mt101_fillNtuples.C File Reference

Fill n-tuples in distinct workers. More...

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Detailed Description

Fill n-tuples in distinct workers.

This tutorial illustrates the basics of how it's possible with ROOT to offload heavy operations on multiple threads and how it's possible to write simultaneously multiple files. The operation performed in this case is the creation of random gaussian numbers. NOTE: this code can be executed in a macro, ACLiC'ed or not, but not yet at the command line prompt.

// Measure time in a scope
class TimerRAII {
   TStopwatch fTimer;
   std::string fMeta;
public:
   TimerRAII(const char *meta): fMeta(meta) {
      fTimer.Start();
   }
   ~TimerRAII() {
      fTimer.Stop();
      std::cout << fMeta << " - real time elapsed " << fTimer.RealTime() << "s" << std::endl;
   }
};

Int_t mt101_fillNtuples(UInt_t nWorkers = 4)
{

   // No nuisance for batch execution
   gROOT->SetBatch();

   // Total amount of numbers
   const UInt_t nNumbers = 20000000U;

   // A simple function to fill ntuples randomly

   auto fillRandom = [](TNtuple & ntuple, TRandom3 & rndm, UInt_t n) {
      for (UInt_t i = 0; i < n; ++i) ntuple.Fill(rndm.Gaus());
   };

   // Perform the operation sequentially ---------------------------------------

   // Create a random generator and and Ntuple to hold the numbers
   TRandom3 rndm(1);
   TFile ofile("mt101_singleCore.root", "RECREATE");
   TNtuple randomNumbers("singleCore", "Random Numbers", "r");

   // Now let's measure how much time we need to fill it up
   {
      TimerRAII t("Sequential execution");
      fillRandom(randomNumbers, rndm, nNumbers);
      randomNumbers.Write();
   }


   // We now go MT! ------------------------------------------------------------

   // The first, fundamental operation to be performed in order to make ROOT
   // thread-aware.
   ROOT::EnableThreadSafety();

   // We define our work item
   auto workItem = [&fillRandom](UInt_t workerID, UInt_t workSize) {
      // One generator, file and ntuple per worker
      TRandom3 workerRndm(workerID); // Change the seed
      TFile ofile(Form("mt101_multiCore_%u.root", workerID), "RECREATE");
      TNtuple workerRandomNumbers("multiCore", "Random Numbers", "r");
      fillRandom(workerRandomNumbers, workerRndm, workSize);
      workerRandomNumbers.Write();
   };

   // Create the collection which will hold the threads, our "pool"
   std::vector<std::thread> workers;

   // We measure time here as well
   {
      TimerRAII t("Parallel execution");

      // We split the work in equal parts
      const auto workSize = nNumbers / nWorkers;

      // Fill the "pool" with workers
      for (UInt_t workerID = 0; workerID < nWorkers; ++workerID) {
         workers.emplace_back(workItem, workerID, workSize);
      }

      // Now join them
      for (auto && worker : workers) worker.join();
   }

   return 0;

}

Author

Danilo Piparo

Definition in file mt101_fillNtuples.C.