From: David Wells Date: Thu, 28 May 2020 17:43:31 +0000 (-0400) Subject: Convert step-28 to use tasks. X-Git-Tag: v9.3.0-rc1~1524^2~2 X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=26205eba684105c855874f91c50770b51a4bb9f0;p=dealii.git Convert step-28 to use tasks. --- diff --git a/examples/step-28/step-28.cc b/examples/step-28/step-28.cc index f5bb6a7602..1035dfed72 100644 --- a/examples/step-28/step-28.cc +++ b/examples/step-28/step-28.cc @@ -1129,7 +1129,7 @@ namespace Step28 // in many of the other tutorial programs in that it has a public // run function and private functions doing all the rest. In // several places, we have to do something for all energy groups, in which - // case we will start threads for each group to let these things run in + // case we will start tasks for each group to let these things run in // parallel if deal.II was configured for multithreading. For strategies of // parallelization, take a look at the @ref threads module. // @@ -1477,31 +1477,36 @@ namespace Step28 // since each of these sums can be computed independently, we actually do // this in parallel. One of the problems is that the function in the // EnergyGroup class that computes the fission source returns a - // value. If we now simply spin off a new thread, we have to later capture - // the return value of the function run on that thread. The way this can be - // done is to use the return value of the Threads::new_thread function, - // which returns an object of type Threads::Thread@ if the function - // spawned returns a double. We can then later ask this object for the - // returned value (when doing so, the Threads::Thread::return_value function - // first waits for the thread to finish if it hasn't done so already). + // value. We would like to add these values together in the loop itself: + // ideally, each task would compute its value and then immediately add it to + // the total. Concurrently summing values in this way requires two features: + //
    + //
  1. We need a way of storing a value such that multiple threads can + // read and write into concurrently in a way that prevents data races + // (i.e., thread-safe reading and writing).
  2. + //
  3. We need a way to increment such a value that is also + // thread-safe.
  4. + //
// - // The way this function then works is to first spawn one thread for each - // energy group we work with, then one-by-one collecting the returned values - // of each thread and return the sum. + // The first feature is available through the template class + // std::atomic. However, the second feature, implemented by + // std::atomic::fetch_add(), is only available in C++20 + // and later: since deal.II supports older versions of the C++ language + // standard we cannot use this feature yet. Hence, instead, we simply write + // each group's value out to an entry in a vector and sum the values at the + // end of the function. template double NeutronDiffusionProblem::get_total_fission_source() const { - std::vector> threads; + std::vector fission_sources(parameters.n_groups); + Threads::TaskGroup<> tasks; for (unsigned int group = 0; group < parameters.n_groups; ++group) - threads.push_back( - Threads::new_thread(&EnergyGroup::get_fission_source, - *energy_groups[group])); + tasks += Threads::new_task<>([&, group]() { + fission_sources[group] = energy_groups[group]->get_fission_source(); + }); + tasks.join_all(); - double fission_source = 0; - for (unsigned int group = 0; group < parameters.n_groups; ++group) - fission_source += threads[group].return_value(); - - return fission_source; + return std::accumulate(fission_sources.begin(), fission_sources.end(), 0.0); } @@ -1525,27 +1530,28 @@ namespace Step28 BlockVector group_error_indicators(n_cells); { - Threads::ThreadGroup threads; + Threads::TaskGroup<> tasks; for (unsigned int group = 0; group < parameters.n_groups; ++group) - threads += Threads::new_thread(&EnergyGroup::estimate_errors, - *energy_groups[group], - group_error_indicators.block(group)); - threads.join_all(); + tasks += Threads::new_task([&, group]() { + energy_groups[group]->estimate_errors( + group_error_indicators.block(group)); + }); } + // The destructor of Threads::TaskGroup joins all threads so we know that + // the computation is done by the time we exit the scope. const float max_error = group_error_indicators.linfty_norm(); const float refine_threshold = 0.3 * max_error; const float coarsen_threshold = 0.01 * max_error; { - Threads::ThreadGroup threads; + Threads::TaskGroup tasks; for (unsigned int group = 0; group < parameters.n_groups; ++group) - threads += Threads::new_thread(&EnergyGroup::refine_grid, - *energy_groups[group], - group_error_indicators.block(group), - refine_threshold, - coarsen_threshold); - threads.join_all(); + tasks += Threads::new_task([&, group]() { + energy_groups[group]->refine_grid(group_error_indicators.block(group), + refine_threshold, + coarsen_threshold); + }); } } @@ -1580,9 +1586,9 @@ namespace Step28 { // We will measure the CPU time that each cycle takes below. The // constructor for Timer calls Timer::start(), so once we create a - // timer we can query it for information. Since we use a thread pool - // to assemble the system matrices, the CPU time we measure (if we run - // with more than one thread) will be larger than the wall time. + // timer we can query it for information. Since many parts of this + // loop are parallelized with tasks, the CPU time we measure (if we + // run with more than one thread) will be larger than the wall time. Timer timer; std::cout << "Cycle " << cycle << ':' << std::endl; @@ -1611,13 +1617,11 @@ namespace Step28 std::cout << energy_groups[group]->n_dofs() << ' '; std::cout << std::endl << std::endl; - - Threads::ThreadGroup threads; + Threads::TaskGroup<> tasks; for (unsigned int group = 0; group < parameters.n_groups; ++group) - threads += - Threads::new_thread(&EnergyGroup::assemble_system_matrix, - *energy_groups[group]); - threads.join_all(); + tasks += Threads::new_task( + [&, group]() { energy_groups[group]->assemble_system_matrix(); }); + tasks.join_all(); double error; unsigned int iteration = 1;