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Reference Guide
mt103_fillNtupleFromMultipleThreads.C File Reference

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Fill the same TNtuple from different threads.

This tutorial illustrates the basics of how it's possible with ROOT to write simultaneously to a single output file using TBufferMerger.

void mt103_fillNtupleFromMultipleThreads()
// Avoid unnecessary output
// Make ROOT thread-safe
// Total number of events
const size_t nEntries = 65535;
// Match number of threads to what the hardware can do
const size_t nWorkers = 4;
// Split work in equal parts
const size_t nEventsPerWorker = nEntries / nWorkers;
// Create the TBufferMerger: this class orchestrates the parallel writing
auto fileName = "mt103_fillNtupleFromMultipleThreads.root";
ROOT::TBufferMerger merger(fileName);
// Define what each worker will do
// We obtain from a merger a TBufferMergerFile, which is nothing more than
// a file which is held in memory and that flushes to the TBufferMerger its
// content.
auto work_function = [&](int seed) {
auto f = merger.GetFile();
TNtuple ntrand("ntrand", "Random Numbers", "r");
TRandom rnd(seed);
for (auto i : ROOT::TSeqI(nEntries))
// Create worker threads
std::vector<std::thread> workers;
for (auto i : ROOT::TSeqI(nWorkers))
workers.emplace_back(work_function, i + 1); // seed==0 means random seed :)
// Make sure workers are done
for (auto &&worker : workers)
#define f(i)
Definition: RSha256.hxx:104
#define gROOT
Definition: TROOT.h:405
TBufferMerger is a class to facilitate writing data in parallel from multiple threads,...
A pseudo container class which is a generator of indices.
Definition: TSeq.hxx:67
A simple TTree restricted to a list of float variables only.
Definition: TNtuple.h:28
This is the base class for the ROOT Random number generators.
Definition: TRandom.h:27
void EnableThreadSafety()
Enable support for multi-threading within the ROOT code in particular, enables the global mutex to ma...
Definition: TROOT.cxx:493
May 2017
Guilherme Amadio

Definition in file mt103_fillNtupleFromMultipleThreads.C.