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Reference Guide
mp101_fillNtuples.C File Reference

Detailed Description

View in nbviewer Open in SWAN 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))
Int_t mp101_fillNtuples()
// No nuisance for batch execution
// 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);
// 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);
return 0;
// Create the pool of workers
ROOT::TProcessExecutor workers(nWorkers);
// Fill the pool with work
workers.Map(workItem, ROOT::TSeqI(nWorkers));
return 0;
int Int_t
Definition: RtypesCore.h:45
unsigned int UInt_t
Definition: RtypesCore.h:46
#define gROOT
Definition: TROOT.h:406
char * Form(const char *fmt,...)
This class provides a simple interface to execute the same task multiple times in parallel,...
A pseudo container class which is a generator of indices.
Definition: TSeq.hxx:66
A ROOT file is a suite of consecutive data records (TKey instances) with a well defined format.
Definition: TFile.h:54
A simple TTree restricted to a list of float variables only.
Definition: TNtuple.h:28
virtual Int_t Fill()
Fill a Ntuple with current values in fArgs.
Definition: TNtuple.cxx:169
Random number generator class based on M.
Definition: TRandom3.h:27
virtual Double_t Gaus(Double_t mean=0, Double_t sigma=1)
Samples a random number from the standard Normal (Gaussian) Distribution with the given mean and sigm...
Definition: TRandom.cxx:274
const Int_t n
Definition: legend1.C:16
January 2016
Danilo Piparo

Definition in file mp101_fillNtuples.C.