// 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 <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.
  //
  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&&, A&&...);

    // 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 the maximum threads or task queue depth arguments are unspecified,
    // then appropriate defaults are used.
    //
    scheduler (size_t max_active,
               size_t init_active = 1,
               size_t max_threads = 0,
               size_t queue_depth = 0)
    {
      startup (max_active, init_active, max_threads, queue_depth);
    }

    // Start the scheduler.
    //
    void
    startup (size_t max_active,
             size_t init_active = 1,
             size_t max_threads = 0,
             size_t queue_depth = 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 ();

    // If initially active thread(s) (besides the one that calls startup())
    // exist before the call to startup(), then they must call join() before
    // executing any tasks. The two common cases where you don't have to call
    // join are a single active thread that calls startup()/shutdown() or
    // active thread(s) that are created after startup().
    //
    void
    join ()
    {
      assert (task_queue_ = nullptr);

      // Lock the mutex to make sure the values set in startup() are visible
      // in this thread.
      //
      lock l (mutex_);
    }

    // If initially active thread(s) participate in multiple schedulers and/or
    // sessions (intervals between startup() and shutdown()), then they must
    // call leave() before joining another scheduler/session. Note that this
    // applies to the active thread that calls shutdown(). Note that a thread
    // can only participate in one scheduler at a time.
    //
    void
    leave ()
    {
      task_queue_ = nullptr;
    }

    // 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
    {
      using func_type = std::decay_t<F>;
      using args_type = std::tuple<std::decay_t<A>...>;

      atomic_count* task_count;
      func_type func;
      args_type 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&, lock&, void*);

    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 (but see join()).
    //
    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.

      // Our task queue is circular with head being the index of the first
      // element and tail -- of the last. Since this makes the empty and one
      // element cases indistinguishable, we also keep the size.
      //
      // The mark is an index somewhere between (figuratively speaking) head
      // and tail, if enabled. If the mark is hit, then it is disabled until
      // the queue becomes empty.
      //
      size_t head = 0;
      size_t mark = 0;
      size_t tail = 0;
      size_t size = 0;

      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)
    {
      size_t& s (tq.size);
      size_t& t (tq.tail);

      if (s != task_queue_depth_)
      {
        //                                         normal  wrap empty
        //                                         |       |    |
        t = s != 0 ? (t != task_queue_depth_ - 1 ? t + 1 : 0) : t;
        s++;
        return &tq.data[t];
      }

      return nullptr;
    }

    bool
    empty_front (task_queue& tq) const {return tq.size == 0;}

    void
    pop_front (task_queue& tq, lock& ql)
    {
      size_t& s (tq.size);
      size_t& h (tq.head);
      size_t& m (tq.mark);

      bool a (h == m); // Adjust mark?
      task_data& td (tq.data[h]);

      //                                         normal  wrap empty
      //                                         |       |    |
      h = s != 1 ? (h != task_queue_depth_ - 1 ? h + 1 : 0) : h;

      if (--s == 0 || a)
        m = h; // Reset or adjust the mark.

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

    bool
    empty_back (task_queue& tq) const
    {
      return tq.size == 0 || tq.mark == task_queue_depth_;
    }

    void
    pop_back (task_queue& tq, lock& ql)
    {
      size_t& s (tq.size);
      size_t& t (tq.tail);
      size_t& m (tq.mark);

      bool a (t == m); // Adjust mark?

      task_data& td (tq.data[t]);

      // Save the old queue mark and set the new one in case the task we are
      // about to run adds sub-tasks.
      //
      size_t om (m);
      m = t; // Where next push() will go.

      //                     normal  wrap                     empty
      //                     |       |                        |
      t = s != 1 ? (t != 0 ? t - 1 : task_queue_depth_ - 1) : t;
      --s;

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

      // Restore the old mark (which we might have to adjust).
      //
      if (s == 0)
        m = t;                 // Reset the mark.
      else if (a)
        m = task_queue_depth_; // Disable the mark.
      else
        // What happens if head goes past the old mark? In this case we will
        // get into the empty queue state before we end up making any (wrong)
        // decisions based on this value. Unfortunately there is no way to
        // detect this (and do some sanity asserts) since things can wrap
        // around.
        //
        // To put it another way, the understanding here is that after the
        // task returns we will either have an empty queue or there will still
        // be tasks between the old mark and and the current tail, something
        // along these lines:
        //
        // OOOOOXXXXOOO
        //   |  |  |
        //   m  h  t
        //
        m = om;
    }

    // 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 thread's task queue.
    //
    static
#ifdef BUTL_CXX11_THREAD_LOCAL
    thread_local
#else
    __thread
#endif
    task_queue* task_queue_;

    task_queue&
    create_queue ();
  };

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

#include <build2/scheduler.txx>

#endif // BUILD2_SCHEDULER