如何：创建使用特定计划程序策略的代理

项目
06/09/2015

代理是以异步方式与其他组件一起解决更大的计算任务的应用程序组件。代理通常具有已设置的生命周期并且会维护状态。

每个代理都可能具有独特的应用程序要求。例如，启用用户交互（检索输入或显示输出）的代理在访问计算资源时可能需要较高的优先级别。利用计划程序策略，可以控制计划程序在管理任务时使用的策略。本主题演示如何创建使用特定计划程序策略的代理。

有关将自定义计划程序策略与异步消息块一起使用的基本示例，请参见如何：指定特定的计划程序策略。

本主题使用异步代理库中的功能（如代理、消息块和消息传递函数）执行工作。有关异步代理库的更多信息，请参见异步代理库。

示例

下面的示例定义了派生自 concurrency::agent 的两个类：permutor 和 printer。 permutor 类计算给定输入字符串的所有排列。 printer 类将进度消息打印到控制台。 permutor 类执行可能会使用所有可用计算资源的计算密集型操作。 printer 类必须及时打印每条进度消息才会有用。

为了向 printer 类提供对计算资源的公平访问权，此示例使用如何：管理计划程序实例中描述的步骤创建了一个具有自定义策略的计划程序实例。自定义策略将线程优先级别指定为最高优先级别类。

为了说明使用具有自定义策略的计划程序的好处，此示例将整个任务执行两次。此示例首先使用默认计划程序安排这两项任务，然后使用默认计划程序安排 permutor 对象，并使用具有自定义策略的计划程序安排 printer 对象。

// permute-strings.cpp 
// compile with: /EHsc
#include <windows.h>
#include <ppl.h>
#include <agents.h>
#include <iostream>
#include <sstream>

using namespace concurrency;
using namespace std;

// Computes all permutations of a given input string. 
class permutor : public agent
{
public:
   explicit permutor(ISource<wstring>& source,
      ITarget<unsigned int>& progress)
      : _source(source)
      , _progress(progress)
   {
   }

   explicit permutor(ISource<wstring>& source,
      ITarget<unsigned int>& progress,
      Scheduler& scheduler)
      : agent(scheduler)
      , _source(source)
      , _progress(progress)
   {
   }

   explicit permutor(ISource<wstring>& source,
      ITarget<unsigned int>& progress,
      ScheduleGroup& group)
      : agent(group)       
      , _source(source)
      , _progress(progress)
   {
   }

protected:
   // Performs the work of the agent. 
   void run()
   {
      // Read the source string from the buffer.
      wstring s = receive(_source);

      // Compute all permutations.
      permute(s);

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

   // Computes the factorial of the given value. 
   unsigned int factorial(unsigned int n)
   {
      if (n == 0)
         return 0;
      if (n == 1)
         return 1;
      return n * factorial(n - 1);
   }

   // Computes the nth permutation of the given wstring.
   wstring permutation(int n, const wstring& s) 
   {
      wstring t(s);

      size_t len = t.length();
      for (unsigned int i = 2; i < len; ++i)
      {
         swap(t[n % i], t[i]);
         n = n / i;
      }
      return t;
   }

   // Computes all permutations of the given string. 
   void permute(const wstring& s)
   {      
      // The factorial gives us the number of permutations. 
      unsigned int permutation_count = factorial(s.length());

      // The number of computed permutations.
      LONG count = 0L;      

      // Tracks the previous percentage so that we only send the percentage 
      // when it changes. 
      unsigned int previous_percent = 0u;

      // Send initial progress message.
      send(_progress, previous_percent);

      // Compute all permutations in parallel.
      parallel_for (0u, permutation_count, [&](unsigned int i) {
         // Compute the permutation.
         permutation(i, s);

         // Send the updated status to the progress reader. 
         unsigned int percent = 100 * InterlockedIncrement(&count) / permutation_count;
         if (percent > previous_percent)
         {
             send(_progress, percent);
             previous_percent = percent;
         }
      });

      // Send final progress message.
      send(_progress, 100u);
   }

private:
   // The buffer that contains the source string to permute.
   ISource<wstring>& _source;

   // The buffer to write progress status to.
   ITarget<unsigned int>& _progress;
};

// Prints progress messages to the console. 
class printer : public agent
{
public:
   explicit printer(ISource<wstring>& source,
      ISource<unsigned int>& progress)
      : _source(source)
      , _progress(progress)
   {
   }

   explicit printer(ISource<wstring>& source,
      ISource<unsigned int>& progress, Scheduler& scheduler)
      : agent(scheduler)
      , _source(source)
      , _progress(progress)
   {
   }

   explicit printer(ISource<wstring>& source,
      ISource<unsigned int>& progress, ScheduleGroup& group)
      : agent(group)       
      , _source(source)
      , _progress(progress)
   {
   }

protected:
   // Performs the work of the agent. 
   void run()
   {
      // Read the source string from the buffer and print a message.
      wstringstream ss;
      ss << L"Computing all permutations of '" << receive(_source) << L"'..." << endl;
      wcout << ss.str();

      // Print each progress message. 
      unsigned int previous_progress = 0u;
      while (true)
      {         
         unsigned int progress = receive(_progress);

         if (progress > previous_progress || progress == 0u)
         { 
            wstringstream ss;
            ss << L'\r' << progress << L"% complete...";
            wcout << ss.str();
            previous_progress = progress;
         }

         if (progress == 100)
            break;
      }
      wcout << endl;

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

private:
   // The buffer that contains the source string to permute.
   ISource<wstring>& _source;

   // The buffer that contains progress status.
   ISource<unsigned int>& _progress;
};

// Computes all permutations of the given string.  
void permute_string(const wstring& source,
   Scheduler& permutor_scheduler, Scheduler& printer_scheduler)
{  
   // Message buffer that contains the source string. 
   // The permutor and printer agents both read from this buffer.
   single_assignment<wstring> source_string;

   // Message buffer that contains the progress status. 
   // The permutor agent writes to this buffer and the printer agent reads 
   // from this buffer.
   unbounded_buffer<unsigned int> progress;

   // Create the agents with the appropriate schedulers.
   permutor agent1(source_string, progress, permutor_scheduler);
   printer agent2(source_string, progress, printer_scheduler);

   // Start the agents.
   agent1.start();
   agent2.start();

   // Write the source string to the message buffer. This will unblock the agents.
   send(source_string, source);

   // Wait for both agents to finish.
   agent::wait(&agent1);
   agent::wait(&agent2);
}

int wmain()
{
   const wstring source(L"Grapefruit");

   // Compute all permutations on the default scheduler.

   Scheduler* default_scheduler = CurrentScheduler::Get();

   wcout << L"With default scheduler: " << endl;
   permute_string(source, *default_scheduler, *default_scheduler);
   wcout << endl;

   // Compute all permutations again. This time, provide a scheduler that 
   // has higher context priority to the printer agent.

   SchedulerPolicy printer_policy(1, ContextPriority, THREAD_PRIORITY_HIGHEST);
   Scheduler* printer_scheduler = Scheduler::Create(printer_policy);

   // Register to be notified when the scheduler shuts down.
   HANDLE hShutdownEvent = CreateEvent(NULL, FALSE, FALSE, NULL);
   printer_scheduler->RegisterShutdownEvent(hShutdownEvent);

   wcout << L"With higher context priority: " << endl;
   permute_string(source, *default_scheduler, *printer_scheduler);
   wcout << endl; 

   // Release the printer scheduler.
   printer_scheduler->Release();

   // Wait for the scheduler to shut down and destroy itself.
   WaitForSingleObject(hShutdownEvent, INFINITE);

   // Close the event handle.
   CloseHandle(hShutdownEvent);
}

该示例产生下面的输出。

虽然这两组任务产生的结果相同，但使用自定义策略的版本可使 printer 对象能够按照提升的优先级别运行，以使其行为的响应能力更强。

编译代码

复制代码示例，并将此代码粘贴到 Visual Studio项目中或一个名为 permute-strings.cpp 的文件中，然后在 Visual Studio命令提示符窗口中运行以下命令。

cl.exe /EHsc permute-strings.cpp

请参见

参考

How-to: Specify Specific Scheduler Policies

概念

计划程序策略

异步代理

通过

如何：创建使用特定计划程序策略的代理

示例

编译代码

请参见

参考

概念

其他资源