very rough shared memory profiling

shell.nix

let
  pkgs = import <nixpkgs> { };
in
with pkgs;
mkShell {
  name = "profile-shmem";
  buildInputs = [ clang_11 boost ];
}

shmem_profile.cpp

#include <algorithm>
#include <random>
#include <iostream>
#include <chrono>
#include <cstdlib>

#include <boost/interprocess/mapped_region.hpp>
#include <boost/interprocess/shared_memory_object.hpp>

namespace shm = boost::interprocess;

int main(int argc, const char* argv[]) {
  std::random_device rd;
  std::mt19937 rng(rd());
  std::uniform_int_distribution<uint8_t> dist(0, 254);

  size_t bytes = 5000000;
  std::vector<uint8_t> data(bytes, 0);

  size_t nruns = 5;

  using unit = std::chrono::microseconds;
  static const char* unit_name = "us";

  std::vector<unit> points;
  points.reserve(nruns);
  
  {
    shm::shared_memory_object shm_obj(shm::open_or_create,"shared_memory", shm::read_write);
    shm::mapped_region region(shm_obj, shm::read_write, 0, bytes);

    shm_obj.truncate(bytes);

    auto address = reinterpret_cast<uint8_t*>(region.get_address());

    for (size_t run = 0; run < nruns; ++run) {
      std::generate(address, address + bytes, [&dist, &rng]() { return dist(rng); });
      auto start = std::chrono::steady_clock::now();
      std::copy(address, address + bytes, data.data());
      auto end = std::chrono::steady_clock::now();
      points.push_back(std::chrono::duration_cast<unit>(end - start));
    }
    shm::shared_memory_object::remove("shared_memory");
  }
  

  using limits_ssize_t = std::numeric_limits<ssize_t>;
  auto avg = std::accumulate(points.cbegin(), points.cend(), unit(0)) / nruns;
  auto min = std::accumulate(points.cbegin(), points.cend(), unit(limits_ssize_t::max()), [](auto a, auto b){return std::min(a, b);});
  auto max = std::accumulate(points.cbegin(), points.cend(), unit(limits_ssize_t::min()), [](auto a, auto b){return std::max(a, b);});
  std::cout << "avg: " << avg.count() << " " << unit_name << std::endl;
  std::cout << "min: " << min.count() << " " << unit_name << std::endl;
  std::cout << "max: " << max.count() << " " << unit_name << std::endl;

  return 0;
}

Enter a nix shell:

$ nix-shell shell.nix

Compile with

$ clang++ -std=c++17 -o shmem_profile shmem_profile.cpp -lrt

Times:

avg: 190 us
min: 167 us
max: 262 us