thread_management.h

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// -----------------------------------------------------------------------------
//
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception OR LGPL-2.1-or-later
// Copyright (C) 2000 - 2025 by the deal.II authors
//
// This file is part of the deal.II library.
//
// Detailed license information governing the source code and contributions
// can be found in LICENSE.md and CONTRIBUTING.md at the top level directory.
//
// -----------------------------------------------------------------------------
 
#ifndef dealii_thread_management_h
#define dealii_thread_management_h
 
 
#include <deal.II/base/config.h>
 
#include <deal.II/base/exceptions.h>
#include <deal.II/base/multithread_info.h>
#include <deal.II/base/mutex.h>
#include <deal.II/base/std_cxx20/type_traits.h>
#include <deal.II/base/template_constraints.h>
 
#ifdef DEAL_II_WITH_TASKFLOW
#  include <taskflow/taskflow.hpp>
#endif
 
#include <atomic>
#include <functional>
#include <future>
#include <list>
#include <memory>
#include <thread>
#include <tuple>
#include <utility>
#include <vector>
 
#ifdef DEAL_II_HAVE_CXX20
#  include <concepts>
#endif
 
 
#ifdef DEAL_II_WITH_TBB
#  include <tbb/task_group.h>
#endif
 
DEAL_II_NAMESPACE_OPEN
 
namespace Threads
{
  template <typename ForwardIterator>
  std::vector<std::pair<ForwardIterator, ForwardIterator>>
  split_range(const ForwardIterator &begin,
              const ForwardIterator &end,
              const unsigned int     n_intervals);
 
  std::vector<std::pair<unsigned int, unsigned int>>
  split_interval(const unsigned int begin,
                 const unsigned int end,
                 const unsigned int n_intervals);
 
  namespace internal
  {
    [[noreturn]] void
    handle_std_exception(const std::exception &exc);
 
    [[noreturn]] void
    handle_unknown_exception();
  } // namespace internal
 
} // namespace Threads
 
/* ----------- implementation of functions in namespace Threads ---------- */
#ifndef DOXYGEN
namespace Threads
{
  template <typename ForwardIterator>
  std::vector<std::pair<ForwardIterator, ForwardIterator>>
  split_range(const ForwardIterator &begin,
              const ForwardIterator &end,
              const unsigned int     n_intervals)
  {
    using IteratorPair = std::pair<ForwardIterator, ForwardIterator>;
 
    // in non-multithreaded mode, we often have the case that this
    // function is called with n_intervals==1, so have a shortcut here
    // to handle that case efficiently
 
    if (n_intervals == 1)
      return (std::vector<IteratorPair>(1, IteratorPair(begin, end)));
 
    // if more than one interval requested, do the full work
    const unsigned int n_elements              = std::distance(begin, end);
    const unsigned int n_elements_per_interval = n_elements / n_intervals;
    const unsigned int residual                = n_elements % n_intervals;
 
    std::vector<IteratorPair> return_values(n_intervals);
 
    return_values[0].first = begin;
    for (unsigned int i = 0; i < n_intervals; ++i)
      {
        if (i != n_intervals - 1)
          {
            return_values[i].second = return_values[i].first;
            // note: the cast is performed to avoid a warning of gcc
            // that in the library `dist>=0' is checked (dist has a
            // template type, which here is unsigned if no cast is
            // performed)
            std::advance(return_values[i].second,
                         static_cast<signed int>(n_elements_per_interval));
            // distribute residual in division equally among the first
            // few subintervals
            if (i < residual)
              ++return_values[i].second;
 
            return_values[i + 1].first = return_values[i].second;
          }
        else
          return_values[i].second = end;
      }
    return return_values;
  }
} // namespace Threads
 
#endif // DOXYGEN
 
namespace Threads
{
  namespace internal
  {
    template <typename RT>
    struct return_value
    {
    private:
      RT   value;
      bool value_is_initialized;
 
    public:
      using reference_type = RT &;
 
      inline return_value()
        : value()
        , value_is_initialized(false)
      {}
 
      inline reference_type
      get()
      {
        Assert(
          value_is_initialized,
          ExcMessage(
            "You cannot read the return value of a thread or task "
            "if that value has not been set. This happens, for example, if "
            "a task or thread threw an exception."));
        return value;
      }
 
      inline void
      set(RT &&v)
      {
        value = std::move(v);
      }
 
      inline void
      set_from(std::future<RT> &v)
      {
        // Get the value from the std::future object. If the future holds
        // an exception, then the assignment fails, we exit the function via the
        // exception right away, and value_is_initialized is not set to true --
        // that's something we can check later on.
        value                = std::move(v.get());
        value_is_initialized = true;
      }
    };
 
 
    template <typename RT>
    struct return_value<RT &>
    {
    private:
      RT  *value;
      bool value_is_initialized;
 
    public:
      using reference_type = RT &;
 
      inline return_value()
        : value(nullptr)
        , value_is_initialized(false)
      {}
 
      inline reference_type
      get() const
      {
        Assert(
          value_is_initialized,
          ExcMessage(
            "You cannot read the return value of a thread or task "
            "if that value has not been set. This happens, for example, if "
            "a task or thread threw an exception."));
        return *value;
      }
 
      inline void
      set(RT &v)
      {
        value = &v;
      }
 
      inline void
      set_from(std::future<RT &> &v)
      {
        // Get the value from the std::future object. If the future holds
        // an exception, then the assignment fails, we exit the function via the
        // exception right away, and value_is_initialized is not set to true --
        // that's something we can check later on.
        value                = &v.get();
        value_is_initialized = true;
      }
    };
 
 
    template <>
    struct return_value<void>
    {
      using reference_type = void;
 
      static inline void
      get()
      {}
 
 
      inline void
      set_from(std::future<void> &)
      {}
    };
  } // namespace internal
 
 
 
  namespace internal
  {
    template <typename T>
    struct maybe_make_ref
    {
      static T
      act(const T &t)
      {
        return t;
      }
    };
 
 
 
    template <typename T>
    struct maybe_make_ref<T &>
    {
      static std::reference_wrapper<T>
      act(T &t)
      {
        return std::ref(t);
      }
    };
 
 
 
    template <typename RT, typename Function>
    DEAL_II_CXX20_REQUIRES(
      (std::invocable<Function> &&
       std::convertible_to<std::invoke_result_t<Function>, RT>))
    void evaluate_and_set_promise(Function &function, std::promise<RT> &promise)
    {
      promise.set_value(function());
    }
 
 
    template <typename Function>
    DEAL_II_CXX20_REQUIRES((std::invocable<Function>))
    void evaluate_and_set_promise(Function           &function,
                                  std::promise<void> &promise)
    {
      function();
      promise.set_value();
    }
  } // namespace internal
 
 
 
  template <typename RT = void>
  class Task
  {
  public:
    Task(const std::function<RT()> &function_object)
    {
      if (MultithreadInfo::n_threads() > 1)
        {
#ifdef DEAL_II_WITH_TASKFLOW
          // If we are creating the task from a thread not managed by the
          // current Taskflow executor, then emplace the new task and run it
          // asynchronously. Otherwise, we would be asking Taskflow to emplace
          // a new task with the same executor, which may lead to deadlocks
          // (see
          // https://taskflow.github.io/taskflow/ExecuteTaskflow.html#ExecuteATaskflowFromAnInternalWorker
          // and the discussion in
          // https://github.com/dealii/dealii/issues/19079). As a consequence,
          // we let the code fall through to the code at the bottom of the
          // function that executes the task right there and then,
          // synchronously.
          //
          // In practice, this means that one can't use task-based programming
          // in a nested way. That's unfortunate but it is what it is for
          // now.
          if (MultithreadInfo::get_taskflow_executor().this_worker_id() < 0)
            {
              task_data = std::make_shared<TaskData>(
                MultithreadInfo::get_taskflow_executor().async(
                  function_object));
              return;
            }
#elif defined(DEAL_II_WITH_TBB)
          // Create a promise object and from it extract a future that
          // we can use to refer to the outcome of the task. For reasons
          // explained below, we can't just create a std::promise object,
          // but have to make do with a pointer to such an object.
          std::unique_ptr<std::promise<RT>> promise =
            std::make_unique<std::promise<RT>>();
          task_data =
            std::make_shared<TaskData>(std::move(promise->get_future()));
 
          // Then start the task, using a task_group object (for just this one
          // task) that is associated with the TaskData object. Note that we
          // have to *copy* the function object being executed so that it is
          // guaranteed to live on the called thread as well -- the copying is
          // facilitated by capturing the 'function_object' variable by value.
          //
          // We also have to *move* the promise object into the new task's
          // memory space because promises can not be copied and we can't refer
          // to it by reference because it's a local variable of the current
          // (surrounding) function that may go out of scope before the promise
          // is ultimately set. This leads to a conundrum: if we had just
          // declared 'promise' as an object of type std::promise, then we could
          // capture it in the lambda function via
          //     [..., promise=std::move(promise)]() {...}
          // and set the promise in the body of the lambda. But setting a
          // promise is a non-const operation on the promise, and so we would
          // actually have to declare the lambda function as 'mutable' because
          // by default, lambda captures are 'const'. That is, we would have
          // to write
          //     [..., promise=std::move(promise)]() mutable {...}
          // But this leads to other problems: It turns out that the
          // tbb::task_group::run() function cannot take mutable lambdas as
          // argument :-(
          //
          // We work around this issue by not declaring the 'promise' variable
          // as an object of type std::promise, but as a pointer to such an
          // object. This pointer we can move, and the *pointer* itself can
          // be 'const' (meaning we can leave the lambda as non-mutable)
          // even though we modify the object *pointed to*. One would think
          // that a std::unique_ptr would be the right choice for this, but
          // that's not true: the resulting lambda function can then be
          // non-mutable, but the lambda function object is not copyable
          // and at least some TBB variants require that as well. So
          // instead we move the std::unique_ptr used above into a
          // std::shared_ptr to be stored within the lambda function object.
          task_data->task_group->run(
            [function_object,
             promise =
               std::shared_ptr<std::promise<RT>>(std::move(promise))]() {
              try
                {
                  internal::evaluate_and_set_promise(function_object, *promise);
                }
              catch (...)
                {
                  try
                    {
                      // store anything thrown in the promise
                      promise->set_exception(std::current_exception());
                    }
                  catch (...)
                    {
                      // set_exception() may throw too. But ignore this on
                      // the task.
                    }
                }
            });
          return;
 
#else
          // If no threading library is supported, just fall back onto C++11
          // facilities. The problem with this is that the standard does
          // not actually say what std::async should do. The first
          // argument to that function can be std::launch::async or
          // std::launch::deferred, or both. The *intent* of the standard's
          // authors was probably that if one sets it to
          //   std::launch::async | std::launch::deferred,
          // that the task is run in a thread pool. But at least as of
          // 2021, GCC doesn't do that: It just runs it on a new thread.
          // If one chooses std::launch::deferred, it runs the task on
          // the same thread but only when one calls join() on the task's
          // std::future object. In the former case, this leads to
          // oversubscription, in the latter case to undersubscription of
          // resources. We choose oversubscription here.
          //
          // The issue illustrates why relying on external libraries
          // with task schedulers is the way to go.
          task_data = std::make_shared<TaskData>(
            std::async(std::launch::async | std::launch::deferred,
                       function_object));
          return;
#endif
        }
      {
        // Only one thread allowed. So let the task run to completion
        // and just emplace a 'ready' future.
        //
        // The design of std::promise/std::future is unclear, but it
        // seems that the intent is to obtain the std::future before
        // we set the std::promise. So create the TaskData object at
        // the top and then run the task and set the returned
        // value. Since everything here happens sequentially, it
        // really doesn't matter in which order all of this is
        // happening.
        std::promise<RT> promise;
        task_data = std::make_shared<TaskData>(promise.get_future());
        try
          {
            internal::evaluate_and_set_promise(function_object, promise);
          }
        catch (...)
          {
            try
              {
                // store anything thrown in the promise
                promise.set_exception(std::current_exception());
              }
            catch (...)
              {
                // set_exception() may throw too. But ignore this on
                // the task.
              }
          }
      }
    }
 
    Task() = default;
 
    Task(const Task &other) = default;
 
    Task(Task &&other) noexcept = default;
 
    Task &
    operator=(const Task &other) = default;
 
    Task &
    operator=(Task &&other) noexcept = default;
 
    void
    join() const
    {
      // Make sure we actually have a task that we can wait for.
      AssertThrow(joinable(), ExcNoTask());
 
      task_data->wait();
    }
 
    bool
    joinable() const
    {
      return (task_data != nullptr);
    }
 
 
    typename internal::return_value<RT>::reference_type
    return_value()
    {
      // Make sure we actually have a task that we can wait for.
      AssertThrow(joinable(), ExcNoTask());
 
      // Then return the promised object. If necessary, wait for the promise to
      // be set.
      return task_data->get();
    }
 
 
    DeclExceptionMsg(ExcNoTask,
                     "The current object is not associated with a task that "
                     "can be joined. It may have been detached, or you "
                     "may have already joined it in the past.");
  private:
    class TaskData
    {
    public:
      TaskData(std::future<RT> &&future) noexcept
        : future(std::move(future))
        , task_has_finished(false)
#ifdef DEAL_II_WITH_TBB
        , task_group(std::make_unique<tbb::task_group>())
#endif
      {}
 
      TaskData(const TaskData &) = delete;
 
      TaskData(TaskData &&) = delete;
 
      TaskData &
      operator=(const TaskData &) = delete;
 
      TaskData &
      operator=(TaskData &&) = delete;
 
      ~TaskData() noexcept
      {
        // Explicitly wait for the results to be ready. This class stores
        // a std::future object, and we could just let the compiler generate
        // the destructor which would then call the destructor of std::future
        // which *may* block until the future is ready. As explained in
        //   https://en.cppreference.com/w/cpp/thread/future/~future
        // this is only a *may*, not a *must*. (The standard does not
        // appear to say anything about it at all.) As a consequence,
        // let's be explicit about waiting.
        //
        // One of the corner cases we have to worry about is that if a task
        // ends by throwing an exception, then wait() will re-throw that
        // exception on the thread that calls it, the first time around
        // someone calls wait() (or the return_value() function of the
        // surrounding class). So if we get to this constructor and an exception
        // is thrown by wait(), then that means that the last Task object
        // referring to a task is going out of scope with nobody having
        // ever checked the return value of the task itself. In that case,
        // one could argue that they would also not have cared about whether
        // an exception is thrown, and that we should simply ignore the
        // exception. This is what we do here. It is also the simplest solution,
        // because we don't know what one should do with the exception to begin
        // with: destructors aren't allowed to throw exceptions, so we can't
        // just rethrow it here if one had been triggered.
        try
          {
            wait();
          }
        catch (...)
          {}
      }
 
      void
      wait()
      {
        // If we have previously already moved the result, then we don't
        // need a lock and can just return.
        if (task_has_finished)
          return;
 
        // Else, we need to go under a lock and try again. A different thread
        // may have waited and finished the task since then, so we have to try
        // a second time. (This is Schmidt's double-checking pattern.)
        std::scoped_lock lock(mutex);
        if (task_has_finished)
          return;
        else
          {
#ifdef DEAL_II_WITH_TASKFLOW
            // We want to call executor.corun_until() to keep scheduling tasks
            // until the task we are waiting for has actually finished. The
            // problem is that TaskFlow documents that you can only call
            // corun_until() on a worker of the executor. In other words, we
            // can call it from *inside* other tasks, but not from the main
            // thread (or other threads that might have been created outside
            // of TaskFlow).
            //
            // Fortunately, we can check whether we are on a worker thread:
            if (MultithreadInfo::get_taskflow_executor().this_worker_id() >= 0)
              MultithreadInfo::get_taskflow_executor().corun_until([this]() {
                return (future.wait_for(std::chrono::seconds(0)) ==
                        std::future_status::ready);
              });
            else
              // We are on a thread not managed by TaskFlow. In that case, we
              // can simply stop the current thread to wait for the task to
              // finish (i.e., for the std::future object to become ready). We
              // can do this because we need not fear that this leads to a
              // deadlock: The current threads is waiting for completion of a
              // task that is running on a completely different set of
              // threads, and so not making any progress here can not deprive
              // these other threads of the ability to schedule their tasks.
              //
              // Indeed, this is even true if the current thread is a worker
              // of one executor and we are waiting for a task running on a
              // different executor: The current task being stopped may block
              // the current executor from scheduling more tasks, but it is
              // unrelated to the tasks of the scheduler for which we are
              // waiting for something, and so that other executor will
              // eventually get around to scheduling the task we are waiting
              // for, at which point the current task will also complete.
              future.wait();
 
#elif defined(DEAL_II_WITH_TBB)
            // If we build on the TBB, then we can't just wait for the
            // std::future object to get ready. Apparently the TBB happily
            // enqueues a task into an arena and then just sits on it without
            // ever executing it unless someone expresses an interest in the
            // task. The way to avoid this is to add the task to a
            // tbb::task_group, and then here wait for the single task
            // associated with that task group.
            //
            // This also makes sense from another perspective. Imagine that
            // we allow at most N threads, and that we create N+1 tasks in such
            // a way that the first N all wait for the (N+1)st task to finish.
            // (See the multithreading/task_17 test for an example.) If they
            // all just sit in their std::future::wait() function, nothing
            // is ever going to happen because the scheduler sees that N tasks
            // are currently running and is never informed that all they're
            // doing is wait for another task to finish. What *needs* to
            // happen is that the wait() or join() function goes back into
            // the scheduler to make sure the scheduler knows that these
            // tasks are not actually using CPU time on the thread they're
            // working on, and that it is time to run other tasks on the
            // same thread -- this is the way we can eventually get that
            // (N+1)st task executed, which then unblocks the other N threads.
            // (Note that this also implies that multiple tasks can be
            // executing on the same thread at the same time -- not
            // concurrently, of course, but with one executing and the others
            // currently waiting for other tasks to finish.)
            //
            // If we get here, we know for a fact that atomically
            // (because under a lock), no other thread has so far
            // determined that we are finished and removed the
            // 'task_group' object. So we know that the pointer is
            // still valid. But we also know that, because below we
            // set the task_has_finished flag to 'true', that no other
            // thread will ever get back to this point and query the
            // 'task_group' object, so we can delete it.
            task_group->wait();
            task_group.reset();
#endif
 
            // Wait for the task to finish and then move its
            // result. (We could have made the set_from() function
            // that we call here wait for the future to be ready --
            // which happens implicitly when it calls future.get() --
            // but that would have required putting an explicit
            // future.wait() into the implementation of
            // internal::return_value<void>::set_from(), which is a
            // bit awkward: that class doesn't actually need to set
            // anything, and so it looks odd to have the explicit call
            // to future.wait() in the set_from() function. Avoid the
            // issue by just explicitly calling future.wait() here.)
            future.wait();
 
            // Acquire the returned object. If the task ended in an
            // exception, `set_from` will call `std::future::get`, which
            // will throw an exception. This leaves `returned_object` in
            // an undefined state, but moreover we would bypass setting
            // `task_has_finished=true` below. So catch the exception
            // for just long enough that we can set that flag, and then
            // re-throw it:
            try
              {
                returned_object.set_from(future);
              }
            catch (...)
              {
                task_has_finished = true;
                throw;
              }
 
            // If we got here, the task has ended without an exception and
            // we can safely set the flag and return.
            task_has_finished = true;
          }
      }
 
 
 
      typename internal::return_value<RT>::reference_type
      get()
      {
        wait();
        return returned_object.get();
      }
 
    private:
      std::mutex mutex;
 
      std::future<RT> future;
 
      std::atomic<bool> task_has_finished;
 
      internal::return_value<RT> returned_object;
 
#ifdef DEAL_II_WITH_TBB
      std::unique_ptr<tbb::task_group> task_group;
 
      friend class Task<RT>;
#endif
    };
 
    std::shared_ptr<TaskData> task_data;
  };
 
 
 
  template <typename RT>
  inline Task<RT>
  new_task(const std::function<RT()> &function)
  {
    return Task<RT>(function);
  }
 
 
 
  template <typename FunctionObjectType>
  DEAL_II_CXX20_REQUIRES((std::invocable<FunctionObjectType>))
  inline auto new_task(FunctionObjectType function_object)
    -> Task<decltype(function_object())>
  {
    using return_type = decltype(function_object());
    ::MultithreadInfo::initialize_multithreading();
    return new_task(std::function<return_type()>(function_object));
  }
 
 
 
  template <typename RT, typename... Args>
  inline Task<RT>
  new_task(RT (*fun_ptr)(Args...), std_cxx20::type_identity_t<Args>... args)
  {
    auto dummy = std::make_tuple(internal::maybe_make_ref<Args>::act(args)...);
    return new_task(
      [dummy, fun_ptr]() -> RT { return std::apply(fun_ptr, dummy); });
  }
 
 
 
  template <
    typename FunctionObject,
    typename... Args,
    typename = std::enable_if_t<std::is_invocable_v<FunctionObject, Args...>>,
    typename = std::enable_if_t<std::is_function_v<FunctionObject> == false>,
    typename =
      std::enable_if_t<std::is_member_pointer_v<FunctionObject> == false>,
    typename = std::enable_if_t<std::is_pointer_v<FunctionObject> == false>>
  inline Task<std::invoke_result_t<FunctionObject, Args...>>
  new_task(const FunctionObject &fun, Args &&...args)
  {
    using RT   = std::invoke_result_t<FunctionObject, Args...>;
    auto dummy = std::make_tuple(std::forward<Args>(args)...);
    return new_task([dummy, fun]() -> RT { return std::apply(fun, dummy); });
  }
 
 
 
  template <typename RT, typename C, typename... Args>
  inline Task<RT>
  new_task(RT (C::*fun_ptr)(Args...),
           std_cxx20::type_identity_t<C> &c,
           std_cxx20::type_identity_t<Args>... args)
  {
    // NOLINTNEXTLINE(modernize-avoid-bind) silence clang-tidy
    return new_task(std::function<RT()>(std::bind(
      fun_ptr, std::ref(c), internal::maybe_make_ref<Args>::act(args)...)));
  }
 
  template <typename RT, typename C, typename... Args>
  inline Task<RT>
  new_task(RT (C::*fun_ptr)(Args...) const,
           std_cxx20::type_identity_t<const C> &c,
           std_cxx20::type_identity_t<Args>... args)
  {
    // NOLINTNEXTLINE(modernize-avoid-bind) silence clang-tidy
    return new_task(std::function<RT()>(std::bind(
      fun_ptr, std::cref(c), internal::maybe_make_ref<Args>::act(args)...)));
  }
 
 
  // ------------------------ TaskGroup -------------------------------------
 
  template <typename RT = void>
  class TaskGroup
  {
  public:
    TaskGroup &
    operator+=(const Task<RT> &t)
    {
      tasks.push_back(t);
      return *this;
    }
 
 
    std::size_t
    size() const
    {
      return tasks.size();
    }
 
    std::vector<RT>
    return_values()
    {
      std::vector<RT> results;
      results.reserve(size());
      for (auto &t : tasks)
        results.emplace_back(std::move(t.return_value()));
      return results;
    }
 
 
    void
    join_all() const
    {
      for (const auto &t : tasks)
        t.join();
    }
 
  private:
    std::list<Task<RT>> tasks;
  };
 
} // namespace Threads
 
DEAL_II_NAMESPACE_CLOSE
#endif