# mt201_parallelHistoFill¶

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.

Author: Danilo Piparo
This notebook tutorial was automatically generated with ROOTBOOK-izer from the macro found in the ROOT repository on Sunday, October 02, 2022 at 09:33 AM.

In [1]:
const UInt_t poolSize = 4U;

In [2]:
ROOT::EnableThreadSafety();


The concrete histogram instances are created in each thread lazily, i.e. only if a method is invoked.

In [3]:
ROOT::TThreadedObject<TH1F> ts_h("myHist", "Filled in parallel", 128, -8, 8);


The function used to fill the histograms in each thread.

In [4]:
auto fillRandomHisto = [&](int seed = 0) {
TRandom3 rndm(seed);
// IMPORTANT!
// 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.

In [5]:
auto seeds = ROOT::TSeqI(1, poolSize + 1);



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.

In [6]:
auto monitor = [&]() {
for (auto i : ROOT::TSeqI(5)) {
auto h = ts_h.SnapshotMerge();
std::cout << "Entries for the snapshot " << h->GetEntries() << std::endl;
}
};
pool.emplace_back(monitor);

Entries for the snapshot 0
Entries for the snapshot 0
Entries for the snapshot 0
Entries for the snapshot 0
Entries for the snapshot 0


The threads filling the histograms

In [7]:
for (auto seed : ROOT::TSeqI(seeds)) {
pool.emplace_back(fillRandomHisto, seed);
}


Wait for the threads to finish

In [8]:
for (auto &&t : pool)
t.join();


Merge the final result

In [9]:
auto sumRandomHisto = ts_h.Merge();

std::cout << "Entries for the total sum " << sumRandomHisto->GetEntries() << std::endl;

auto c = new TCanvas();
sumRandomHisto->DrawClone();
return 0;

Entries for the total sum 4e+06


Draw all canvases

In [10]:
gROOT->GetListOfCanvases()->Draw()