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]:

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);
   // 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);

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.

In [6]:
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;
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)

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();
return 0;
Entries for the total sum 4e+06

Draw all canvases

In [10]: