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Reference Guide
mt201_parallelHistoFill.C File Reference

Detailed Description

View in nbviewer Open in SWAN Parallel fill of a histogram.

This tutorial shows how a histogram can be filled in parallel with a multithreaded approach. The difference with the multiprocess case, see mp201, is that here we cannot count on the copy-on-write mechanism, but we rather need to protect the histogram resource with a TThreadedObject class. The result of the filling is monitored with the SnapshotMerge method. This method is not thread safe: in presence of ROOT histograms, the system will not crash but the result is not uniquely defined.

const UInt_t poolSize = 4U;
Int_t mt201_parallelHistoFill()
// The concrete histogram instances are created in each thread
// lazily, i.e. only if a method is invoked.
ROOT::TThreadedObject<TH1F> ts_h("myHist", "Filled in parallel", 128, -8, 8);
// The function used to fill the histograms in each thread.
auto fillRandomHisto = [&](int seed = 0) {
TRandom3 rndm(seed);
// It is important to realise that a copy on the stack of the object we
// would like to perform operations on is the most efficient way of
// accessing it, in particular in presence of a tight loop like the one
// below where any overhead put on top of the Fill function call would
// have an impact.
auto histogram = ts_h.Get();
for (auto i : ROOT::TSeqI(1000000)) {
histogram->Fill(rndm.Gaus(0, 1));
// The seeds for the random number generators.
auto seeds = ROOT::TSeqI(1, poolSize + 1);
std::vector<std::thread> pool;
// A monitoring thread. This is here only to illustrate the functionality of
// the SnapshotMerge method.
// It allows "to spy" the multithreaded calculation without the need
// of interrupting it.
auto monitor = [&]() {
for (auto i : ROOT::TSeqI(5)) {
std::this_thread::sleep_for(std::chrono::duration<double, std::nano>(500));
auto h = ts_h.SnapshotMerge();
std::cout << "Entries for the snapshot " << h->GetEntries() << std::endl;
// The threads filling the histograms
for (auto seed : ROOT::TSeqI(seeds)) {
pool.emplace_back(fillRandomHisto, seed);
// Wait for the threads to finish
for (auto &&t : pool)
// Merge the final result
auto sumRandomHisto = ts_h.Merge();
std::cout << "Entries for the total sum " << sumRandomHisto->GetEntries() << std::endl;
auto c = new TCanvas();
return 0;
January 2016
Danilo Piparo

Definition in file mt201_parallelHistoFill.C.

#define c(i)
Definition: RSha256.hxx:101
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
void EnableThreadSafety()
Enables the global mutex to make ROOT thread safe/aware.
Definition: TROOT.cxx:494
TSeq< int > TSeqI
Definition: TSeq.hxx:194
int Int_t
Definition: RtypesCore.h:45
Random number generator class based on M.
Definition: TRandom3.h:27
#define h(i)
Definition: RSha256.hxx:106
unsigned int UInt_t
Definition: RtypesCore.h:46
A wrapper to make object instances thread private, lazily.
Definition: TThreadedObject.hxx:151
The Canvas class.
Definition: TCanvas.h:23
A pseudo container class which is a generator of indices.
Definition: TSeq.hxx:66