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Reference Guide
mp101_fillNtuples.C
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1 /// \file
2 /// \ingroup tutorial_multicore
3 /// \notebook -nodraw
4 /// Fill n-tuples in distinct workers.
5 /// This tutorial illustrates the basics of how it's possible with ROOT to
6 /// offload heavy operations on multiple processes and how it's possible to write
7 /// simultaneously multiple files. The operation performed in this case is the
8 /// creation of random gaussian numbers.
9 ///
10 /// \macro_code
11 ///
12 /// \date January 2016
13 /// \author Danilo Piparo
14 
15 // Some useful constants and functions
16 
17 // Total amount of numbers
18 const UInt_t nNumbers = 20000000U;
19 
20 // The number of workers
21 const UInt_t nWorkers = 4U;
22 
23 // We split the work in equal parts
24 const auto workSize = nNumbers / nWorkers;
25 
26 // A simple function to fill ntuples randomly
27 void fillRandom(TNtuple &ntuple, TRandom3 &rndm, UInt_t n)
28 {
29  for (auto i : ROOT::TSeqI(n))
30  ntuple.Fill(rndm.Gaus());
31 }
32 
33 Int_t mp101_fillNtuples()
34 {
35 
36  // No nuisance for batch execution
37  gROOT->SetBatch();
38 
39  //---------------------------------------
40  // Perform the operation sequentially
41 
42  // Create a random generator and and Ntuple to hold the numbers
43  TRandom3 rndm(1);
44  TFile ofile("mp101_singleCore.root", "RECREATE");
45  TNtuple randomNumbers("singleCore", "Random Numbers", "r");
46  fillRandom(randomNumbers, rndm, nNumbers);
47  randomNumbers.Write();
48  ofile.Close();
49 
50  //---------------------------------------
51  // We now go MP!
52 
53  // We define our work item
54  auto workItem = [](UInt_t workerID) {
55  // One generator, file and ntuple per worker
56  TRandom3 workerRndm(workerID); // Change the seed
57  TFile ofile(Form("mp101_multiCore_%u.root", workerID), "RECREATE");
58  TNtuple workerRandomNumbers("multiCore", "Random Numbers", "r");
59  fillRandom(workerRandomNumbers, workerRndm, workSize);
60  workerRandomNumbers.Write();
61  return 0;
62  };
63 
64  // Create the pool of workers
65  ROOT::TProcessExecutor workers(nWorkers);
66 
67  // Fill the pool with work
68  workers.Map(workItem, ROOT::TSeqI(nWorkers));
69 
70  return 0;
71 }
n
const Int_t n
Definition: legend1.C:16
TRandom::Gaus
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
TNtuple
A simple TTree restricted to a list of float variables only.
Definition: TNtuple.h:28
Form
char * Form(const char *fmt,...)
Int_t
int Int_t
Definition: RtypesCore.h:45
TRandom3
Random number generator class based on M.
Definition: TRandom3.h:27
UInt_t
unsigned int UInt_t
Definition: RtypesCore.h:46
TFile
A ROOT file is a suite of consecutive data records (TKey instances) with a well defined format.
Definition: TFile.h:54
ROOT::TProcessExecutor
This class provides a simple interface to execute the same task multiple times in parallel,...
Definition: TProcessExecutor.hxx:38
ROOT::TSeq
A pseudo container class which is a generator of indices.
Definition: TSeq.hxx:66
TNtuple::Fill
virtual Int_t Fill()
Fill a Ntuple with current values in fArgs.
Definition: TNtuple.cxx:169
gROOT
#define gROOT
Definition: TROOT.h:406