multithread_root_tuples.cpp

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#include <TFile.h>
#include <TTree.h>
#include <ROOT/TThreadedObject.hxx>
#include <TROOT.h>
 
#include <thread>
#include <mutex>
#include <vector>
#include <iostream>
#include <string>
 
// example of writing ROOT TTrees in multiple threads
// to compile:
// g++ -std=c++17 -O2 multithread_root_tuples.cpp $(root-config --cflags --libs) -o multithread_root_tuples
 
 
// Mutex for thread-safe std::cout
std::mutex cout_mutex;
 
void writeTuple(int thread_id) {
    std::string filename = "tuple_thread_" + std::to_string(thread_id) + ".root";
 
    TFile file(filename.c_str(), "RECREATE");
    if (file.IsZombie()) {
        std::lock_guard<std::mutex> lock(cout_mutex);
        std::cerr << "Error opening file: " << filename << std::endl;
        return;
    }
 
    TTree tree("mytree", "Thread-safe example TTree");
 
    std::vector<int> ids;
    std::vector<float> values;
 
    tree.Branch("ids", &ids);
    tree.Branch("values", &values);
 
    // AUTO FLUSH AND AUTOSAVE: flush every ~10 MB (adjust as needed)
    tree.SetAutoFlush(10 * 1024 * 1024); // 10 MB
    tree.SetAutoSave(30 * 1024 * 1024);  // 30 MB autosave snapshots
 
    const int total_events = 10000000;
    for (int entry = 0; entry < total_events; ++entry) {
        ids.clear();
        values.clear();
 
        int num_items = 10;
        for (int i = 0; i < num_items; ++i) {
            ids.push_back(entry * num_items + i);
            values.push_back(1.23f * i + thread_id);
        }
 
        tree.Fill();
    }
 
    tree.Write(); // Write entire tree metadata and baskets
    file.Close();
 
    std::lock_guard<std::mutex> lock(cout_mutex);
    std::cout << "Thread " << thread_id << " wrote file: " << filename << std::endl;
}
 
int main() {
    ROOT::EnableThreadSafety();
 
    const int num_threads = 8;
    std::vector<std::thread> threads;
 
    for (int i = 0; i < num_threads; ++i) {
        threads.emplace_back(writeTuple, i);
    }
 
    for (auto& t : threads) {
        t.join();
    }
 
    std::cout << "All threads finished writing ROOT files." << std::endl;
    return 0;
}