mp101_fillNtuples.C

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/// \file
/// \ingroup tutorial_multicore
/// \notebook -nodraw
/// Fill n-tuples in distinct workers.
/// This tutorial illustrates the basics of how it's possible with ROOT to
/// offload heavy operations on multiple processes and how it's possible to write
/// simultaneously multiple files. The operation performed in this case is the
/// creation of random gaussian numbers.
///
/// \macro_code
///
/// \date January 2016
/// \author Danilo Piparo

// Some useful constants and functions

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

// The number of workers
const UInt_t nWorkers = 4U;

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

// A simple function to fill ntuples randomly
void fillRandom(TNtuple &ntuple, TRandom3 &rndm, UInt_t n)
{
   for (auto i : ROOT::TSeqI(n))
      ntuple.Fill(rndm.Gaus());
}

Int_t mp101_fillNtuples()
{

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

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

   // Create a random generator and and Ntuple to hold the numbers
   TRandom3 rndm(1);
   TFile ofile("mp101_singleCore.root", "RECREATE");
   TNtuple randomNumbers("singleCore", "Random Numbers", "r");
   fillRandom(randomNumbers, rndm, nNumbers);
   randomNumbers.Write();
   ofile.Close();

   //---------------------------------------
   // We now go MP!

   // We define our work item
   auto workItem = [](UInt_t workerID) {
      // One generator, file and ntuple per worker
      TRandom3 workerRndm(workerID); // Change the seed
      TFile ofile(Form("mp101_multiCore_%u.root", workerID), "RECREATE");
      TNtuple workerRandomNumbers("multiCore", "Random Numbers", "r");
      fillRandom(workerRandomNumbers, workerRndm, workSize);
      workerRandomNumbers.Write();
      return 0;
   };

   // Create the pool of workers
   ROOT::TProcessExecutor workers(nWorkers);

   // Fill the pool with work
   workers.Map(workItem, ROOT::TSeqI(nWorkers));

   return 0;
}