mt101_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 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.

 
// 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 mt101_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 MT! ------------------------------------------------------------
 
   // 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 ofile(Form("mt101_multiCore_%u.root", workerID), "RECREATE");
      TNtuple workerRandomNumbers("multiCore", "Random Numbers", "r");
      fillRandom(workerRandomNumbers, workerRndm, workSize);
      workerRandomNumbers.Write();
      return 0;
   };
 
   // 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;
}

Date

January 2016

Author

Danilo Piparo

Definition in file mt101_fillNtuples.C.