Logo ROOT   6.08/07
Reference Guide
mtbb101_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.

/// \date January 2016
// Some useful constants and functions
// Total amount of numbers
const UInt_t nNumbers = 20000000U;
// The number of workers
const UInt_t nThreads = 4U;
// We split the work in equal parts
const auto workSize = nNumbers / nThreads;
// 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 mtbb101_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::TThreadExecutor pool(nThreads);
// Fill the pool with work
pool.Map(workItem, ROOT::TSeqI(nThreads));
return 0;
}
Author
Danilo Piparo

Definition in file mtbb101_fillNtuples.C.