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Reference Guide
mt001_fillHistos.C
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1/// \file
2/// \ingroup tutorial_multicore
3/// \notebook
4/// Fill histograms in parallel and write them on file.
5/// The simplest meaningful possible example which shows ROOT thread awareness.
6///
7/// \macro_code
8///
9/// \date January 2016
10/// \author Danilo Piparo
11
12// Total amount of numbers
13const UInt_t nNumbers = 20000000U;
14
15// The number of workers
16const UInt_t nWorkers = 4U;
17
18Int_t mt001_fillHistos()
19{
20
21 // The first, fundamental operation to be performed in order to make ROOT
22 // thread-aware.
24
25 // We define our work item
26 auto workItem = [](UInt_t workerID) {
27 // One generator, file and ntuple per worker
28 TRandom3 workerRndm(workerID); // Change the seed
29 TFile f(Form("myFile_mt001_%u.root", workerID), "RECREATE");
30 TH1F h(Form("myHisto_%u", workerID), "The Histogram", 64, -4, 4);
31 for (UInt_t i = 0; i < nNumbers; ++i) {
32 h.Fill(workerRndm.Gaus());
33 }
34 h.Write();
35 };
36
37 // Create the collection which will hold the threads, our "pool"
38 std::vector<std::thread> workers;
39
40 // Fill the "pool" with workers
41 for (auto workerID : ROOT::TSeqI(nWorkers)) {
42 workers.emplace_back(workItem, workerID);
43 }
44
45 // Now join them
46 for (auto &&worker : workers)
47 worker.join();
48
49 return 0;
50}
#define f(i)
Definition: RSha256.hxx:104
#define h(i)
Definition: RSha256.hxx:106
int Int_t
Definition: RtypesCore.h:45
unsigned int UInt_t
Definition: RtypesCore.h:46
char * Form(const char *fmt,...)
Formats a string in a circular formatting buffer.
Definition: TString.cxx:2456
A pseudo container class which is a generator of indices.
Definition: TSeq.hxx:67
A ROOT file is a suite of consecutive data records (TKey instances) with a well defined format.
Definition: TFile.h:54
1-D histogram with a float per channel (see TH1 documentation)}
Definition: TH1.h:577
Random number generator class based on M.
Definition: TRandom3.h:27
void EnableThreadSafety()
Enables the global mutex to make ROOT thread safe/aware.
Definition: TROOT.cxx:493