ceph/src/boost/libs/histogram/examples/guide_parallel_filling.cpp

   1 // Copyright 2018-2019 Hans Dembinski
   2 //
   3 // Distributed under the Boost Software License, Version 1.0.
   4 // (See accompanying file LICENSE_1_0.txt
   5 // or copy at http://www.boost.org/LICENSE_1_0.txt)
   6
   7 //[ guide_parallel_filling
   8
   9 #include <boost/histogram.hpp>
  10 #include <boost/histogram/algorithm/sum.hpp>
  11 #include <cassert>
  12 #include <functional>
  13 #include <thread>
  14 #include <vector>
  15
  16 // dummy fill function, to be executed in parallel by several threads
  17 template <typename Histogram>
  18 void fill(Histogram& h) {
  19   for (unsigned i = 0; i < 1000; ++i) { h(i % 10); }
  20 }
  21
  22 int main() {
  23   using namespace boost::histogram;
  24
  25   /*
  26     Create histogram with container of thread-safe counters for parallel filling in
  27     several threads. Only filling is thread-safe, other guarantees are not given.
  28   */
  29   auto h = make_histogram_with(dense_storage<accumulators::thread_safe<unsigned>>(),
  30                                axis::integer<>(0, 10));
  31
  32   /*
  33     Run the fill function in parallel from different threads. This is safe when a
  34     thread-safe accumulator and a storage with thread-safe cell access are used.
  35   */
  36   auto fill_h = [&h]() { fill(h); };
  37   std::thread t1(fill_h);
  38   std::thread t2(fill_h);
  39   std::thread t3(fill_h);
  40   std::thread t4(fill_h);
  41   t1.join();
  42   t2.join();
  43   t3.join();
  44   t4.join();
  45
  46   // Without a thread-safe accumulator, this number may be smaller.
  47   assert(algorithm::sum(h) == 4000);
  48 }
  49
  50 //]