imt101_parTreeProcessing.C

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/// \file
/// \ingroup tutorial_multicore
/// Illustrate the usage of the TTreeProcessorMT::Process method.
/// Such method provides an implicit parallelisation of the reading and processing of a TTree.
/// In particular, when invoking Process, the user provides a function that iterates on a subrange
/// of the tree via a TTreeReader. Multiple tasks will be spawned, one for each sub-range, so that
/// the processing of the tree is parallelised. Since two invocations of the user function can
/// potentially run in parallel, the function code must be thread safe.
/// The example also introduces a new class, ROOT::TThreadedObject, which makes objects
/// thread private. With the help of this class, histograms can be filled safely inside the
/// user function and then merged at the end to get the final result. 
///
/// \macro_code
///
/// \author Enric Tejedor
/// \date 26/09/2016


int imt101_parTreeProcessing()
{
  // First enable implicit multi-threading globally, so that the implicit parallelisation is on.
  // The parameter of the call specifies the number of threads to use.
  int nthreads = 4;
  ROOT::EnableImplicitMT(nthreads);

  // Create one TThreadedObject per histogram to fill during the processing of the tree
  ROOT::TThreadedObject<TH1F> ptHist("pt_dist","p_{T} Distribution;p_{T};dN/p_{T}dp_{T}", 100, 0, 5);
  ROOT::TThreadedObject<TH1F> pzHist("pz_dist","p_{Z} Distribution;p_{Z};dN/dp_{Z}", 100, 0, 5);
  ROOT::TThreadedObject<TH2F> pxpyHist("px_py","p_{X} vs p_{Y} Distribution;p_{X};p_{Y}", 100, -5., 5., 100, -5., 5.);

  // Create a TTreeProcessorMT: specify the file and the tree in it
  ROOT::TTreeProcessorMT tp("http://root.cern.ch/files/tp_process_imt.root",
                            "events");

  // Define the function that will process a subrange of the tree.
  // The function must receive only one parameter, a TTreeReader,
  // and it must be thread safe. To enforce the latter requirement,
  // TThreadedObject histograms will be used.
  auto myFunction = [&](TTreeReader &myReader) {
     TTreeReaderValue<std::vector<ROOT::Math::PxPyPzEVector>> tracksRV(myReader, "tracks");

     // For performance reasons, a copy of the pointer associated to this thread on the
     // stack is used
     auto myPtHist = ptHist.Get();
     auto myPzHist = pzHist.Get();
     auto myPxPyHist = pxpyHist.Get();

     while (myReader.Next()) {
        auto tracks = *tracksRV;
        for (auto&& track : tracks) {
           myPtHist->Fill(track.Pt(), 1./track.Pt());
           myPxPyHist->Fill(track.Px(), track.Py());

           myPzHist->Fill(track.Pz());
        }
     }
  };

  // Launch the parallel processing of the tree
  tp.Process(myFunction);

  // Use the TThreadedObject::Merge method to merge the thread private histograms
  // into the final result
  auto ptHistMerged   = ptHist.Merge();
  auto pzHistMerged   = pzHist.Merge();
  auto pxpyHistMerged = pxpyHist.Merge();

  return 0;
}