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mt201_parallelHistoFill.C File Reference

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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;
#define c(i)
Definition RSha256.hxx:101
#define h(i)
Definition RSha256.hxx:106
int Int_t
Definition RtypesCore.h:45
unsigned int UInt_t
Definition RtypesCore.h:46
A wrapper to make object instances thread private, lazily.
The Canvas class.
Definition TCanvas.h:23
Random number generator class based on M.
Definition TRandom3.h:27
tbb::task_arena is an alias of tbb::interface7::task_arena, which doesn't allow to forward declare tb...
void EnableThreadSafety()
Enable support for multi-threading within the ROOT code in particular, enables the global mutex to ma...
Definition TROOT.cxx:501
TSeq< int > TSeqI
Definition TSeq.hxx:203
January 2016
Danilo Piparo

Definition in file mt201_parallelHistoFill.C.