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imt001_parBranchProcessing.C
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1 /// \file
2 /// \ingroup tutorial_multicore
3 /// \notebook
4 /// Demonstrate how to activate and use the implicit parallelisation of TTree::GetEntry.
5 /// Such parallelisation creates one task per top-level branch of the tree being read.
6 /// In this example, most of the branches are floating point numbers, which are very fast to read.
7 /// This parallelisation can be used, though, on bigger trees with many (complex) branches, which
8 /// are more likely to benefit from speedup gains.
9 ///
10 /// \macro_code
11 ///
12 /// \date 26/09/2016
13 /// \author Enric Tejedor
14 
15 int imt001_parBranchProcessing()
16 {
17  // First enable implicit multi-threading globally, so that the implicit parallelisation is on.
18  // The parameter of the call specifies the number of threads to use.
19  int nthreads = 4;
20  ROOT::EnableImplicitMT(nthreads);
21 
22  // Open the file containing the tree
23  auto file = TFile::Open("http://root.cern.ch/files/h1/dstarmb.root");
24 
25  // Get the tree
26  auto tree = file->Get<TTree>("h42");
27 
28  const auto nEntries = tree->GetEntries();
29 
30  // Read the branches in parallel.
31  // Note that the interface does not change, the parallelisation is internal
32  for (auto i : ROOT::TSeqUL(nEntries)) {
33  tree->GetEntry(i); // parallel read
34  }
35 
36  // IMT parallelisation can be disabled for a specific tree
37  tree->SetImplicitMT(false);
38 
39  // If now GetEntry is invoked on the tree, the reading is sequential
40  for (auto i : ROOT::TSeqUL(nEntries)) {
41  tree->GetEntry(i); // sequential read
42  }
43 
44  // Parallel reading can be re-enabled
45  tree->SetImplicitMT(true);
46 
47  // IMT can be also disabled globally.
48  // As a result, no tree will run GetEntry in parallel
50 
51  // This is still sequential: the global flag is disabled, even if the
52  // flag for this particular tree is enabled
53  for (auto i : ROOT::TSeqUL(nEntries)) {
54  tree->GetEntry(i); // sequential read
55  }
56 
57  return 0;
58 }
ROOT::DisableImplicitMT
void DisableImplicitMT()
Disables the implicit multi-threading in ROOT (see EnableImplicitMT).
Definition: TROOT.cxx:542
tree
Definition: tree.py:1
TTree
A TTree represents a columnar dataset.
Definition: TTree.h:79
TFile::Open
static TFile * Open(const char *name, Option_t *option="", const char *ftitle="", Int_t compress=ROOT::RCompressionSetting::EDefaults::kUseCompiledDefault, Int_t netopt=0)
Create / open a file.
Definition: TFile.cxx:3997
ROOT::EnableImplicitMT
void EnableImplicitMT(UInt_t numthreads=0)
Enable ROOT's implicit multi-threading for all objects and methods that provide an internal paralleli...
Definition: TROOT.cxx:525
file
Definition: file.py:1
ROOT::TSeq
A pseudo container class which is a generator of indices.
Definition: TSeq.hxx:66