mp_fillNtuples.C File Reference

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 processes and how it's possible to write simultaneously multiple files. The operation performed in this case is the creation of random gaussian numbers.

 
// 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 mp_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;
}

Date

January 2016

Author

Danilo Piparo

Definition in file mp_fillNtuples.C.