// file      : build2/scheduler -*- C++ -*-
// copyright : Copyright (c) 2014-2016 Code Synthesis Ltd
// license   : MIT; see accompanying LICENSE file

#ifndef BUILD2_SCHEDULER
#define BUILD2_SCHEDULER

#include <mutex>
#include <tuple>
#include <atomic>
#include <cerrno>
#include <cassert>
#include <type_traits>        // aligned_storage, etc
#include <condition_variable>

#include <build2/types>
#include <build2/utility>

namespace build2
{
  // Scheduler of tasks and threads. Works best for "substantial" tasks (e.g.,
  // running a process), where in comparison thread synchronization overhead
  // is negligible.
  //
  // A thread (called "master") may need to perform several tasks which can be
  // done in parallel (e.g., update all the prerequisites or run all the
  // tests). To acomplish this, the master, via a call to async(), can ask the
  // scheduler to run a task in another thread (called "helper"). If a helper
  // is available, then the task is executed asynchronously by such helper.
  // Otherwise, the task is executed synchronously as part of the async()
  // call. Once the master thread has scheduled all the tasks, it calls wait()
  // to await for their completion.
  //
  // The scheduler makes sure that only a certain number of threads (for
  // example, the number of available hardware threads) are "active" at any
  // given time (thus the reason why async() may choose to perform the task
  // synchronously). When a master thread calls wait(), it is "suspended"
  // until all its asynchronous tasks are completed (at which point it becomes
  // "ready"). A suspension of a master results in either another ready master
  // being "resumed" or another helper thread becoming available.
  //
  // On completion of a task a helper thread returns to the scheduler which
  // can again lead either to a ready master being resumed (in which case the
  // helper is suspended) or the helper becoming available to perform another
  // task.
  //
  // Note that suspended threads are not reused as helpers. Rather, a new
  // helper thread is always created if none is available. This is done to
  // allow a ready master to continue as soon as possible. If it were reused
  // as a helper, then it could be blocked on a nested wait() further down the
  // stack. This means that the number of threads created by the scheduler
  // will normally exceed the maximum active allowed. It should not, however,
  // get excessive provided the tasks are not too deeply nested.
  //
  // @@ Actually, this is not quite correct: this seems to be a function of
  // both depth and width of the task tree. Perhaps we should use a hybrid
  // approach: when the number of helper threads reaches a certain limit
  // we switch to reusing suspended threads? Also, a thread can rake its
  // own and its subtask's queues directly in wait() since wait() won't
  // return until they are done.
  //
  class scheduler
  {
  public:
    using atomic_count = std::atomic<size_t>;

    // F should return void and not throw any exceptions. The way the result
    // of a task is communicated back to the master thread is ad hoc, usually
    // via "out" arguments. Such result(s) can only be retrieved by the master
    // once its task count reaches 0.
    //
    // The argument passing semantics is the same as for std::thread. In
    // particular, lvalue-references are passed as copies (use ref()/cref()
    // for the by-reference semantics).
    //
    // If the scheduler is shutdown, throw system_error(ECANCELED).
    //
    template <typename F, typename... A>
    void
    async (atomic_count& task_count, F&& f, A&&... a)
    {
      using task = task_type<F, A...>;

      static_assert (sizeof (task) <= sizeof (task_data),
                     "insufficient space");

      static_assert (std::is_trivially_destructible<task>::value,
                     "not trivially destructible");

      // Push into the queue unless we are running serially or the queue is
      // full.
      //
      task_data* td (nullptr);

      if (max_active_ != 1)
      {
        task_queue& tq (task_queue_ != nullptr
                        ? *task_queue_
                        : create_queue ());

        lock ql (tq.mutex);

        if (tq.shutdown)
          throw system_error (ECANCELED, std::system_category ());

        if ((td = push (tq)) != nullptr)
        {
          // Package the task.
          //
          new (&td->data) task {
            &task_count,
            decay_copy (forward <F> (f)),
            {decay_copy (forward <A> (a))...}};

          td->thunk = &task_thunk<F, A...>;
        }
        else
          tq.stat_full++;
      }

      // If serial/full, then run the task synchronously. In this case
      // there is no need to mess with task count.
      //
      if (td == nullptr)
      {
        forward<F> (f) (forward<A> (a)...);
        return;
      }

      // Increment the task count.
      //
      task_count.fetch_add (1, std::memory_order_release);

      lock l (mutex_);
      task_ = true;

      // If there is a spare active thread, wake up (or create) the helper.
      //
      if (active_ < max_active_)
        activate_helper (l);
    }

    // Wait until the task count reaches 0. If the scheduler is shutdown
    // while waiting, throw system_error(ECANCELED).
    //
    void
    wait (atomic_count& task_count);

    // Startup and shutdown.
    //
  public:
    // Unless already shut down, call shutdown() but ignore errors.
    //
    ~scheduler ();

    // Create a shut down scheduler.
    //
    scheduler () = default;

    // Create a started up scheduler.
    //
    // The initial active argument is the number of threads to assume are
    // already active (e.g., the calling thread). It must not be 0 (since
    // someone has to schedule the first task).
    //
    // If maximum threads is unspecified, then a generally appropriate default
    // limit it used.
    //
    scheduler (size_t max_active,
               size_t init_active = 1,
               size_t max_threads = 0)
    {
      startup (max_active, init_active, max_threads);
    }

    // Note: naturally not thread-safe.
    //
    void
    startup (size_t max_active,
             size_t init_active = 1,
             size_t max_threads = 0);

    // Wait for all the helper threads to terminate. Throw system_error on
    // failure. Note that the initially active threads are not waited for.
    // Return scheduling statistics.
    //
    struct stat
    {
      size_t thread_max_active     = 0; // max # of active threads allowed.
      size_t thread_max_total      = 0; // max # of total threads allowed.
      size_t thread_helpers        = 0; // # of helper threads created.
      size_t thread_max_waiting    = 0; // max # of waiters at any time.

      size_t task_queue_depth      = 0; // # of entries in a queue (capacity).
      size_t task_queue_full       = 0; // # of times task queue was full.

      size_t wait_queue_slots      = 0; // # of wait slots (buckets).
      size_t wait_queue_collisions = 0; // # of times slot had been occupied.
    };

    stat
    shutdown ();

    // Return the number of hardware threads or 0 if unable to determine.
    //
    static size_t
    hardware_concurrency ()
    {
      return std::thread::hardware_concurrency ();
    }

  private:
    using lock = std::unique_lock<std::mutex>;

    void
    activate_helper (lock&);

    void
    create_helper (lock&);

    // We restrict ourselves to a single pointer as an argument in hope of
    // a small object optimization.
    //
    static void
    helper (void*);

    void
    suspend (atomic_count& task_count);

    void
    resume (atomic_count& task_count);

    // Task encapsulation.
    //
    template <typename F, typename... A>
    struct task_type
    {
      atomic_count* task_count;
      std::decay_t<F> func;
      std::tuple<std::decay_t<A>...> args;

      template <size_t... i>
      void
      thunk (std::index_sequence<i...>)
      {
        move (func) (std::get<i> (move (args))...);
      }
    };

    template <typename F, typename... A>
    static void
    task_thunk (scheduler& s, lock& ql, void* td)
    {
      using task = task_type<F, A...>;

      // Move the data and release the lock.
      //
      task t (move (*static_cast<task*> (td)));
      ql.unlock ();

      t.thunk (std::index_sequence_for<A...> ());

      atomic_count& tc (*t.task_count);
      if (--tc == 0)
        s.resume (tc); // Resume a waiter, if any.
    }

    template <typename T>
    static std::decay_t<T>
    decay_copy (T&& x) {return forward<T> (x);}

  private:
    std::mutex mutex_;

    bool shutdown_ = true;   // Shutdown flag.
    bool task_ = false;      // Task queued flag (see below).

    // The constraints that we must maintain:
    //
    //                  active <= max_active
    // (init_active + helpers) <= max_threads
    //
    // Note that the first three are immutable between startup() and
    // shutdown() so can be accessed without a lock (@@ What about
    // the case of multiple initially active?).
    //
    size_t init_active_ = 0; // Initially active threads.
    size_t max_active_  = 0; // Maximum number of active threads.
    size_t max_threads_ = 0; // Maximum number of total threads.

    size_t helpers_     = 0; // Number of helper threads created so far.

    // Every thread that we manage must be accounted for in one of these
    // counters. And their sum should equal (init_active + helpers).
    //
    size_t active_   = 0;  // Active master threads executing a task.
    size_t idle_     = 0;  // Idle helper threads waiting for a task.
    size_t waiting_  = 0;  // Suspended master threads waiting for their tasks.
    size_t ready_    = 0;  // Ready master thread waiting to become active.
    size_t starting_ = 0;  // Helper threads starting up.

    std::condition_variable idle_condv_;  // Idle helpers queue.
    std::condition_variable ready_condv_; // Ready masters queue.

    // Statistics counters.
    //
    size_t stat_max_waiters_;
    size_t stat_wait_collisions_;

    // Wait queue.
    //
    // A wait slot blocks a bunch of threads. When they are (all) unblocked,
    // they re-examine their respective conditions and either carry on or
    // block again.
    //
    // The wait queue is a shard of slots. A thread picks a slot based on the
    // address of its task count variable. How many slots do we need? This
    // depends on the number of waiters that we can have which cannot be
    // greater than the total number of threads.
    //
    struct wait_slot
    {
      std::mutex mutex;
      std::condition_variable condv;
      size_t waiters = 0;
      bool shutdown = true;
    };

    size_t wait_queue_size_; // Multiple of max_threads.
    unique_ptr<wait_slot[]> wait_queue_;

    // Task queue.
    //
    // Each queue has its own mutex. If the task_ flag above is true then
    // there *might* be a task in one of the queues. If it is false, then
    // it means there are either no tasks or someone is busy working on
    // them.
    //
    // For now we only support trivially-destructible tasks.
    //
    struct task_data
    {
      std::aligned_storage<sizeof (void*) * 7>::type data;
      void (*thunk) (scheduler&, lock&, void*);
    };

    // We have two requirements: Firstly, we want to keep the master thread
    // (the one that called wait()) busy working though its own queue for as
    // long as possible before (if at all) it "reincarnates" as a helper. The
    // main reason for this is the limited number of helpers we can create.
    //
    // Secondly, we don't want to block wait() longer than necessary since the
    // master thread can do some work with the result. Plus, overall, we want
    // to "unwind" task hierarchies as soon as possible since they hold up
    // resources such as thread's stack. All this means that the master thread
    // can only work through tasks that it has queued at this "level" of the
    // async()/wait() calls since we know that wait() cannot return until
    // they are done.
    //
    // To satisfy the first requirement, the master and helper threads get the
    // tasks from different ends of the queue: master from the back while
    // helpers from the front. And the master always adds new tasks to the
    // back.
    //
    // To satisfy the second requirement, the master thread stores the index
    // of the first task it has queued at this "level" and makes sure it
    // doesn't try to deque any task beyond that.
    //
    size_t task_queue_depth_; // Multiple of max_active.

    struct task_queue
    {
      std::mutex mutex;
      bool shutdown = false;

      size_t stat_full = 0; // Number of times pop() returned NULL.

      // head <= stop <= tail
      //
      size_t head = 0; // Index of the first element.
      size_t stop = 0; // Index of the element to stop at in pop_back().
      size_t tail = 0; // Index of the one past last element.

      unique_ptr<task_data[]> data;

      task_queue (size_t depth): data (new task_data[depth]) {}
    };

    // Task queue API. Expects the queue mutex to be locked.
    //

    // Push a new task to the queue returning a pointer to the task data to be
    // filled or NULL if the queue is full.
    //
    task_data*
    push (task_queue& tq)
    {
      return tq.tail != task_queue_depth_ ? &tq.data[tq.tail++] : nullptr;
    }

    bool
    empty_front (task_queue& tq) const {return tq.head == tq.tail;}

    void
    pop_front (task_queue& tq, lock& ql)
    {
      task_data& td (tq.data[tq.head++]);

      if (tq.head == tq.tail)
        tq.stop = tq.head = tq.tail = 0; // Reset.
      else if (tq.head > tq.stop)
        tq.stop = tq.head;

      // The thunk moves the task data to its stack, releases the lock,
      // and continues to execute the task.
      //
      td.thunk (*this, ql, &td.data);
    }

    bool
    empty_back (task_queue& tq) const {return tq.stop == tq.tail;}

    void
    pop_back (task_queue& tq, lock& ql)
    {
      task_data& td (tq.data[--tq.tail]);

      if (tq.head == tq.tail)
        tq.stop = tq.head = tq.tail = 0;

      td.thunk (*this, ql, &td.data);
    }

    // Each thread has its own queue. Instead of allocating all max_threads of
    // them up front, we will reserve the space but will only construct queues
    // on demand.
    //
    vector<unique_ptr<task_queue>> task_queues_;

    // TLS cache of each thread's task queue.
    //
    thread_local static task_queue* task_queue_;

    task_queue&
    create_queue ();
  };

  // Main (and only) scheduler. Started up and shut down in main().
  //
  extern scheduler sched;
}

#endif // BUILD2_SCHEDULER