concurrentfill.cxx

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/// \file
/// \ingroup tutorial_v7
///
/// \macro_code
///
/// \date 2015-07-09
/// \warning This is part of the ROOT 7 prototype! It will change without notice. It might trigger earthquakes. Feedback
/// is welcome!
/// \author Axel Naumann <axel@cern.ch>

/*************************************************************************
 * Copyright (C) 1995-2015, Rene Brun and Fons Rademakers.               *
 * All rights reserved.                                                  *
 *                                                                       *
 * For the licensing terms see $ROOTSYS/LICENSE.                         *
 * For the list of contributors see $ROOTSYS/README/CREDITS.             *
 *************************************************************************/

#include "ROOT/THist.hxx"
#include "ROOT/THistConcurrentFill.hxx"

#include <iostream>
#include <future>
#include <random>

using namespace ROOT;

double wasteCPUTime(std::mt19937 &gen)
{
   // Simulate number crunching through gen and ridiculous num bits
   return std::generate_canonical<double, 100>(gen) + std::generate_canonical<double, 100>(gen) +
          std::generate_canonical<double, 100>(gen) + std::generate_canonical<double, 100>(gen) +
          std::generate_canonical<double, 100>(gen);
}

using Filler_t = Experimental::THistConcurrentFiller<Experimental::TH2D, 1024>;

/// This function is called within each thread: it spends some CPU time and then
/// fills a number into the histogram, through the Filler_t. This is repeated
/// several times.
void theTask(Filler_t filler)
{
   std::mt19937 gen;

   for (int i = 0; i < 3000000; ++i)
      filler.Fill({wasteCPUTime(gen), wasteCPUTime(gen)});
}

/// This example fills a histogram concurrently, from several threads.
void concurrentHistFill(Experimental::TH2D &hist)
{
   // THistConcurrentFillManager allows multiple threads to fill the histogram
   // concurrently.
   //
   // Details: each thread's Fill() calls are buffered. once the buffer is full,
   // the THistConcurrentFillManager locks and flushes the buffer into the
   // histogram.
   Experimental::THistConcurrentFillManager<Experimental::TH2D> fillMgr(hist);

   std::array<std::thread, 8> threads;

   // Let the threads fill the histogram concurrently.
   for (auto &thr: threads) {
      // Each thread calls fill(), passing a dedicated filler per thread.
      thr = std::thread(theTask, fillMgr.MakeFiller());
   }

   // Join them.
   for (auto &thr: threads)
      thr.join();
}

void concurrentfill()
{
   // This histogram will be filled from several threads.
   Experimental::TH2D hist{{100, 0., 1.}, {{0., 1., 2., 3., 10.}}};

   concurrentHistFill(hist);

   std::cout << hist.GetEntries() << '\n';
}