const UInt_t poolSize = 4U;

TH1::AddDirectory(false);
ROOT::TThreadExecutor pool(poolSize);
auto fillRandomHisto = [](int seed = 0) {
   TRandom3 rndm(seed);
   auto h = new TH1F("myHist", "Filled in parallel", 128, -8, 8);
   for (auto i : ROOT::TSeqI(1000000)) {
      h->Fill(rndm.Gaus(0, 1));
   }
   return h;
};

auto seeds = ROOT::TSeqI(23);
ROOT::ExecutorUtils::ReduceObjects<TH1F *> redfunc;
auto sumRandomHisto = pool.MapReduce(fillRandomHisto, seeds, redfunc);

auto c = new TCanvas();
sumRandomHisto->Draw();
return 0;

gROOT->GetListOfCanvases()->Draw()