go和c ++之间的地图性能比较[关闭]

Question

I don't understand how golang is outperforming c++ in this operation by 10 times, even the map lookup is 3 times faster in go than c++.

this is the c++ snippet

#include <iostream>
#include <unordered_map>
#include <chrono>

std::chrono::nanoseconds elapsed(std::chrono::steady_clock::time_point start) {
    std::chrono::steady_clock::time_point now = std::chrono::high_resolution_clock::now();
    return std::chrono::duration_cast<std::chrono::nanoseconds>(now - start);
}
void make_map(int times) {
    std::unordered_map<double, double> hm;
    double c = 0.0;
    for (int i = 0; i < times; i++) {
        hm[c] = c + 10.0;
        c += 1.0;
    }
}

int main() {
    std::chrono::steady_clock::time_point start_time = std::chrono::high_resolution_clock::now();
    make_map(10000000);
    printf("elapsed %lld", elapsed(start_time).count());
}

this is the golang snippet:

func makeMap() {
    o := make(map[float64]float64)
    var i float64 = 0
    x := time.Now()
    for ; i <= 10000000; i++ {
        o[i] = i+ 10
    }
    TimeTrack(x)
}
func TimeTrack(start time.Time) {
    elapsed := time.Since(start)

    // Skip this function, and fetch the PC and file for its parent.
    pc, _, _, _ := runtime.Caller(1)

    // Retrieve a function object this functions parent.
    funcObj := runtime.FuncForPC(pc)

    // Regex to extract just the function name (and not the module path).
    runtimeFunc := regexp.MustCompile(`^.*\.(.*)$`)
    name := runtimeFunc.ReplaceAllString(funcObj.Name(), "$1")

    log.Println(fmt.Sprintf("%s took %s", name, elapsed))
}

What I'd like to know is how to optimize the c++ to achieve better performance.

Answer 1

Updated to measure similar operations for both cpp and go. It starts measurment before calling the map-making function and ends it when the function returns. Both versions reserve space in the map and return the created map (from which a couple of numbers are printed).

Slightly modified cpp:

#include <iostream>
#include <unordered_map>
#include <chrono>

std::unordered_map<double, double> make_map(double times) {
    std::unordered_map<double, double> m(times);

    for (double c = 0; c < times; ++c) {
        m[c] = c + 10.0;
    }
    return m;
}

int main() {
    std::chrono::high_resolution_clock::time_point start_time = std::chrono::high_resolution_clock::now();
    auto m = make_map(10000000);
    std::chrono::high_resolution_clock::time_point end_time = std::chrono::high_resolution_clock::now();
    auto elapsed = std::chrono::duration_cast<std::chrono::nanoseconds>(end_time-start_time);
    std::cout << elapsed.count()/1000000000. << "s
";
    std::cout << m[10] << "
"
              << m[9999999] << "
";    
}

% g++ -DNDEBUG -std=c++17 -Ofast -o perf perf.cpp
% ./perf
2.81886s
20
1e+07

Slightly modified go version:

package main

import (
    "fmt"
    "time"
)

func make_map(elem float64) map[float64]float64 {
    m := make(map[float64]float64, int(elem))
    var i float64 = 0
    for ; i < elem; i++ {
        m[i] = i + 10
    }
    return m
}

func main() {
    start_time := time.Now()
    r := make_map(10000000)
    end_time := time.Now()
    fmt.Println(end_time.Sub(start_time))
    fmt.Println(r[10])
    fmt.Println(r[9999999])
}

% go build -a perf.go
% ./perf
1.967707381s
20
1.0000009e+07

It doesn't look like a tie as it did before the update. One thing slowing the cpp version down is the default hashing function for double. When replacing it with a really bad (but fast) hasher, I got the time down to 1.89489s.

struct bad_hasher {
    size_t operator()(const double& d) const {
        static_assert(sizeof(double)==sizeof(size_t));

        return
            *reinterpret_cast<const size_t*>( reinterpret_cast<const std::byte*>(&d) );
    }
};