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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
15int 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}
A pseudo container class which is a generator of indices.
Definition: TSeq.hxx:67
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:4053
A TTree represents a columnar dataset.
Definition: TTree.h:79
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:527
void DisableImplicitMT()
Disables the implicit multi-threading in ROOT (see EnableImplicitMT).
Definition: TROOT.cxx:544
Definition: file.py:1
Definition: tree.py:1