Memory access - sequential vs. random

memaccess.cpp

#include <iostream>
#include <sstream>
#include <iomanip>
#include <vector>
#include <chrono>
#include <random>
#include <limits>

int main()
{
    std::mt19937 mt { 0xfeedbeef };
    std::vector<unsigned> testdata;
    const size_t testsize = 1024 * 1024 * 25;
    std::uniform_int_distribution<unsigned> datadist { 0, std::numeric_limits<unsigned>::max() };
    std::stringstream ss;
    
    for(size_t i = 0; i < testsize; i++) {
        testdata.push_back(datadist(mt));
    }
    
    unsigned total = 0;
    auto start = std::chrono::high_resolution_clock::now();
    for(auto val : testdata) {
        total += val;
    }
    auto end = std::chrono::high_resolution_clock::now();
    ss << total;
    std::cout << "sequential: " << std::right << std::setw(9) << std::fixed << std::setprecision(3) << (double)testsize * sizeof(unsigned) / 1024 / 1024 /
        std::chrono::duration_cast<std::chrono::nanoseconds>(end - start).count() * 1e9 << " MB/s\n";

    unsigned total2 = 0;
    std::uniform_int_distribution<unsigned> indexdist { 0, testsize - 1 };
    auto start2 = std::chrono::high_resolution_clock::now();
    for(size_t i = 0; i < testsize; i++) {
        total2 += testdata[indexdist(mt)];
    }
    auto end2 = std::chrono::high_resolution_clock::now();
    ss << total2;
    std::cout << "random:     " << std::right << std::setw(9) << std::fixed << std::setprecision(3) << (double)testsize * sizeof(unsigned) / 1024 / 1024 /
        std::chrono::duration_cast<std::chrono::nanoseconds>(end2 - start2).count() * 1e9 << " MB/s\n";
}

results.txt

$ g++ -Wall -Wextra -pedantic -std=c++1y -O3 -march=native memaccess.cpp -o memaccess && ./memaccess
sequential: 14283.674 MB/s
random:       164.464 MB/s