src/hotspot/share/gc/g1/g1RemSetTrackingPolicy.cpp

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 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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#include "precompiled.hpp"
#include "gc/g1/g1CollectionSetChooser.hpp"
#include "gc/g1/g1RemSetTrackingPolicy.hpp"
#include "gc/g1/heapRegion.inline.hpp"
#include "gc/g1/heapRegionRemSet.inline.hpp"
#include "runtime/safepoint.hpp"

bool G1RemSetTrackingPolicy::needs_scan_for_rebuild(HeapRegion* r) const {
  // All non-free and non-young regions need to be scanned for references;
  // At every gc we gather references to other regions in young.
  // Free regions trivially do not need scanning because they do not contain live
  // objects.
  return !(r->is_young() || r->is_free());
}

void G1RemSetTrackingPolicy::update_at_allocate(HeapRegion* r) {
  if (r->is_young()) {
    // Always collect remembered set for young regions.
    r->rem_set()->set_state_complete();
  } else if (r->is_humongous()) {
    // Collect remembered sets for humongous regions by default to allow eager reclaim.
    r->rem_set()->set_state_complete();
  } else if (r->is_old()) {
    // By default, do not create remembered set for new old regions.
    r->rem_set()->set_state_untracked();
  } else {
    guarantee(false, "Unhandled region %u with heap region type %s", r->hrm_index(), r->get_type_str());
  }
}

void G1RemSetTrackingPolicy::update_at_free(HeapRegion* r) {
  /* nothing to do */
}

static void print_before_rebuild(HeapRegion* r, bool selected_for_rebuild, size_t total_live_bytes, size_t live_bytes) {
  log_trace(gc, remset, tracking)("Before rebuild region %u "
                                  "(tams: " PTR_FORMAT ") "
                                  "total_live_bytes %zu "
                                  "selected %s "
                                  "(live_bytes %zu "
                                  "type %s)",
                                  r->hrm_index(),
                                  p2i(r->top_at_mark_start()),
                                  total_live_bytes,
                                  BOOL_TO_STR(selected_for_rebuild),
                                  live_bytes,
                                  r->get_type_str());
}

bool G1RemSetTrackingPolicy::update_humongous_before_rebuild(HeapRegion* r, bool is_live) {
  assert(SafepointSynchronize::is_at_safepoint(), "should be at safepoint");
  assert(r->is_humongous(), "Region %u should be humongous", r->hrm_index());

  assert(!r->rem_set()->is_updating(), "Remembered set of region %u is updating before rebuild", r->hrm_index());

  bool selected_for_rebuild = false;
  // For humongous regions, to be of interest for rebuilding the remembered set the following must apply:
  // - We always try to update the remembered sets of humongous regions containing
  // type arrays as they might have been reset after full gc.
  if (is_live && cast_to_oop(r->humongous_start_region()->bottom())->is_typeArray() && !r->rem_set()->is_tracked()) {
    r->rem_set()->set_state_updating();
    selected_for_rebuild = true;
  }

  size_t const live_bytes = is_live ? HeapRegion::GrainBytes : 0;
  print_before_rebuild(r, selected_for_rebuild, live_bytes, live_bytes);

  return selected_for_rebuild;
}

bool G1RemSetTrackingPolicy::update_before_rebuild(HeapRegion* r, size_t live_bytes_below_tams) {
  assert(SafepointSynchronize::is_at_safepoint(), "should be at safepoint");
  assert(!r->is_humongous(), "Region %u is humongous", r->hrm_index());

  // Only consider updating the remembered set for old gen regions.
  if (!r->is_old()) {
    return false;
  }

  assert(!r->rem_set()->is_updating(), "Remembered set of region %u is updating before rebuild", r->hrm_index());

  size_t live_bytes_above_tams = (r->top() - r->top_at_mark_start()) * HeapWordSize;
  size_t total_live_bytes = live_bytes_below_tams + live_bytes_above_tams;

  bool selected_for_rebuild = false;
  // For old regions, to be of interest for rebuilding the remembered set the following must apply:
  // - They must contain some live data in them.
  // - Only need to rebuild non-complete remembered sets.
  // - Otherwise only add those old gen regions which occupancy is low enough that there
  // is a chance that we will ever evacuate them in the mixed gcs.
  if ((total_live_bytes > 0) &&
      G1CollectionSetChooser::region_occupancy_low_enough_for_evac(total_live_bytes) &&
      !r->rem_set()->is_tracked()) {

    r->rem_set()->set_state_updating();
    selected_for_rebuild = true;
  }

  print_before_rebuild(r, selected_for_rebuild, total_live_bytes, live_bytes_below_tams);

  return selected_for_rebuild;
}

void G1RemSetTrackingPolicy::update_after_rebuild(HeapRegion* r) {
  assert(SafepointSynchronize::is_at_safepoint(), "should be at safepoint");

  if (r->is_old_or_humongous()) {
    if (r->rem_set()->is_updating()) {
      r->rem_set()->set_state_complete();
    }
    G1CollectedHeap* g1h = G1CollectedHeap::heap();
    // We can drop remembered sets of humongous regions that have a too large remembered set:
    // We will never try to eagerly reclaim or move them anyway until the next concurrent
    // cycle as e.g. remembered set entries will always be added.
    if (r->is_starts_humongous() && !g1h->is_potential_eager_reclaim_candidate(r)) {
      // Handle HC regions with the HS region.
      g1h->humongous_obj_regions_iterate(r,
                                         [&] (HeapRegion* r) {
                                           assert(!r->is_continues_humongous() || r->rem_set()->is_empty(),
                                                  "Continues humongous region %u remset should be empty", r->hrm_index());
                                           r->rem_set()->clear(true /* only_cardset */);
                                         });
    }
    G1ConcurrentMark* cm = G1CollectedHeap::heap()->concurrent_mark();
    log_trace(gc, remset, tracking)("After rebuild region %u "
                                    "(tams " PTR_FORMAT " "
                                    "liveness %zu "
                                    "remset occ %zu "
                                    "size %zu)",
                                    r->hrm_index(),
                                    p2i(r->top_at_mark_start()),
                                    cm->live_bytes(r->hrm_index()),
                                    r->rem_set()->occupied(),
                                    r->rem_set()->mem_size());
  }
}