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Reference Guide
mt001_fillHistos.C File Reference

Detailed Description

View in nbviewer Open in SWAN Fill histograms in parallel and write them on file. The simplest meaningful possible example which shows ROOT thread awareness.

// Total amount of numbers
const UInt_t nNumbers = 20000000U;
// The number of workers
const UInt_t nWorkers = 4U;
Int_t mt001_fillHistos()
{
// The first, fundamental operation to be performed in order to make ROOT
// thread-aware.
// We define our work item
auto workItem = [](UInt_t workerID) {
// One generator, file and ntuple per worker
TRandom3 workerRndm(workerID); // Change the seed
TFile f(Form("myFile_mt001_%u.root", workerID), "RECREATE");
TH1F h(Form("myHisto_%u", workerID), "The Histogram", 64, -4, 4);
for (UInt_t i = 0; i < nNumbers; ++i) {
h.Fill(workerRndm.Gaus());
}
h.Write();
};
// Create the collection which will hold the threads, our "pool"
std::vector<std::thread> workers;
// Fill the "pool" with workers
for (auto workerID : ROOT::TSeqI(nWorkers)) {
workers.emplace_back(workItem, workerID);
}
// Now join them
for (auto &&worker : workers)
worker.join();
return 0;
}
Date
January 2016
Author
Danilo Piparo

Definition in file mt001_fillHistos.C.

f
#define f(i)
Definition: RSha256.hxx:122
ROOT::EnableThreadSafety
void EnableThreadSafety()
Enables the global mutex to make ROOT thread safe/aware.
Definition: TROOT.cxx:494
Form
char * Form(const char *fmt,...)
Int_t
int Int_t
Definition: RtypesCore.h:45
TRandom3
Definition: TRandom3.h:27
h
#define h(i)
Definition: RSha256.hxx:124
UInt_t
unsigned int UInt_t
Definition: RtypesCore.h:46
TFile
Definition: TFile.h:54
TH1F
1-D histogram with a float per channel (see TH1 documentation)}
Definition: TH1.h:572
ROOT::TSeq
A pseudo container class which is a generator of indices.
Definition: TSeq.hxx:66