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mp101_fillNtuples.C File Reference

Detailed Description

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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 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;
}
int Int_t
Definition RtypesCore.h:45
unsigned int UInt_t
Definition RtypesCore.h:46
#define gROOT
Definition TROOT.h:407
char * Form(const char *fmt,...)
Formats a string in a circular formatting buffer.
Definition TString.cxx:2467
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:67
A ROOT file is composed of a header, followed by consecutive data records (TKey instances) with a wel...
Definition TFile.h:53
A simple TTree restricted to a list of float variables only.
Definition TNtuple.h:28
Random number generator class based on M.
Definition TRandom3.h:27
const Int_t n
Definition legend1.C:16
This file contains a specialised ROOT message handler to test for diagnostic in unit tests.
Date
January 2016
Author
Danilo Piparo

Definition in file mp101_fillNtuples.C.