www.contoso.com/",
      "http://www.consolidatedmessenger.com/",
      "http://www.fabrikam.com/", 
      "https://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/",
      "https://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;
}

此示例将在具有四个处理器的计算机上产生以下输出：

Downloading http://www.adatum.com/...
Downloading https://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 https://www.contoso.com/...
Downloading http://www.consolidatedmessenger.com/...
Downloading http://www.fabrikam.com/...
Downloading https://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 https://www.proseware.com/...
Downloading http://www.fineartschool.net...
Downloading http://www.tailspintoys.com/...
Downloaded 1801040 bytes in 3276 ms.

当启用过度订阅时，此示例运行的速度会更快，这是因为当其他任务等待延迟操作完成时，将会运行附加任务。

编译代码

将示例代码复制并将其粘贴在 Visual Studio 项目中，或将它粘贴到一个文件，名为下载 oversubscription.cpp 和 Visual Studio 的命令提示符窗口中运行的以下任一命令的然后。

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

cl.exe /EHsc /MT download-oversubscription.cpp

可靠编程

当不再需要过度订阅时，始终禁用过度订阅。考虑一个不处理由另一个函数引发的异常的函数。如果在此函数返回之前不禁用过度订阅，则任何附加的并行工作也将过度订阅当前上下文。

可以使用“获取资源即初始化”(RAII) 模式，将过度订阅限制为某个给定范围。在 RAII 模式下，将在堆栈上分配一个数据结构。该数据结构在创建时将会初始化或获取一个资源，而且该数据结构在销毁时将会销毁或释放该资源。RAII 模式可确保在封闭范围退出之前调用析构函数。因此，在引发异常时或在函数包含多个 return 语句时，将可以正确地管理资源。

下面的示例定义了一个名为 scoped_blocking_signal 的结构。scoped_blocking_signal 结构的构造函数启用过度订阅，而析构函数禁用过度订阅。

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 来确保在函数返回之前禁用过度订阅。这项技术可确保 download 方法不会出现异常。

// 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()));
}

通过

如何：使用过度订阅抵消延迟

示例

编译代码

可靠编程

请参见

参考

概念

其他资源