imt001_parBranchProcessing.C File Reference

Detailed Description

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.

/// \date 26/09/2016


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;
}

Author

Enric Tejedor

Definition in file imt001_parBranchProcessing.C.