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Reference Guide
mp102_readNtuplesFillHistosAndFit.C File Reference

Detailed Description

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Read n-tuples in distinct workers, fill histograms, merge them and fit.

We express parallelism with multiprocessing as it is done with multithreading in mt102_readNtuplesFillHistosAndFit.

Int_t mp102_readNtuplesFillHistosAndFit()
// No nuisance for batch execution
// Perform the operation sequentially
TChain inputChain("multiCore");
if (inputChain.GetNtrees() <= 0) {
Printf(" No files in the TChain: did you run mp101_fillNtuples.C before?");
return 1;
TH1F outHisto("outHisto", "Random Numbers", 128, -4, 4);
inputChain.Draw("r >> outHisto");
// We now go MP!
// TProcessExecutor offers an interface to directly process trees and chains without
// the need for the user to go through the low level implementation of a
// map-reduce.
// We adapt our parallelisation to the number of input files
const auto nFiles = inputChain.GetListOfFiles()->GetEntries();
// This is the function invoked during the processing of the trees.
auto workItem = [](TTreeReader &reader) {
TTreeReaderValue<Float_t> randomRV(reader, "r");
auto partialHisto = new TH1F("outHistoMP", "Random Numbers", 128, -4, 4);
while (reader.Next()) {
return partialHisto;
// Create the pool of processes
ROOT::TTreeProcessorMP workers(nFiles);
// Process the TChain
auto sumHistogram = workers.Process(inputChain, workItem, "multiCore");
sumHistogram->Fit("gaus", 0);
return 0;
int Int_t
Definition: RtypesCore.h:45
#define gROOT
Definition: TROOT.h:404
void Printf(const char *fmt,...)
Formats a string in a circular formatting buffer and prints the string.
Definition: TString.cxx:2470
This class provides an interface to process a TTree dataset in parallel with multi-process technology...
A chain is a collection of files containing TTree objects.
Definition: TChain.h:33
1-D histogram with a float per channel (see TH1 documentation)}
Definition: TH1.h:574
A simple, robust and fast interface to read values from ROOT columnar datasets such as TTree,...
Definition: TTreeReader.h:44
January 2016
Danilo Piparo

Definition in file mp102_readNtuplesFillHistosAndFit.C.