panda3d/panda/src/pipeline/threadSimpleManager.cxx

/**
 * PANDA 3D SOFTWARE
 * Copyright (c) Carnegie Mellon University.  All rights reserved.
 *
 * All use of this software is subject to the terms of the revised BSD
 * license.  You should have received a copy of this license along
 * with this source code in a file named "LICENSE."
 *
 * @file threadSimpleManager.cxx
 * @author drose
 * @date 2007-06-19
 */

#include "threadSimpleManager.h"

#ifdef THREAD_SIMPLE_IMPL

#include "threadSimpleImpl.h"
#include "blockerSimple.h"
#include "mainThread.h"

#ifdef WIN32
#ifndef WIN32_LEAN_AND_MEAN
#define WIN32_LEAN_AND_MEAN 1
#endif
#include <windows.h>
#endif

bool ThreadSimpleManager::_pointers_initialized;
ThreadSimpleManager *ThreadSimpleManager::_global_ptr;

/**
 *
 */
ThreadSimpleManager::
ThreadSimpleManager() :
  _simple_thread_epoch_timeslice
  ("simple-thread-epoch-timeslice", 0.05,
   PRC_DESC("When SIMPLE_THREADS is defined, this defines the amount of time, "
            "in seconds, that should be considered the "
            "typical timeslice for one epoch (to run all threads once).")),
  _simple_thread_volunteer_delay
  ("simple-thread-volunteer-delay", 0.0,
   PRC_DESC("When SIMPLE_THREADS is defined, this defines the amount of time, "
            "in seconds, for which a task that voluntarily yields should "
            "be delayed.")),
  _simple_thread_yield_sleep
  ("simple-thread-yield-sleep", 0.001,
   PRC_DESC("When SIMPLE_THREADS is defined, this defines the amount of time, "
            "in seconds, for which the process should be put to sleep when "
            "yielding the timeslice to the system.")),
  _simple_thread_window
  ("simple-thread-window", 1.0,
   PRC_DESC("When SIMPLE_THREADS is defined, this defines the amount of time, "
            "in seconds, over which to average all the threads' runtimes, "
            "for the purpose of scheduling threads.")),
  _simple_thread_low_weight
  ("simple-thread-low-weight", 0.2,
   PRC_DESC("When SIMPLE_THREADS is defined, this determines the relative "
            "amount of time that is given to threads with priority TP_low.")),
  _simple_thread_normal_weight
  ("simple-thread-normal-weight", 1.0,
   PRC_DESC("When SIMPLE_THREADS is defined, this determines the relative "
            "amount of time that is given to threads with priority TP_normal.")),
  _simple_thread_high_weight
  ("simple-thread-high-weight", 5.0,
   PRC_DESC("When SIMPLE_THREADS is defined, this determines the relative "
            "amount of time that is given to threads with priority TP_high.")),
  _simple_thread_urgent_weight
  ("simple-thread-urgent-weight", 10.0,
   PRC_DESC("When SIMPLE_THREADS is defined, this determines the relative "
            "amount of time that is given to threads with priority TP_urgent."))
{
  _tick_scale = 1000000.0;
  _total_ticks = 0;
  _current_thread = nullptr;
  _clock = TrueClock::get_global_ptr();
  _waiting_for_exit = nullptr;

  // Install these global pointers so very low-level code (code defined before
  // the pipeline directory) can yield when necessary.
  global_thread_yield = &Thread::force_yield;
  global_thread_consider_yield = &Thread::consider_yield;
}

/**
 * Adds the indicated thread to the ready queue.  The thread will be executed
 * when its turn comes.  If the thread is not the currently executing thread,
 * its _jmp_context should be filled appropriately.
 *
 * If volunteer is true, the thread is volunteering to sleep before its
 * timeslice has been used up.  If volunteer is false, the thread would still
 * be running if it could.
 */
void ThreadSimpleManager::
enqueue_ready(ThreadSimpleImpl *thread, bool volunteer) {
  // We actually add it to _next_ready, so that we can tell when we have
  // processed every thread in a given epoch.
  if (!volunteer) {
    _next_ready.push_back(thread);

  } else {
    // Unless it's a volunteer, in which case we actually put it to sleep for
    // the duration of the timeslice, so it won't interfere with timeslice
    // accounting for the remaining ready threads.
    double now = get_current_time();
    thread->_wake_time = now + _simple_thread_volunteer_delay;
    _volunteers.push_back(thread);
    push_heap(_volunteers.begin(), _volunteers.end(), CompareStartTime());
  }
}

/**
 * Adds the indicated thread to the sleep queue, until the indicated number of
 * seconds have elapsed.  Then the thread will be automatically moved to the
 * ready queue.
 */
void ThreadSimpleManager::
enqueue_sleep(ThreadSimpleImpl *thread, double seconds) {
  if (thread_cat->is_debug()) {
    thread_cat.debug()
      << *_current_thread->_parent_obj << " sleeping for "
      << seconds << " seconds\n";
  }

  double now = get_current_time();
  thread->_wake_time = now + seconds;
  _sleeping.push_back(thread);
  push_heap(_sleeping.begin(), _sleeping.end(), CompareStartTime());
}

/**
 * Adds the indicated thread to the blocked queue for the indicated blocker.
 * The thread will be awoken by a later call to unblock_one() or
 * unblock_all().
 */
void ThreadSimpleManager::
enqueue_block(ThreadSimpleImpl *thread, BlockerSimple *blocker) {
  _blocked[blocker].push_back(thread);
  blocker->_flags |= BlockerSimple::F_has_waiters;
}

/**
 * Unblocks one thread waiting on the indicated blocker, if any.  Returns true
 * if anything was unblocked, false otherwise.
 */
bool ThreadSimpleManager::
unblock_one(BlockerSimple *blocker) {
  Blocked::iterator bi = _blocked.find(blocker);
  if (bi != _blocked.end()) {
    nassertr(blocker->_flags & BlockerSimple::F_has_waiters, false);

    FifoThreads &threads = (*bi).second;
    nassertr(!threads.empty(), false);
    ThreadSimpleImpl *thread = threads.front();
    threads.pop_front();
    _ready.push_back(thread);
    if (threads.empty()) {
      blocker->_flags &= ~BlockerSimple::F_has_waiters;
      _blocked.erase(bi);
    }

    return true;
  }

  return false;
}

/**
 * Unblocks all threads waiting on the indicated blocker.  Returns true if
 * anything was unblocked, false otherwise.
 */
bool ThreadSimpleManager::
unblock_all(BlockerSimple *blocker) {
  Blocked::iterator bi = _blocked.find(blocker);
  if (bi != _blocked.end()) {
    nassertr(blocker->_flags & BlockerSimple::F_has_waiters, false);

    FifoThreads &threads = (*bi).second;
    nassertr(!threads.empty(), false);
    while (!threads.empty()) {
      ThreadSimpleImpl *thread = threads.front();
      threads.pop_front();
      _ready.push_back(thread);
    }
    blocker->_flags &= ~BlockerSimple::F_has_waiters;
    _blocked.erase(bi);
    return true;
  }
  return false;
}

/**
 * Adds the indicated thread to the finished queue.  The manager will drop the
 * reference count on the indicated thread at the next epoch.  (A thread can't
 * drop its own reference count while it is running, since that might
 * deallocate its own stack.)
 */
void ThreadSimpleManager::
enqueue_finished(ThreadSimpleImpl *thread) {
  _finished.push_back(thread);
}

/**
 * Moves the indicated thread to the head of the ready queue.  If it is not
 * already on the ready queue, does nothing.
 */
void ThreadSimpleManager::
preempt(ThreadSimpleImpl *thread) {
  FifoThreads::iterator ti;
  ti = find(_ready.begin(), _ready.end(), thread);
  if (ti != _ready.end()) {
    _ready.erase(ti);
    _ready.push_front(thread);
  }
}

/**
 * Switches out the currently executing thread and chooses a new thread for
 * execution.  Before calling this, the current thread should have already re-
 * enqueued itself with a call to enqueue(), if it intends to run again.
 *
 * This will fill in the current thread's _jmp_context member appropriately,
 * and then change the global current_thread pointer.
 */
void ThreadSimpleManager::
next_context() {
  // Delete any threads that need it.  We can't delete the current thread,
  // though.
  while (!_finished.empty() && _finished.front() != _current_thread) {
    ThreadSimpleImpl *finished_thread = _finished.front();
    _finished.pop_front();
    unref_delete(finished_thread->_parent_obj);
  }

  ++_current_thread->_context_switches;

  // Mark the current thread's resume point.

#ifdef HAVE_PYTHON
  // Save the current Python thread state.
  _current_thread->_python_state = thread_state_swap(nullptr);
#endif  // HAVE_PYTHON

#ifdef DO_PSTATS
  Thread::PStatsCallback *pstats_callback = _current_thread->_parent_obj->get_pstats_callback();
  if (pstats_callback != nullptr) {
    pstats_callback->deactivate_hook(_current_thread->_parent_obj);
  }
#endif  // DO_PSTATS

  save_thread_context(_current_thread->_context, st_choose_next_context, this);
  // Pass 2: we have returned into the context, and are now resuming the
  // current thread.

#ifdef DO_PSTATS
  if (pstats_callback != nullptr) {
    pstats_callback->activate_hook(_current_thread->_parent_obj);
  }
#endif  // DO_PSTATS

#ifdef HAVE_PYTHON
  thread_state_swap(_current_thread->_python_state);
#endif  // HAVE_PYTHON
}

/**
 * Blocks until all running threads (other than the current thread) have
 * finished.  This only works when called from the main thread; if called on
 * any other thread, nothing will happen.
 */
void ThreadSimpleManager::
prepare_for_exit() {
  if (!_current_thread->_parent_obj->is_exact_type(MainThread::get_class_type())) {
    if (thread_cat->is_debug()) {
      thread_cat.debug()
        << "Ignoring prepare_for_exit called from "
        << *(_current_thread->_parent_obj) << "\n";
    }
    return;
  }

  if (thread_cat->is_debug()) {
    thread_cat.debug()
      << "prepare_for_exit\n";
  }

  nassertv(_waiting_for_exit == nullptr);
  _waiting_for_exit = _current_thread;

  // At this point, any non-joinable threads on any of the queues are
  // automatically killed.
  kill_non_joinable(_ready);

  Blocked::iterator bi = _blocked.begin();
  while (bi != _blocked.end()) {
    Blocked::iterator bnext = bi;
    ++bnext;
    BlockerSimple *blocker = (*bi).first;
    FifoThreads &threads = (*bi).second;
    kill_non_joinable(threads);
    if (threads.empty()) {
      blocker->_flags &= ~BlockerSimple::F_has_waiters;
      _blocked.erase(bi);
    }
    bi = bnext;
  }

  kill_non_joinable(_sleeping);
  kill_non_joinable(_volunteers);

  next_context();

  // Delete any remaining threads.
  while (!_finished.empty() && _finished.front() != _current_thread) {
    ThreadSimpleImpl *finished_thread = _finished.front();
    _finished.pop_front();
    unref_delete(finished_thread->_parent_obj);
  }
}

/**
 * Sets the initial value of the current_thread pointer, i.e.  the main
 * thread.  It is valid to call this method only exactly once.
 */
void ThreadSimpleManager::
set_current_thread(ThreadSimpleImpl *current_thread) {
  nassertv(_current_thread == nullptr);
  _current_thread = current_thread;
}

/**
 * Removes the indicated thread from the accounting, for instance just before
 * the thread destructs.
 */
void ThreadSimpleManager::
remove_thread(ThreadSimpleImpl *thread) {
  TickRecords new_records;
  TickRecords::iterator ri;
  for (ri = _tick_records.begin(); ri != _tick_records.end(); ++ri) {
    if ((*ri)._thread != thread) {
      // Keep this record.
      new_records.push_back(*ri);
    } else {
      // Lose this record.
      nassertv(_total_ticks >= (*ri)._tick_count);
      _total_ticks -= (*ri)._tick_count;
    }
  }

  _tick_records.swap(new_records);
}

/**
 * Calls the appropriate system sleep function to sleep the whole process for
 * the indicated number of seconds.
 */
void ThreadSimpleManager::
system_sleep(double seconds) {
#ifdef WIN32
  Sleep((int)(seconds * 1000 + 0.5));

#else
  /*
  struct timespec rqtp;
  rqtp.tv_sec = time_t(seconds);
  rqtp.tv_nsec = long((seconds - (double)rqtp.tv_sec) * 1000000000.0 + 0.5);
  nanosleep(&rqtp, NULL);
  */

  // We use select() as the only way that seems to actually yield the
  // timeslice.  sleep() and nanosleep() don't appear to do the trick.
  struct timeval tv;
  tv.tv_sec = time_t(seconds);
  tv.tv_usec = long((seconds - (double)tv.tv_sec) * 1000000.0 + 0.5);
  select(0, nullptr, nullptr, nullptr, &tv);
#endif  // WIN32
}

/**
 * Writes a list of threads running and threads blocked.
 */
void ThreadSimpleManager::
write_status(std::ostream &out) const {
  out << "Currently running: " << *_current_thread->_parent_obj << "\n";

  out << "Ready:";
  FifoThreads::const_iterator ti;
  Sleeping::const_iterator si;
  for (ti = _ready.begin(); ti != _ready.end(); ++ti) {
    out << " " << *(*ti)->_parent_obj;
  }
  for (ti = _next_ready.begin(); ti != _next_ready.end(); ++ti) {
    out << " " << *(*ti)->_parent_obj;
  }
  for (si = _volunteers.begin(); si != _volunteers.end(); ++si) {
    out << " " << *(*si)->_parent_obj;
  }
  out << "\n";

  double now = get_current_time();

  out << "Sleeping:";
  // Copy and sort for convenience.
  Sleeping s2 = _sleeping;
  sort(s2.begin(), s2.end(), CompareStartTime());
  for (si = s2.begin(); si != s2.end(); ++si) {
    out << " " << *(*si)->_parent_obj << "(" << (*si)->_wake_time - now
        << "s)";
  }
  out << "\n";

  Blocked::const_iterator bi;
  for (bi = _blocked.begin(); bi != _blocked.end(); ++bi) {
    BlockerSimple *blocker = (*bi).first;
    const FifoThreads &threads = (*bi).second;
    out << "On blocker " << blocker << ":\n";
    FifoThreads::const_iterator ti;
    for (ti = threads.begin(); ti != threads.end(); ++ti) {
      ThreadSimpleImpl *thread = (*ti);
      out << " " << *thread->_parent_obj;
#ifdef DEBUG_THREADS
      out << " (";
      thread->_parent_obj->output_blocker(out);
      out << ")";
#endif  // DEBUG_THREADS
    }
    out << "\n";
  }
}

/**
 * Calls the appropriate system function to yield the whole process to any
 * other system processes.
 */
void ThreadSimpleManager::
system_yield() {
  if (!_pointers_initialized) {
    // Ignore this call before we construct the global ThreadSimpleManager.
    return;
  }

  if (thread_cat->is_debug()) {
    thread_cat.debug()
      << "system_yield\n";
  }

  // There seem to be some issues with modern operating systems not wanting to
  // actually yield the timeslice in response to sleep(0). In particular,
  // Windows and OSX both seemed to do nothing in that call.  Whatever.  We'll
  // force the point by explicitly sleeping for 1 ms in both cases.  This is
  // user-configurable in case 1 ms is too much (though on Windows that's all
  // the resolution you have).
  system_sleep(_global_ptr->_simple_thread_yield_sleep);
}

/**
 * Returns elapsed time in seconds from some undefined epoch, via whatever
 * clock the manager is using for all thread timing.
 */
double ThreadSimpleManager::
get_current_time() const {
  return _clock->get_short_raw_time();
}

/**
 * Should be called at startup to initialize the simple threading system.
 */
void ThreadSimpleManager::
init_pointers() {
  if (!_pointers_initialized) {
    _pointers_initialized = true;
    _global_ptr = new ThreadSimpleManager;
    Thread::get_main_thread();
  }
}

/**
 * Select the next context to run.  Continuing the work of next_context().
 */
void ThreadSimpleManager::
st_choose_next_context(struct ThreadContext *from_context, void *data) {
  ThreadSimpleManager *self = (ThreadSimpleManager *)data;
  self->choose_next_context(from_context);
}

/**
 * Select the next context to run.  Continuing the work of next_context().
 */
void ThreadSimpleManager::
choose_next_context(struct ThreadContext *from_context) {
  double now = get_current_time();

  do_timeslice_accounting(_current_thread, now);
  _current_thread = nullptr;

  if (!_sleeping.empty() || !_volunteers.empty()) {
    if (_ready.empty() && _next_ready.empty()) {
      // All of our threads are currently sleeping.  Therefore, wake the
      // volunteer(s) immediately.
      wake_all_sleepers(_volunteers);

      // We should also yield the whole process now, to be polite to the rest
      // of the system.
      system_yield();
      now = get_current_time();
    }
    wake_sleepers(_sleeping, now);
    wake_sleepers(_volunteers, now);
  }

  bool new_epoch = !_ready.empty() && _next_ready.empty();

  // Choose a new thread to execute.
  while (true) {
    // If there are no threads, sleep.
    while (_ready.empty()) {
      if (!_next_ready.empty()) {
        // We've finished an epoch.
        _ready.swap(_next_ready);

        if (new_epoch && !_tick_records.empty()) {
          // Pop the oldest timeslice record off when we finish an epoch
          // without executing any threads, to ensure we don't get caught in
          // an "all threads reached budget" loop.
          if (thread_cat->is_debug()) {
            thread_cat.debug()
              << "All threads exceeded budget.\n";
          }
          TickRecord &record = _tick_records.front();
          _total_ticks -= record._tick_count;

          if (record._thread->_run_ticks >= record._tick_count) {
            // Ensure we don't go negative.
            record._thread->_run_ticks -= record._tick_count;
          } else {
            // It is possible for this to happen if the application has been
            // paused for more than 2^31 ticks.
            record._thread->_run_ticks = 0;
          }
          _tick_records.pop_front();
        }
        new_epoch = true;

      } else if (!_volunteers.empty()) {
        // There are some volunteers.  Wake them.  Also wake any sleepers that
        // need it.
        if (thread_cat->is_debug()) {
          thread_cat.debug()
            << "Waking volunteers.\n";
        }
        // We should yield the whole process now, to be polite to the rest of
        // the system.
        system_yield();
        now = get_current_time();
        wake_all_sleepers(_volunteers);
        wake_sleepers(_sleeping, now);

      } else if (!_sleeping.empty()) {
        // All threads are sleeping.
        double wait = _sleeping.front()->_wake_time - now;
        if (wait > 0.0) {
          if (thread_cat->is_debug()) {
            thread_cat.debug()
              << "Sleeping all threads " << wait << " seconds\n";
          }
          system_sleep(wait);
        }
        now = get_current_time();
        wake_sleepers(_sleeping, now);
        wake_sleepers(_volunteers, now);

      } else {
        // No threads are ready!
        if (_waiting_for_exit != nullptr) {
          // This is a shutdown situation.  In this case, we quietly abandoned
          // the remaining blocked threads, if any, and switch back to the
          // main thread to finish shutting down.
          _ready.push_back(_waiting_for_exit);
          _waiting_for_exit = nullptr;
          break;
        }

        // No threads are ready to run, but we're not explicitly shutting
        // down.  This is an error condition, an unintentional deadlock.
        if (!_blocked.empty()) {
          thread_cat->error()
            << "Deadlock!  All threads blocked.\n";
          report_deadlock();
          abort();
        }

        // No threads are queued anywhere.  This is some kind of internal
        // error, since normally the main thread, at least, should be queued
        // somewhere.
        thread_cat->error()
          << "All threads disappeared!\n";
        exit(0);
      }
    }

    ThreadSimpleImpl *chosen_thread = _ready.front();
    _ready.pop_front();

    double timeslice = determine_timeslice(chosen_thread);
    if (timeslice > 0.0) {
      // This thread is ready to roll.  Break out of the loop.
      chosen_thread->_start_time = now;
      chosen_thread->_stop_time = now + timeslice;
      _current_thread = chosen_thread;
      break;
    }

    // This thread is not ready to wake up yet.  Put it back for next epoch.
    // It doesn't count as a volunteer, though--its timeslice was used up.
    _next_ready.push_back(chosen_thread);
  }

  // All right, the thread is ready to roll.  Begin.
  if (thread_cat->is_debug()) {
    size_t blocked_count = 0;
    Blocked::const_iterator bi;
    for (bi = _blocked.begin(); bi != _blocked.end(); ++bi) {
      const FifoThreads &threads = (*bi).second;
      blocked_count += threads.size();
    }

    double timeslice = _current_thread->_stop_time - _current_thread->_start_time;
    thread_cat.debug()
      << "Switching to " << *_current_thread->_parent_obj
      << " for " << timeslice << " s ("
      << _ready.size() << " + " << _next_ready.size()
      << " + " << _volunteers.size()
      << " other threads ready, " << blocked_count
      << " blocked, " << _sleeping.size() << " sleeping)\n";
  }

  switch_to_thread_context(from_context, _current_thread->_context);

  // Shouldn't get here.
  nassertv(false);
  abort();
}

/**
 * Records the amount of time the indicated thread has run, and updates the
 * moving average.
 */
void ThreadSimpleManager::
do_timeslice_accounting(ThreadSimpleImpl *thread, double now) {
  double elapsed = now - thread->_start_time;
  if (thread_cat.is_debug()) {
    thread_cat.debug()
      << *thread->_parent_obj << " ran for " << elapsed << " s of "
      << thread->_stop_time - thread->_start_time << " requested.\n";
  }

  // Clamp the elapsed time at 0.  (If it's less than 0, the clock is running
  // backwards, ick.)
  elapsed = std::max(elapsed, 0.0);

  unsigned int ticks = (unsigned int)(elapsed * _tick_scale + 0.5);
  thread->_run_ticks += ticks;

  // Now remove any old records.
  unsigned int ticks_window = (unsigned int)(_simple_thread_window * _tick_scale + 0.5);
  while (_total_ticks > ticks_window) {
    nassertv(!_tick_records.empty());
    TickRecord &record = _tick_records.front();
    _total_ticks -= record._tick_count;
    if (record._thread->_run_ticks >= record._tick_count) {
      // Ensure we don't go negative.
      record._thread->_run_ticks -= record._tick_count;
    } else {
      // It is possible for this to happen if the application has been paused
      // for more than 2^31 ticks.
      record._thread->_run_ticks = 0;
    }
    _tick_records.pop_front();
  }

  // Finally, record the new record.
  TickRecord record;
  record._tick_count = ticks;
  record._thread = thread;
  _tick_records.push_back(record);
  _total_ticks += ticks;
}


/**
 * Moves any threads due to wake up from the sleeping queue to the ready
 * queue.
 */
void ThreadSimpleManager::
wake_sleepers(ThreadSimpleManager::Sleeping &sleepers, double now) {
  while (!sleepers.empty() && sleepers.front()->_wake_time <= now) {
    ThreadSimpleImpl *thread = sleepers.front();
    pop_heap(sleepers.begin(), sleepers.end(), CompareStartTime());
    sleepers.pop_back();
    _ready.push_back(thread);
  }
}

/**
 * Moves all threads from the indicated sleeping queue to the ready queue,
 * regardless of wake time.
 */
void ThreadSimpleManager::
wake_all_sleepers(ThreadSimpleManager::Sleeping &sleepers) {
  while (!sleepers.empty()) {
    ThreadSimpleImpl *thread = sleepers.front();
    pop_heap(sleepers.begin(), sleepers.end(), CompareStartTime());
    sleepers.pop_back();
    _ready.push_back(thread);
  }
}

/**
 *
 */
void ThreadSimpleManager::
report_deadlock() {
  Blocked::const_iterator bi;
  for (bi = _blocked.begin(); bi != _blocked.end(); ++bi) {
    BlockerSimple *blocker = (*bi).first;
    const FifoThreads &threads = (*bi).second;
    thread_cat.info()
      << "On blocker " << blocker << ":\n";
    FifoThreads::const_iterator ti;
    for (ti = threads.begin(); ti != threads.end(); ++ti) {
      ThreadSimpleImpl *thread = (*ti);
      thread_cat.info()
        << "  " << *thread->_parent_obj;
#ifdef DEBUG_THREADS
      thread_cat.info(false) << " (";
      thread->_parent_obj->output_blocker(thread_cat.info(false));
      thread_cat.info(false) << ")";
#endif  // DEBUG_THREADS
      thread_cat.info(false) << "\n";
    }
  }
}

/**
 * Determines the amount of time that should be allocated to the next
 * timeslice of this thread, based on its priority weight and the amount of
 * time it has run recently relative to other threads.
 */
double ThreadSimpleManager::
determine_timeslice(ThreadSimpleImpl *chosen_thread) {
  if (_ready.empty() && _next_ready.empty()) {
    // This is the only ready thread.  It gets the full timeslice.
    return _simple_thread_epoch_timeslice;
  }

  // Count up the total runtime and weight of all ready threads.
  unsigned int total_ticks = chosen_thread->_run_ticks;
  double total_weight = chosen_thread->_priority_weight;

  FifoThreads::const_iterator ti;
  for (ti = _ready.begin(); ti != _ready.end(); ++ti) {
    total_ticks += (*ti)->_run_ticks;
    total_weight += (*ti)->_priority_weight;
  }
  for (ti = _next_ready.begin(); ti != _next_ready.end(); ++ti) {
    total_ticks += (*ti)->_run_ticks;
    total_weight += (*ti)->_priority_weight;
  }

  nassertr(total_weight != 0.0, 0.0);
  double budget_ratio = chosen_thread->_priority_weight / total_weight;

  if (total_ticks == 0) {
    // This must be the first thread.  Special case.
    return budget_ratio * _simple_thread_epoch_timeslice;
  }

  double run_ratio = (double)chosen_thread->_run_ticks / (double)total_ticks;
  double remaining_ratio = budget_ratio - run_ratio;

  if (thread_cat->is_debug()) {
    thread_cat.debug()
      << *chosen_thread->_parent_obj << " accrued "
      << chosen_thread->_run_ticks / _tick_scale << " s of "
      << total_ticks / _tick_scale << "; budget is "
      << budget_ratio * total_ticks / _tick_scale << ".\n";
    if (remaining_ratio <= 0.0) {
      thread_cat.debug()
        << "Exceeded budget.\n";
    }
  }

  return remaining_ratio * _simple_thread_epoch_timeslice;
}

/**
 * Removes any non-joinable threads from the indicated queue and marks them
 * killed.
 */
void ThreadSimpleManager::
kill_non_joinable(ThreadSimpleManager::FifoThreads &threads) {
  FifoThreads new_threads;
  FifoThreads::iterator ti;
  for (ti = threads.begin(); ti != threads.end(); ++ti) {
    ThreadSimpleImpl *thread = (*ti);
    if (thread->_joinable) {
      new_threads.push_back(thread);
    } else {
      if (thread_cat->is_debug()) {
        thread_cat.debug()
          << "Killing " << *thread->_parent_obj << "\n";
      }
      thread->_status = ThreadSimpleImpl::TS_killed;
      enqueue_finished(thread);
    }
  }

  threads.swap(new_threads);
}

/**
 * Removes any non-joinable threads from the indicated queue and marks them
 * killed.
 */
void ThreadSimpleManager::
kill_non_joinable(ThreadSimpleManager::Sleeping &threads) {
  Sleeping new_threads;
  Sleeping::iterator ti;
  for (ti = threads.begin(); ti != threads.end(); ++ti) {
    ThreadSimpleImpl *thread = (*ti);
    if (thread->_joinable) {
      new_threads.push_back(thread);
    } else {
      if (thread_cat->is_debug()) {
        thread_cat.debug()
          << "Killing " << *thread->_parent_obj << "\n";
      }
      thread->_status = ThreadSimpleImpl::TS_killed;
      enqueue_finished(thread);
    }
  }
  make_heap(new_threads.begin(), new_threads.end(), CompareStartTime());
  threads.swap(new_threads);
}

#endif // THREAD_SIMPLE_IMPL