Java GC 源碼分析(2)

Mark-Sweep Policy

---

• 類繼承關係：CollectorPolicy=>GenCollectorPolicy =>TwoGenerationCollectorPolicy

• TwoGenerationCollectorPolicy類：

class TwoGenerationCollectorPolicy : public GenCollectorPolicy {
 protected:
  size_t _min_gen1_size;
  size_t _initial_gen1_size;
  size_t _max_gen1_size;

  void initialize_flags();
  void initialize_size_info();

  DEBUG_ONLY(void assert_flags();)
  DEBUG_ONLY(void assert_size_info();)

 public:
  TwoGenerationCollectorPolicy() : GenCollectorPolicy(), _min_gen1_size(0),
    _initial_gen1_size(0), _max_gen1_size(0) {}

  // Accessors
  size_t min_gen1_size()     { return _min_gen1_size; }
  size_t initial_gen1_size() { return _initial_gen1_size; }
  size_t max_gen1_size()     { return _max_gen1_size; }

  // Inherited methods
  TwoGenerationCollectorPolicy* as_two_generation_policy() { return this; }

  int number_of_generations()          { return 2; }
  BarrierSet::Name barrier_set_name()  { return BarrierSet::CardTableModRef; }

  virtual CollectorPolicy::Name kind() {
    return CollectorPolicy::TwoGenerationCollectorPolicyKind;
  }

  // Returns true if gen0 sizes were adjusted
  bool adjust_gen0_sizes(size_t* gen0_size_ptr, size_t* gen1_size_ptr,
                         const size_t heap_size);
};

---

• Mark-Sweep Policy

class MarkSweepPolicy : public TwoGenerationCollectorPolicy {
 protected:
  void initialize_alignments();
  void initialize_generations();

 public:
  MarkSweepPolicy() {}

  MarkSweepPolicy* as_mark_sweep_policy() { return this; }

  void initialize_gc_policy_counters();
};

• 內存對齊函數

#define align_size_up_(size, alignment) (((size) + ((alignment) - 1)) & ~((alignment) - 1))

• 初始化收集策略

CollectorPolicy::initialize_all()

  virtual void initialize_all() {
    initialize_alignments();
    initialize_flags();
    initialize_size_info();
  }

• CollectorPolicy::initialize_flags()
  
  • 參數驗證
  • -Xmx和-Xms參數的對齊：

  _min_heap_byte_size = align_size_up(_min_heap_byte_size, _heap_alignment);
  uintx aligned_initial_heap_size = align_size_up(InitialHeapSize, _heap_alignment);
  uintx aligned_max_heap_size = align_size_up(MaxHeapSize, _heap_alignment);

---

• 代空間初始化

void MarkSweepPolicy::initialize_generations() {
  _generations = NEW_C_HEAP_ARRAY3(GenerationSpecPtr, number_of_generations(), mtGC, 0, AllocFailStrategy::RETURN_NULL);
  if (_generations == NULL) {
    vm_exit_during_initialization("Unable to allocate gen spec");
  }

  if (UseParNewGC) {
    _generations[0] = new GenerationSpec(Generation::ParNew, _initial_gen0_size, _max_gen0_size);
  } else {
    _generations[0] = new GenerationSpec(Generation::DefNew, _initial_gen0_size, _max_gen0_size);
  }
  _generations[1] = new GenerationSpec(Generation::MarkSweepCompact, _initial_gen1_size, _max_gen1_size);

  if (_generations[0] == NULL || _generations[1] == NULL) {
    vm_exit_during_initialization("Unable to allocate gen spec");
  }
}

---

Serial GC的收集過程

void DefNewGeneration::collect(bool   full,
                               bool   clear_all_soft_refs,
                               size_t size,
                               bool   is_tlab) {
  assert(full || size > 0, "otherwise we don't want to collect");

  GenCollectedHeap* gch = GenCollectedHeap::heap();

  _gc_timer->register_gc_start();
  DefNewTracer gc_tracer;
  gc_tracer.report_gc_start(gch->gc_cause(), _gc_timer->gc_start());

  _next_gen = gch->next_gen(this);

  // If the next generation is too full to accommodate promotion
  // from this generation, pass on collection; let the next generation
  // do it.
  if (!collection_attempt_is_safe()) {
    if (Verbose && PrintGCDetails) {
      gclog_or_tty->print(" :: Collection attempt not safe :: ");
    }
    gch->set_incremental_collection_failed(); // Slight lie: we did not even attempt one
    return;
  }
  assert(to()->is_empty(), "Else not collection_attempt_is_safe");

  init_assuming_no_promotion_failure();

  GCTraceTime t1(GCCauseString("GC", gch->gc_cause()), PrintGC && !PrintGCDetails, true, NULL);
  // Capture heap used before collection (for printing).
  size_t gch_prev_used = gch->used();

  gch->trace_heap_before_gc(&gc_tracer);

  SpecializationStats::clear();

  // These can be shared for all code paths
  IsAliveClosure is_alive(this);
  ScanWeakRefClosure scan_weak_ref(this);

  age_table()->clear();
  to()->clear(SpaceDecorator::Mangle);

  gch->rem_set()->prepare_for_younger_refs_iterate(false);

  assert(gch->no_allocs_since_save_marks(0),
         "save marks have not been newly set.");

  // Not very pretty.
  CollectorPolicy* cp = gch->collector_policy();

  FastScanClosure fsc_with_no_gc_barrier(this, false);
  FastScanClosure fsc_with_gc_barrier(this, true);

  KlassScanClosure klass_scan_closure(&fsc_with_no_gc_barrier,
                                      gch->rem_set()->klass_rem_set());

  set_promo_failure_scan_stack_closure(&fsc_with_no_gc_barrier);
  FastEvacuateFollowersClosure evacuate_followers(gch, _level, this,
                                                  &fsc_with_no_gc_barrier,
                                                  &fsc_with_gc_barrier);

  assert(gch->no_allocs_since_save_marks(0),
         "save marks have not been newly set.");

  int so = SharedHeap::SO_AllClasses | SharedHeap::SO_Strings | SharedHeap::SO_CodeCache;

  gch->gen_process_strong_roots(_level,
                                true,  // Process younger gens, if any,
                                       // as strong roots.
                                true,  // activate StrongRootsScope
                                true,  // is scavenging
                                SharedHeap::ScanningOption(so),
                                &fsc_with_no_gc_barrier,
                                true,   // walk *all* scavengable nmethods
                                &fsc_with_gc_barrier,
                                &klass_scan_closure);

  // "evacuate followers".
  evacuate_followers.do_void();

  FastKeepAliveClosure keep_alive(this, &scan_weak_ref);
  ReferenceProcessor* rp = ref_processor();
  rp->setup_policy(clear_all_soft_refs);
  const ReferenceProcessorStats& stats =
  rp->process_discovered_references(&is_alive, &keep_alive, &evacuate_followers,
                                    NULL, _gc_timer);
  gc_tracer.report_gc_reference_stats(stats);

  if (!_promotion_failed) {
    // Swap the survivor spaces.
    eden()->clear(SpaceDecorator::Mangle);
    from()->clear(SpaceDecorator::Mangle);
    if (ZapUnusedHeapArea) {
      // This is now done here because of the piece-meal mangling which
      // can check for valid mangling at intermediate points in the
      // collection(s).  When a minor collection fails to collect
      // sufficient space resizing of the young generation can occur
      // an redistribute the spaces in the young generation.  Mangle
      // here so that unzapped regions don't get distributed to
      // other spaces.
      to()->mangle_unused_area();
    }
    swap_spaces();

    assert(to()->is_empty(), "to space should be empty now");

    adjust_desired_tenuring_threshold();

    // A successful scavenge should restart the GC time limit count which is
    // for full GC's.
    AdaptiveSizePolicy* size_policy = gch->gen_policy()->size_policy();
    size_policy->reset_gc_overhead_limit_count();
    if (PrintGC && !PrintGCDetails) {
      gch->print_heap_change(gch_prev_used);
    }
    assert(!gch->incremental_collection_failed(), "Should be clear");
  } else {
    assert(_promo_failure_scan_stack.is_empty(), "post condition");
    _promo_failure_scan_stack.clear(true); // Clear cached segments.

    remove_forwarding_pointers();
    if (PrintGCDetails) {
      gclog_or_tty->print(" (promotion failed) ");
    }
    // Add to-space to the list of space to compact
    // when a promotion failure has occurred.  In that
    // case there can be live objects in to-space
    // as a result of a partial evacuation of eden
    // and from-space.
    swap_spaces();   // For uniformity wrt ParNewGeneration.
    from()->set_next_compaction_space(to());
    gch->set_incremental_collection_failed();

    // Inform the next generation that a promotion failure occurred.
    _next_gen->promotion_failure_occurred();
    gc_tracer.report_promotion_failed(_promotion_failed_info);

    // Reset the PromotionFailureALot counters.
    NOT_PRODUCT(Universe::heap()->reset_promotion_should_fail();)
  }
  // set new iteration safe limit for the survivor spaces
  from()->set_concurrent_iteration_safe_limit(from()->top());
  to()->set_concurrent_iteration_safe_limit(to()->top());
  SpecializationStats::print();

  // We need to use a monotonically non-decreasing time in ms
  // or we will see time-warp warnings and os::javaTimeMillis()
  // does not guarantee monotonicity.
  jlong now = os::javaTimeNanos() / NANOSECS_PER_MILLISEC;
  update_time_of_last_gc(now);

  gch->trace_heap_after_gc(&gc_tracer);
  gc_tracer.report_tenuring_threshold(tenuring_threshold());

  _gc_timer->register_gc_end();

  gc_tracer.report_gc_end(_gc_timer->gc_end(), _gc_timer->time_partitions());
}