为什么对于基于RB树的C ++ std :: set的插入时间基准,我得到的是常数而不是对数曲线?

时间:2018-08-21 15:51:17

标签: c++ stl

我在Heap vs Binary Search Tree (BST)处将BST和Heap进行了比较,但是当我尝试对两者进行基准测试并比较结果时,我无法解释BST的数据。

首先,我确认标准库确实使用了红黑树:What is the underlying data structure of a STL set in C++?

然后我运行了这个基准测试。

main.cpp

#include <chrono>
#include <iostream>
#include <random>
#include <set>

int main(int argc, char **argv) {
    size_t i, n;
    std::set<int> bst;
    std::random_device dev;
    unsigned int seed = dev();
    std::mt19937 prng(seed);
    std::uniform_int_distribution<> dist;

    if (argc > 1) {
        n = std::stoi(argv[1]);
    } else {
        n = 1000000;
    }
    for (i = 0; i < n; ++i) {
        auto random_value = dist(prng);
        auto start = std::chrono::steady_clock::now();
        bst.insert(random_value);
        auto end = std::chrono::steady_clock::now();
        auto dt_bst = end - start;
        std::cout << random_value << " "
            << std::chrono::duration_cast<std::chrono::nanoseconds>(dt_bst).count() << std::endl;
    }
}

plot.gnuplot:

#!/usr/bin/env gnuplot
set terminal png size 1024, 1024
set output "bst_vs_heap.png"
set title "BST insert time"
set xlabel "size"
set ylabel "nanoseconds"
plot "main.dat" using 2 notitle

编译,运行和绘图:

g++ -ggdb3 -O3 -std=c++17 -Wall -Wextra -pedantic-errors -o main.out main.cpp
./main.out 10000000 > main.dat
./plot.gnuplot

结果:

enter image description here

为什么在理论数据结构中我看不到一条很好的对数曲线,而是一条带有一些离群值的恒定线?

Ubuntu 18.04,GCC 7.3,Intel i7-7820HQ CPU,DDR4 2400 MHz RAM,联想Thinkpad P51。

1 个答案:

答案 0 :(得分:10)

如注释中所述,时钟可能不够准确,因此我尝试将一堆插入片段分组在一起并计时以改善信噪比,并且它起作用了,我现在可以看到对数:< / p>

#include <chrono>
#include <iostream>
#include <random>
#include <set>

int main(int argc, char **argv) {
    size_t i, j, n, granule;
    std::set<int> bst;
    std::random_device dev;
    unsigned int seed = dev();
    std::mt19937 prng(seed);
    std::uniform_int_distribution<> dist;
    int *randoms;

    if (argc > 1) {
        n = std::stoi(argv[1]);
    } else {
        n = 1000000;
    }
    if (argc > 2) {
        granule = std::stoi(argv[2]);
    } else {
        granule = 10;
    }
    randoms = new int[granule];
    for (i = 0; i < n / granule; ++i) {
        for (j = 0; j < granule; ++j) {
            randoms[j] = dist(prng);
        }
        auto start = std::chrono::high_resolution_clock::now();
        for (j = 0; j < granule; ++j) {
            bst.insert(randoms[j]);
        }
        auto end = std::chrono::high_resolution_clock::now();
        auto dt_bst = end - start;
        std::cout << std::chrono::duration_cast<std::chrono::nanoseconds>(dt_bst).count() << std::endl;
    }
    delete[] randoms;
}

命令:

./main.out 100000000 10000

图:

enter image description here