操作員=減慢模擬

我正在運行聚合物的蒙特卡羅模擬。系統當前狀態的整個配置由名為 的物件給出Grid。這是我的定義Grid：

class Grid{

public:
    std::vector <Polymer> PolymersInGrid;                    // all the polymers in the grid 
    int x;                                                   // length of x-edge of grid 
    int y;                                                   // length of y-edge of grid 
    int z;                                                   // length of z-edge of grid 
    double kT;                                               // energy factor 
    double Emm_n ;                                           // monomer-solvent when Not aligned 
    double Emm_a ;                                           // monomer-solvent when Aligned
    double Ems;                                              // monomer-solvent interaction
    double Energy;                                           // energy of grid 
    std::map <std::vector <int>, Particle> OccupancyMap;     // a map that gives the particle given the location
    

    Grid(int xlen, int ylen, int zlen, double kT_, double Emm_a_, double Emm_n_, double Ems_): x (xlen), y (ylen), z (zlen), kT (kT_), Emm_n(Emm_n_), Emm_a (Emm_a_), Ems (Ems_) {        // Constructor of class
        // this->instantiateOccupancyMap(); 
    };


    // Destructor of class 
    ~Grid(){                                    

    }; 

    // assignment operator that allows for a correct transfer of properties. Important to functioning of program. 
    Grid& operator=(Grid other){
        std::swap(PolymersInGrid, other.PolymersInGrid); 
        std::swap(Energy, other.Energy); 
        std::swap(OccupancyMap, other.OccupancyMap);
        return *this; 
    } 
.
.
.
}

如果需要，Polymer我可以深入了解物件的詳細資訊。Particle

在我的驅動程式代碼中，這就是我要做的：定義最大迭代次數。

1. 定義一個完整的 Grid G。
2. G創建被呼叫的副本G_。
3. 我正在擾亂G_.
4. G_如果根據 Metropolis 標準接受擾動，我分配G_給G( G=G_)。
5. 重復步驟 1-4，直到達到最大迭代次數。

這是我的驅動程式代碼：

auto start = std::chrono::high_resolution_clock::now(); 
Grid G_ (G); 
    int acceptance_count = 0; 
    for (int i{1}; i< (Nmov 1); i  ){

        // choose a move 
        G_ = MoveChooser(G, v);  

        if ( MetropolisAcceptance (G.Energy, G_.Energy, G.kT) ) {
            // accepted
            // replace old config with new config

            acceptance_count  ; 
            std::cout << "Number of acceptances is " << acceptance_count << std::endl;
            G = G_;
        }


        else {
            // continue;
        }

        if (i % dfreq == 0){
            G.dumpPositionsOfPolymers (i, dfile) ;
            G.dumpEnergyOfGrid(i, efile, call) ; 
        }
        // G.PolymersInGrid.at(0).printChainCoords();


    }
    
    
    
    auto stop = std::chrono::high_resolution_clock::now(); 
    
    auto duration = std::chrono::duration_cast<std::chrono::milliseconds> (stop-start); 

    std::cout << "\n\nTime taken for simulation: " << duration.count() << " milliseconds" << std::endl;

This is the interesting part: if I run the simulation using condition that do not have lots of "acceptances" (low temperatures, bad solvent), the simulation runs pretty fast. However, if there are a large number of acceptances, the simulation gets incredibly slow. My hypothesis is that my assignment operator = is slowing down my simulation. I ran some tests:

number of acceptances = 25365, wall-clock time = 717770 milliseconds (!)

number of acceptances = 2165, wall-clock time = 64412 milliseconds

number of acceptances = 3000, wall-clock time = 75550 milliseconds

And the trend continues. Could anyone advise me on how to possibly make this more efficient? Is there a way to bypass the slowdown I am experiencing, I think, due to the = operator?

I would really appreciate any advice you have for me!

uj5u.com熱心網友回復：

您當然可以做的一件事來提高性能是強制移動_G而不是應對G：

 G = std::move(G_);

畢竟，在這個階段你不再需要G_了。

旁注。您不需要復制所有成員資料的事實operator=表明您的設計Grid遠非完美，但是，如果程式很小并且您確定您可以控制一切，請保留它。無論如何，而不是 using operator=，您應該定義和使用具有有意義名稱的成員函式，例如 " fast_and_dirty_swap" 等 :-) 然后您可以定義operator=@Jarod42 建議的方式，即 using = default。

我在 C 11 之前使用的另一種方法是對指標進行操作。在這種情況下，一個將有兩個Grids，一個是“真實的”，一個被視為緩沖區或沙箱，并且在接受時會簡單地交換指標，因此填充的“緩沖區”MoveChooser將成為真實的，當前的Grid。

偽代碼：

• 創建兩個緩沖區previous和current，每個緩沖區都能夠存盤模擬狀態
• 初始化 current
• 創建兩個指標, p_prev = &previous,p_curr = &currenrt
• 盡可能多的步驟
  • 計算下一個狀態*p_curr并將其存盤在其中*p_prev（例如monte_carlo_step(p_curr, p_prev)
  • 交換指標：現在當前系統狀態是 atp_curr和之前的 at p_prev。
• 分析存盤在 *p_curr

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