如何：使用 Alloc 和 Free 改善記憶體效能

文章
10/12/2023

本檔說明如何使用 concurrency：：Alloc 和 concurrency：：Free 函式來改善記憶體效能。它會比較針對每個指定 new 和 delete 運算子的三種不同類型，平行反轉陣列元素所需的時間。

Alloc當多個執行緒經常呼叫 Alloc 和 Free 時，和 Free 函式最有用。執行時間會針對每個執行緒保留個別的記憶體快取;因此，執行時間會管理記憶體，而不需要使用鎖定或記憶體屏障。

範例：指定新運算子和刪除運算子的類型

下列範例顯示三種各指定 new 和 delete 運算子的類型。類別會使用全域 new 和 delete 運算子、 malloc_free 類別 new_delete 使用 C Runtime malloc 和 free 函式，而類別 Alloc_Free 則使用並行執行時間和 AllocFree 函式。

// A type that defines the new and delete operators. These operators 
// call the global new and delete operators, respectively.
class new_delete
{
public:
   static void* operator new(size_t size)
   {
      return ::operator new(size);
   }
   
   static void operator delete(void *p)
   {
      return ::operator delete(p);
   }

   int _data;
};

// A type that defines the new and delete operators. These operators 
// call the C Runtime malloc and free functions, respectively.
class malloc_free
{
public:
   static void* operator new(size_t size)
   {
      return malloc(size);
   }
   static void operator delete(void *p)
   {
      return free(p);
   }

   int _data;
};

// A type that defines the new and delete operators. These operators 
// call the Concurrency Runtime Alloc and Free functions, respectively.
class Alloc_Free
{
public:
   static void* operator new(size_t size)
   {
      return Alloc(size);
   }
   static void operator delete(void *p)
   {
      return Free(p);
   }

   int _data;
};

範例：交換和reverse_array函式

下列範例顯示 swap 和 reverse_array 函式。函 swap 式會在指定的索引處交換陣列的內容。它會從堆積配置暫存變數的記憶體。函 reverse_array 式會建立大型陣列，並計算平行反轉該陣列數次所需的時間。

// Exchanges the contents of a[index1] with a[index2].
template<class T>
void swap(T* a, int index1, int index2)
{
   // For illustration, allocate memory from the heap.
   // This is useful when sizeof(T) is large.
   T* temp = new T;
   
   *temp = a[index1];
   a[index1] = a[index2];
   a[index2] = *temp;
   
   delete temp;
}

// Computes the time that it takes to reverse the elements of a 
// large array of the specified type.
template <typename T>
__int64 reverse_array()
{
    const int size = 5000000;
    T* a = new T[size];   
    
    __int64 time = 0;
    const int repeat = 11;

    // Repeat the operation several times to amplify the time difference.
    for (int i = 0; i < repeat; ++i)
    {
        time += time_call([&] {
            parallel_for(0, size/2, [&](int index) 
            {
                swap(a, index, size-index-1); 
            });
        });
    }

    delete[] a;
    return time;
}

範例：wmain 函式

下列範例顯示函 wmain 式，其會計算函式對 new_delete 、 malloc_free 和 Alloc_Free 型別採取行動所需的時間 reverse_array ，每個類型都會使用不同的記憶體配置。

int wmain()
{  
   // Compute the time that it takes to reverse large arrays of 
   // different types.

   // new_delete
   wcout << L"Took " << reverse_array<new_delete>() 
         << " ms with new/delete." << endl;

   // malloc_free
   wcout << L"Took " << reverse_array<malloc_free>() 
         << " ms with malloc/free." << endl;

   // Alloc_Free
   wcout << L"Took " << reverse_array<Alloc_Free>() 
         << " ms with Alloc/Free." << endl;
}

完成程式碼範例

完整的範例如下。

// allocators.cpp
// compile with: /EHsc 
#include <windows.h>
#include <ppl.h>
#include <iostream>

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;
}

// A type that defines the new and delete operators. These operators 
// call the global new and delete operators, respectively.
class new_delete
{
public:
   static void* operator new(size_t size)
   {
      return ::operator new(size);
   }
   
   static void operator delete(void *p)
   {
      return ::operator delete(p);
   }

   int _data;
};

// A type that defines the new and delete operators. These operators 
// call the C Runtime malloc and free functions, respectively.
class malloc_free
{
public:
   static void* operator new(size_t size)
   {
      return malloc(size);
   }
   static void operator delete(void *p)
   {
      return free(p);
   }

   int _data;
};

// A type that defines the new and delete operators. These operators 
// call the Concurrency Runtime Alloc and Free functions, respectively.
class Alloc_Free
{
public:
   static void* operator new(size_t size)
   {
      return Alloc(size);
   }
   static void operator delete(void *p)
   {
      return Free(p);
   }

   int _data;
};

// Exchanges the contents of a[index1] with a[index2].
template<class T>
void swap(T* a, int index1, int index2)
{
   // For illustration, allocate memory from the heap.
   // This is useful when sizeof(T) is large.
   T* temp = new T;
   
   *temp = a[index1];
   a[index1] = a[index2];
   a[index2] = *temp;
   
   delete temp;
}

// Computes the time that it takes to reverse the elements of a 
// large array of the specified type.
template <typename T>
__int64 reverse_array()
{
    const int size = 5000000;
    T* a = new T[size];   
    
    __int64 time = 0;
    const int repeat = 11;

    // Repeat the operation several times to amplify the time difference.
    for (int i = 0; i < repeat; ++i)
    {
        time += time_call([&] {
            parallel_for(0, size/2, [&](int index) 
            {
                swap(a, index, size-index-1); 
            });
        });
    }

    delete[] a;
    return time;
}

int wmain()
{  
   // Compute the time that it takes to reverse large arrays of 
   // different types.

   // new_delete
   wcout << L"Took " << reverse_array<new_delete>() 
         << " ms with new/delete." << endl;

   // malloc_free
   wcout << L"Took " << reverse_array<malloc_free>() 
         << " ms with malloc/free." << endl;

   // Alloc_Free
   wcout << L"Took " << reverse_array<Alloc_Free>() 
         << " ms with Alloc/Free." << endl;
}

此範例會針對具有四個處理器的電腦產生下列範例輸出。

Took 2031 ms with new/delete.
Took 1672 ms with malloc/free.
Took 656 ms with Alloc/Free.

在此範例中，使用 Alloc 和函式的類型可提供最佳的記憶體效能，因為 Alloc 和 FreeFree 函式已針對從多個執行緒頻繁配置和釋放記憶體區塊而優化。

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 allocators.cpp 的檔案中，然後在 Visual Studio 命令提示字元視窗中執行下列命令。

cl.exe /EHsc allocators.cpp

另請參閱

記憶體管理函式
 Alloc 函式
 Free 函式

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