std::queue<T, list<T>>::size() 在 O(n) 中很慢?

2022-01-21 00:00:00 queue performance c++ stl

我的代码使用队列时遇到了意外的性能行为.我意识到当队列中有更多元素时性能会下降.事实证明，使用 size() 方法是原因.这是一些显示问题的代码:

I experienced unexpected performance behavior of my code which uses a queue. I realized that performance degraded when more elements were in the queue. It turned out that usage of the size() method was the reason. Here is some code that shows the problem:

#include <queue>
#include <list>
#include <iostream>

#include "Stopwatch.h"

using namespace std;

struct BigStruct
{
    int x[100];
};
int main()
{
    CStopwatch queueTestSw;

    typedef BigStruct QueueElementType;
    typedef std::queue<QueueElementType, std::list<QueueElementType> > QueueType;
    //typedef std::queue<QueueElementType > QueueType; //no surprise, this queue is fast and constant
    QueueType m_queue;

    for (int i=0;i<22000;i++)
        m_queue.push(QueueElementType());
    CStopwatch sw;
    sw.Start();
    int dummy;
    while (!m_queue.empty())
    {
        //remove 1000 elements:
        for (int i=0;i<1000;i++)
        {
            m_queue.pop();
        }
        //call size() 1000 times and see how long it takes
        sw = CStopwatch();
        sw.Start();
        for (int i=0;i<1000;i++)
        {   
            if (m_queue.size() == 123456)
            {
                dummy++;
            }
        }
        std::cout << m_queue.size() << " items left. time: " << sw.GetElapsedTimeInSeconds() << std::endl;  
    }   
    return dummy;


}

其中 CStopwatch 是一个使用 clock_gettime(CLOCK_REALTIME, ..) 的类.结果是:

Where CStopwatch is a class which uses clock_gettime(CLOCK_REALTIME, ..). The result is:

21000 items left. time: 1.08725
20000 items left. time: 0.968897
19000 items left. time: 0.818259
18000 items left. time: 0.71495
17000 items left. time: 0.583725
16000 items left. time: 0.497451
15000 items left. time: 0.422712
14000 items left. time: 0.352949
13000 items left. time: 0.30133
12000 items left. time: 0.227588
11000 items left. time: 0.178617
10000 items left. time: 0.124512
9000 items left. time: 0.0893425
8000 items left. time: 0.0639174
7000 items left. time: 0.0476441
6000 items left. time: 0.033177
5000 items left. time: 0.0276136
4000 items left. time: 0.022112
3000 items left. time: 0.0163013
2000 items left. time: 0.0101932
1000 items left. time: 0.00506138

这似乎与 http://www.cplusplus.com/reference/stl/queue/size/相矛盾:

复杂性:常数.

如果结构更大，问题会更严重.我正在使用 GCC 4.3.2.

The problem is worse if the struct is bigger. I am using GCC 4.3.2.

你能解释一下吗?有没有办法使用列表来解决这个问题?

Can you explain this? Is there a way to solve this using the list?

(请注意，我需要使用列表作为队列的底层容器，因为我需要恒定的时间插入复杂度.)

(Please note that I need to use the list as underlying container of the queue because I need constant time insertion complexity.)

推荐答案

queue是一个容器adaptor，所以你要明白复杂度描述可能只参考适配器自己做的工作(这确实是恒定的，即只是将调用传递给底层容器).

queue is a container adaptor, so you have to understand that the complexity descriptions may refer only to the work the adaptor does itself (which is indeed constant, namely just passing the call through to the underlying container).

例如，查看此参考:size() 调用底层容器的 size() 函数.对于 list，这在 C++98/03 中复杂度为 O(n)，在 C++11 中复杂度为 O(1).

For example, see this reference: size() calls the underlying container's size() function. For a list, this has complexity O(n) in C++98/03, and O(1) in C++11.

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