c++ async : how to shuffle a vector in multithread

Question

Running a multithreaded program, I noticed that the program was running faster using 1 thread compared to 4 threads, despite the CPU having 4 cores.

After investigating, I found out that the issue appears only when shuffling something.

Below the minimal program I created to reproduce the problem:

#include <math.h>
#include <future>
#include <ctime>
#include <vector>
#include <iostream>
#include <algorithm>

#define NB_JOBS 5000.0
#define MAX_CORES 8

static bool _no_shuffle(int nb_jobs){
  bool b=false;
  for(int i=0;i<nb_jobs;i++){
    std::vector<float> v;
    for(float i=0;i<100.0;i+=0.01) v.push_back(i);
    float sum = 0;
    // no meaning, just using CPU
    for(int i=0;i<v.size();i++) sum+=pow(sin(v[i]),1.1);
    if(sum==100) b=true;
  }
  return b;

}

static bool _shuffle(int nb_jobs){
  bool b=false;
  for(int i=0;i<nb_jobs;i++){
    std::vector<float> v;
    for(float i=0;i<100.0;i+=0.01) v.push_back(i);
    std::random_shuffle(v.begin(), v.end()); // !!! 
    if (v[0]==0.0) b=true;
  }
  return b;
}

static double _duration(int nb_cores){

  auto start = std::chrono::system_clock::now();

  int nb_jobs_per_core = rint ( NB_JOBS / (float)nb_cores );

  std::vector < std::future<bool> > futures;
  for(int i=0;i<nb_cores;i++){
    futures.push_back( std::async(std::launch::async,_shuffle,nb_jobs_per_core));
  }
  for (auto &e: futures) {
    bool foo = e.get();
  }

  auto end = std::chrono::system_clock::now();
  std::chrono::duration<double> elapsed = end - start;

  return elapsed.count();

}


int main(){

  for(int nb_cores=1 ; nb_cores<=MAX_CORES ; nb_cores++){

    double duration = _duration(nb_cores);
    std::cout << nb_cores << " threads: " << duration << " seconds\n";

  }

  return 0;

}

which prints:

1 threads: 1.18503 seconds
2 threads: 9.6502 seconds
3 threads: 3.64973 seconds
4 threads: 9.8834 seconds
5 threads: 10.7937 seconds
6 threads: 11.6447 seconds
7 threads: 11.9236 seconds
8 threads: 12.1254 seconds

Using threads slow down the program !

on the other hand, when replacing:

std::async(std::launch::async,_shuffle,nb_jobs_per_core));

with:

std::async(std::launch::async,_no_shuffle,nb_jobs_per_core));

then:

1 threads: 3.24132 seconds
2 threads: 1.62207 seconds
3 threads: 1.1745 seconds
4 threads: 1.09769 seconds
5 threads: 1.03182 seconds
6 threads: 0.892274 seconds
7 threads: 0.782815 seconds
8 threads: 0.704777 seconds

which seems as expected, indicating shuffling is indeed the issue.

Is shuffling not thread friendly, and if so, how to shuffle a vector in a multi-threaded program ?