src/share/vm/gc_implementation/g1/g1MMUTracker.cpp

/*
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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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#include "precompiled.hpp"
#include "gc_implementation/g1/g1MMUTracker.hpp"
#include "runtime/mutexLocker.hpp"
#include "utilities/ostream.hpp"

#define _DISABLE_MMU                             0

// can't rely on comparing doubles with tolerating a small margin for error
#define SMALL_MARGIN 0.0000001
#define is_double_leq_0(_value) ( (_value) < SMALL_MARGIN )
#define is_double_leq(_val1, _val2) is_double_leq_0((_val1) - (_val2))
#define is_double_geq(_val1, _val2) is_double_leq_0((_val2) - (_val1))

/***** ALL TIMES ARE IN SECS!!!!!!! *****/

G1MMUTracker::G1MMUTracker(double time_slice, double max_gc_time) :
  _time_slice(time_slice),
  _max_gc_time(max_gc_time) { }

G1MMUTrackerQueue::G1MMUTrackerQueue(double time_slice, double max_gc_time) :
  G1MMUTracker(time_slice, max_gc_time),
  _head_index(0),
  _tail_index(trim_index(_head_index+1)),
  _no_entries(0) { }

void G1MMUTrackerQueue::remove_expired_entries(double current_time) {
  double limit = current_time - _time_slice;
  while (_no_entries > 0) {
    if (is_double_geq(limit, _array[_tail_index].end_time())) {
      _tail_index = trim_index(_tail_index + 1);
      --_no_entries;
    } else
      return;
  }
  guarantee(_no_entries == 0, "should have no entries in the array");
}

double G1MMUTrackerQueue::calculate_gc_time(double current_time) {
  double gc_time = 0.0;
  double limit = current_time - _time_slice;
  for (int i = 0; i < _no_entries; ++i) {
    int index = trim_index(_tail_index + i);
    G1MMUTrackerQueueElem *elem = &_array[index];
    if (elem->end_time() > limit) {
      if (elem->start_time() > limit)
        gc_time += elem->duration();
      else
        gc_time += elem->end_time() - limit;
    }
  }
  return gc_time;
}

void G1MMUTrackerQueue::add_pause(double start, double end, bool gc_thread) {
  double longest_allowed = longest_pause_internal(start);
  if (longest_allowed < 0.0)
    longest_allowed = 0.0;
  double duration = end - start;

  remove_expired_entries(end);
  if (_no_entries == QueueLength) {
    // OK, we've filled up the queue. There are a few ways
    // of dealing with this "gracefully"
    //   increase the array size (:-)
    //   remove the oldest entry (this might allow more GC time for
    //     the time slice than what's allowed) - this is what we
    //     currently do
    //   consolidate the two entries with the minimum gap between them
    //     (this might allow less GC time than what's allowed)

    // In the case where ScavengeALot is true, such overflow is not
    // uncommon; in such cases, we can, without much loss of precision
    // or performance (we are GC'ing most of the time anyway!),
    // simply overwrite the oldest entry in the tracker.

    _head_index = trim_index(_head_index + 1);
    assert(_head_index == _tail_index, "Because we have a full circular buffer");
    _tail_index = trim_index(_tail_index + 1);
  } else {
    _head_index = trim_index(_head_index + 1);
    ++_no_entries;
  }
  _array[_head_index] = G1MMUTrackerQueueElem(start, end);
}

// basically the _internal call does not remove expired entries
// this is for trying things out in the future and a couple
// of other places (debugging)

double G1MMUTrackerQueue::longest_pause(double current_time) {
  if (_DISABLE_MMU)
    return _max_gc_time;

  MutexLockerEx x(MMUTracker_lock, Mutex::_no_safepoint_check_flag);
  remove_expired_entries(current_time);

  return longest_pause_internal(current_time);
}

double G1MMUTrackerQueue::longest_pause_internal(double current_time) {
  double target_time = _max_gc_time;

  while( 1 ) {
    double gc_time =
      calculate_gc_time(current_time + target_time);
    double diff = target_time + gc_time - _max_gc_time;
    if (!is_double_leq_0(diff)) {
      target_time -= diff;
      if (is_double_leq_0(target_time)) {
        target_time = -1.0;
        break;
      }
    } else {
      break;
    }
  }

  return target_time;
}

// basically the _internal call does not remove expired entries
// this is for trying things out in the future and a couple
// of other places (debugging)

double G1MMUTrackerQueue::when_sec(double current_time, double pause_time) {
  if (_DISABLE_MMU)
    return 0.0;

  MutexLockerEx x(MMUTracker_lock, Mutex::_no_safepoint_check_flag);
  remove_expired_entries(current_time);

  return when_internal(current_time, pause_time);
}

double G1MMUTrackerQueue::when_internal(double current_time,
                                        double pause_time) {
  // if the pause is over the maximum, just assume that it's the maximum
  double adjusted_pause_time =
    (pause_time > max_gc_time()) ? max_gc_time() : pause_time;
  double earliest_end = current_time + adjusted_pause_time;
  double limit = earliest_end - _time_slice;
  double gc_time = calculate_gc_time(earliest_end);
  double diff = gc_time + adjusted_pause_time - max_gc_time();
  if (is_double_leq_0(diff))
    return 0.0;

  int index = _tail_index;
  while ( 1 ) {
    G1MMUTrackerQueueElem *elem = &_array[index];
    if (elem->end_time() > limit) {
      if (elem->start_time() > limit)
        diff -= elem->duration();
      else
        diff -= elem->end_time() - limit;
      if (is_double_leq_0(diff))
        return  elem->end_time() + diff + _time_slice - adjusted_pause_time - current_time;
    }
    index = trim_index(index+1);
    guarantee(index != trim_index(_head_index + 1), "should not go past head");
  }
}