src/hotspot/share/gc/g1/g1MMUTracker.cpp

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#include "gc/g1/g1MMUTracker.hpp"
#include "gc/g1/g1Trace.hpp"
#include "logging/log.hpp"
#include "runtime/mutexLocker.hpp"
#include "utilities/ostream.hpp"

// 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),
  _head_index(0),
  _tail_index(trim_index(_head_index+1)),
  _no_entries(0) { }

void G1MMUTracker::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 G1MMUTracker::calculate_gc_time(double current_timestamp) {
  double gc_time = 0.0;
  double limit = current_timestamp - _time_slice;
  for (int i = 0; i < _no_entries; ++i) {
    int index = trim_index(_tail_index + i);
    G1MMUTrackerElem *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 G1MMUTracker::add_pause(double start, double end) {
  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] = G1MMUTrackerElem(start, end);

  // Current entry needs to be added before calculating the value
  double slice_time = calculate_gc_time(end);
  G1MMUTracer::report_mmu(_time_slice, slice_time, _max_gc_time);

  if (slice_time < _max_gc_time) {
    log_debug(gc, mmu)("MMU: %.1lfms (%.1lfms/%.1lfms)",
                       slice_time * 1000.0, _max_gc_time * 1000.0, _time_slice * 1000);
  } else {
    log_info(gc, mmu)("MMU target violated: %.1lfms (%.1lfms/%.1lfms)",
                      slice_time * 1000.0, _max_gc_time * 1000.0, _time_slice * 1000);
  }
}

//                                                current_timestamp
//                       GC events               /  pause_time
//                 /     |     \     \          | /  /
// -------------[----]-[---]--[--]---[---]------|[--]-----> Time
//              |         |                         |
//              |         |                         |
//              |<- limit |                         |
//              |         |<- balance_timestamp     |
//              |         ^                         |
//              |                                   |
//              |<--------  _time_slice   --------->|
//
// The MMU constraint requires that we can spend up to `max_gc_time()` on GC
// pauses inside a window of `_time_slice` length. Therefore, we have a GC
// budget of `max_gc_time() - pause_time`, which is to be accounted for by past
// GC events.
//
// Focusing on GC events that are inside [limit, current_timestamp], we iterate
// over them from the newest to the oldest (right-to-left in the diagram) and
// try to locate the timestamp annotated with ^, so that the accumulated GC
// time inside [balance_timestamp, current_timestamp] is equal to the budget.
// Next, return `balance_timestamp - limit`.
//
// When there are not enough GC events, i.e. we have a surplus budget, a new GC
// pause can start right away, so return 0.
double G1MMUTracker::when_sec(double current_timestamp, double pause_time) const {
  assert(pause_time > 0.0, "precondition");

  // If the pause is over the maximum, just assume that it's the maximum.
  pause_time = MIN2(pause_time, max_gc_time());

  double gc_budget = max_gc_time() - pause_time;

  double limit = current_timestamp + pause_time - _time_slice;
  // Iterate from newest to oldest.
  for (int i = 0; i < _no_entries; ++i) {
    int index = trim_index(_head_index - i);
    const G1MMUTrackerElem *elem = &_array[index];
    // Outside the window.
    if (elem->end_time() <= limit) {
      break;
    }

    double duration = (elem->end_time() - MAX2(elem->start_time(), limit));
    // This duration would exceed (strictly greater than) the budget.
    if (duration > gc_budget) {
      // This timestamp captures the instant the budget is balanced (or used up).
      double balance_timestamp = elem->end_time() - gc_budget;
      assert(balance_timestamp >= limit, "inv");
      return balance_timestamp - limit;
    }

    gc_budget -= duration;
  }

  // Not enough gc time spent inside the window, we have a budget surplus.
  return 0;
}