mt_fillHistos.C

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/// \file
/// \ingroup tutorial_analysis_parallel
/// \notebook
/// Fill histograms in parallel and write them on file
/// with a multithreaded approach using std::thread.
/// This is the simplest meaningful example which shows 
/// ROOT thread awareness.
///
/// \macro_code
///
/// \date January 2016
/// \author Danilo Piparo
 
// Total amount of numbers
const UInt_t nNumbers = 20000000U;
 
// The number of workers
const UInt_t nWorkers = 4U;
 
Int_t mt_fillHistos()
{
 
   // The first, fundamental operation to be performed in order to make ROOT
   // thread-aware.
   ROOT::EnableThreadSafety();
 
   // We define our work item
   auto workItem = [](UInt_t workerID) {
      // One generator, file and ntuple per worker
      TRandom3 workerRndm(workerID); // Change the seed
      TFile f(Form("myFile_mt001_%u.root", workerID), "RECREATE");
      TH1F h(Form("myHisto_%u", workerID), "The Histogram", 64, -4, 4);
      for (UInt_t i = 0; i < nNumbers; ++i) {
         h.Fill(workerRndm.Gaus());
      }
      h.Write();
   };
 
   // Create the collection which will hold the threads, our "pool"
   std::vector<std::thread> workers;
 
   // Fill the "pool" with workers
   for (auto workerID : ROOT::TSeqI(nWorkers)) {
      workers.emplace_back(workItem, workerID);
   }
 
   // Now join them
   for (auto &&worker : workers)
      worker.join();
 
   return 0;
}