imt001_parBranchProcessing.C

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/// \file
/// \ingroup tutorial_multicore
/// \notebook
/// Demonstrate how to activate and use the implicit parallelisation of TTree::GetEntry.
/// Such parallelisation creates one task per top-level branch of the tree being read.
/// In this example, most of the branches are floating point numbers, which are very fast to read.
///  This parallelisation can be used, though, on bigger trees with many (complex) branches, which
///  are more likely to benefit from speedup gains.
///
/// \macro_code
///
/// \date 26/09/2016
/// \author Enric Tejedor


int imt001_parBranchProcessing()
{
  // 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);

  // Open the file containing the tree
  TFile *file = TFile::Open("http://root.cern.ch/files/h1/dstarmb.root");

  // Get the tree
  TTree *tree = nullptr;
  file->GetObject<TTree>("h42", tree);

  // Read the branches in parallel.
  // Note that the interface does not change, the parallelisation is internal
  for (Long64_t i = 0; i < tree->GetEntries(); ++i) {
     tree->GetEntry(i);  // parallel read
  }

  // IMT parallelisation can be disabled for a specific tree
  tree->SetImplicitMT(false);

  // If now GetEntry is invoked on the tree, the reading is sequential
  for (Long64_t i = 0; i < tree->GetEntries(); ++i) {
     tree->GetEntry(i);  // sequential read
  }

  // Parallel reading can be re-enabled
  tree->SetImplicitMT(true);

  // IMT can be also disabled globally.
  // As a result, no tree will run GetEntry in parallel
  ROOT::DisableImplicitMT();

  // This is still sequential: the global flag is disabled, even if the
  // flag for this particular tree is enabled
  for (Long64_t i = 0; i < tree->GetEntries(); ++i) {
     tree->GetEntry(i);  // sequential read
  }

  return 0;
}