view src/share/vm/memory/referenceProcessor.hpp @ 2740:4dfb2df418f2

6484982: G1: process references during evacuation pauses Summary: G1 now uses two reference processors - one is used by concurrent marking and the other is used by STW GCs (both full and incremental evacuation pauses). In an evacuation pause, the reference processor is embedded into the closures used to scan objects. Doing so causes causes reference objects to be 'discovered' by the reference processor. At the end of the evacuation pause, these discovered reference objects are processed - preserving (and copying) referent objects (and their reachable graphs) as appropriate. Reviewed-by: ysr, jwilhelm, brutisso, stefank, tonyp
author johnc
date Thu, 22 Sep 2011 10:57:37 -0700
parents eca1193ca245
children d1bdeef3e3e2
line wrap: on
line source
 * Copyright (c) 2001, 2011, Oracle and/or its affiliates. All rights reserved.
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 * 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.
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit if you need additional information or have any
 * questions.


#include "memory/referencePolicy.hpp"
#include "oops/instanceRefKlass.hpp"

// ReferenceProcessor class encapsulates the per-"collector" processing
// of java.lang.Reference objects for GC. The interface is useful for supporting
// a generational abstraction, in particular when there are multiple
// generations that are being independently collected -- possibly
// concurrently and/or incrementally.  Note, however, that the
// ReferenceProcessor class abstracts away from a generational setting
// by using only a heap interval (called "span" below), thus allowing
// its use in a straightforward manner in a general, non-generational
// setting.
// The basic idea is that each ReferenceProcessor object concerns
// itself with ("weak") reference processing in a specific "span"
// of the heap of interest to a specific collector. Currently,
// the span is a convex interval of the heap, but, efficiency
// apart, there seems to be no reason it couldn't be extended
// (with appropriate modifications) to any "non-convex interval".

// forward references
class ReferencePolicy;
class AbstractRefProcTaskExecutor;

// List of discovered references.
class DiscoveredList {
  DiscoveredList() : _len(0), _compressed_head(0), _oop_head(NULL) { }
  oop head() const     {
     return UseCompressedOops ?  oopDesc::decode_heap_oop(_compressed_head) :
  HeapWord* adr_head() {
    return UseCompressedOops ? (HeapWord*)&_compressed_head :
  void set_head(oop o) {
    if (UseCompressedOops) {
      // Must compress the head ptr.
      _compressed_head = oopDesc::encode_heap_oop(o);
    } else {
      _oop_head = o;
  bool   is_empty() const       { return head() == NULL; }
  size_t length()               { return _len; }
  void   set_length(size_t len) { _len = len;  }
  void   inc_length(size_t inc) { _len += inc; assert(_len > 0, "Error"); }
  void   dec_length(size_t dec) { _len -= dec; }
  // Set value depending on UseCompressedOops. This could be a template class
  // but then we have to fix all the instantiations and declarations that use this class.
  oop       _oop_head;
  narrowOop _compressed_head;
  size_t _len;

// Iterator for the list of discovered references.
class DiscoveredListIterator {
  DiscoveredList&    _refs_list;
  HeapWord*          _prev_next;
  oop                _prev;
  oop                _ref;
  HeapWord*          _discovered_addr;
  oop                _next;
  HeapWord*          _referent_addr;
  oop                _referent;
  OopClosure*        _keep_alive;
  BoolObjectClosure* _is_alive;

  oop                _first_seen; // cyclic linked list check

  size_t             _processed;
  size_t             _removed;

  inline DiscoveredListIterator(DiscoveredList&    refs_list,
                                OopClosure*        keep_alive,
                                BoolObjectClosure* is_alive):
#ifdef ASSERT
#ifndef PRODUCT
{ }

  // End Of List.
  inline bool has_next() const { return _ref != NULL; }

  // Get oop to the Reference object.
  inline oop obj() const { return _ref; }

  // Get oop to the referent object.
  inline oop referent() const { return _referent; }

  // Returns true if referent is alive.
  inline bool is_referent_alive() const {
    return _is_alive->do_object_b(_referent);

  // Loads data for the current reference.
  // The "allow_null_referent" argument tells us to allow for the possibility
  // of a NULL referent in the discovered Reference object. This typically
  // happens in the case of concurrent collectors that may have done the
  // discovery concurrently, or interleaved, with mutator execution.
  void load_ptrs(DEBUG_ONLY(bool allow_null_referent));

  // Move to the next discovered reference.
  inline void next() {
    _prev_next = _discovered_addr;
    _prev = _ref;

  // Remove the current reference from the list
  void remove();

  // Make the Reference object active again.
  void make_active();

  // Make the referent alive.
  inline void make_referent_alive() {
    if (UseCompressedOops) {
    } else {

  // Update the discovered field.
  inline void update_discovered() {
    // First _prev_next ref actually points into DiscoveredList (gross).
    if (UseCompressedOops) {
      if (!oopDesc::is_null(*(narrowOop*)_prev_next)) {
    } else {
      if (!oopDesc::is_null(*(oop*)_prev_next)) {

  // NULL out referent pointer.
  void clear_referent();

  // Statistics
  inline size_t processed() const { return _processed; }
  inline size_t removed() const   { return _removed; }

  inline void move_to_next() {
    if (_ref == _next) {
      // End of the list.
      _ref = NULL;
    } else {
      _ref = _next;
    assert(_ref != _first_seen, "cyclic ref_list found");


class ReferenceProcessor : public CHeapObj {
  // Compatibility with pre-4965777 JDK's
  static bool _pending_list_uses_discovered_field;

  MemRegion   _span;                    // (right-open) interval of heap
                                        // subject to wkref discovery

  bool        _discovering_refs;        // true when discovery enabled
  bool        _discovery_is_atomic;     // if discovery is atomic wrt
                                        // other collectors in configuration
  bool        _discovery_is_mt;         // true if reference discovery is MT.

  // If true, setting "next" field of a discovered refs list requires
  // write barrier(s).  (Must be true if used in a collector in which
  // elements of a discovered list may be moved during discovery: for
  // example, a collector like Garbage-First that moves objects during a
  // long-term concurrent marking phase that does weak reference
  // discovery.)
  bool        _discovered_list_needs_barrier;

  BarrierSet* _bs;                      // Cached copy of BarrierSet.
  bool        _enqueuing_is_done;       // true if all weak references enqueued
  bool        _processing_is_mt;        // true during phases when
                                        // reference processing is MT.
  int         _next_id;                 // round-robin mod _num_q counter in
                                        // support of work distribution

  // For collectors that do not keep GC liveness information
  // in the object header, this field holds a closure that
  // helps the reference processor determine the reachability
  // of an oop. It is currently initialized to NULL for all
  // collectors except for CMS and G1.
  BoolObjectClosure* _is_alive_non_header;

  // Soft ref clearing policies
  // . the default policy
  static ReferencePolicy*   _default_soft_ref_policy;
  // . the "clear all" policy
  static ReferencePolicy*   _always_clear_soft_ref_policy;
  // . the current policy below is either one of the above
  ReferencePolicy*          _current_soft_ref_policy;

  // The discovered ref lists themselves

  // The active MT'ness degree of the queues below
  int             _num_q;
  // The maximum MT'ness degree of the queues below
  int             _max_num_q;
  // Arrays of lists of oops, one per thread
  DiscoveredList* _discoveredSoftRefs;
  DiscoveredList* _discoveredWeakRefs;
  DiscoveredList* _discoveredFinalRefs;
  DiscoveredList* _discoveredPhantomRefs;

  static int number_of_subclasses_of_ref() { return (REF_PHANTOM - REF_OTHER); }

  int num_q()                              { return _num_q; }
  int max_num_q()                          { return _max_num_q; }
  void set_active_mt_degree(int v)         { _num_q = v; }
  DiscoveredList* discovered_soft_refs()   { return _discoveredSoftRefs; }

  ReferencePolicy* setup_policy(bool always_clear) {
    _current_soft_ref_policy = always_clear ?
      _always_clear_soft_ref_policy : _default_soft_ref_policy;
    _current_soft_ref_policy->setup();   // snapshot the policy threshold
    return _current_soft_ref_policy;

  // Process references with a certain reachability level.
  void process_discovered_reflist(DiscoveredList               refs_lists[],
                                  ReferencePolicy*             policy,
                                  bool                         clear_referent,
                                  BoolObjectClosure*           is_alive,
                                  OopClosure*                  keep_alive,
                                  VoidClosure*                 complete_gc,
                                  AbstractRefProcTaskExecutor* task_executor);

  void process_phaseJNI(BoolObjectClosure* is_alive,
                        OopClosure*        keep_alive,
                        VoidClosure*       complete_gc);

  // Work methods used by the method process_discovered_reflist
  // Phase1: keep alive all those referents that are otherwise
  // dead but which must be kept alive by policy (and their closure).
  void process_phase1(DiscoveredList&     refs_list,
                      ReferencePolicy*    policy,
                      BoolObjectClosure*  is_alive,
                      OopClosure*         keep_alive,
                      VoidClosure*        complete_gc);
  // Phase2: remove all those references whose referents are
  // reachable.
  inline void process_phase2(DiscoveredList&    refs_list,
                             BoolObjectClosure* is_alive,
                             OopClosure*        keep_alive,
                             VoidClosure*       complete_gc) {
    if (discovery_is_atomic()) {
      // complete_gc is ignored in this case for this phase
      pp2_work(refs_list, is_alive, keep_alive);
    } else {
      assert(complete_gc != NULL, "Error");
      pp2_work_concurrent_discovery(refs_list, is_alive,
                                    keep_alive, complete_gc);
  // Work methods in support of process_phase2
  void pp2_work(DiscoveredList&    refs_list,
                BoolObjectClosure* is_alive,
                OopClosure*        keep_alive);
  void pp2_work_concurrent_discovery(
                DiscoveredList&    refs_list,
                BoolObjectClosure* is_alive,
                OopClosure*        keep_alive,
                VoidClosure*       complete_gc);
  // Phase3: process the referents by either clearing them
  // or keeping them alive (and their closure)
  void process_phase3(DiscoveredList&    refs_list,
                      bool               clear_referent,
                      BoolObjectClosure* is_alive,
                      OopClosure*        keep_alive,
                      VoidClosure*       complete_gc);

  // Enqueue references with a certain reachability level
  void enqueue_discovered_reflist(DiscoveredList& refs_list, HeapWord* pending_list_addr);

  // "Preclean" all the discovered reference lists
  // by removing references with strongly reachable referents.
  // The first argument is a predicate on an oop that indicates
  // its (strong) reachability and the second is a closure that
  // may be used to incrementalize or abort the precleaning process.
  // The caller is responsible for taking care of potential
  // interference with concurrent operations on these lists
  // (or predicates involved) by other threads. Currently
  // only used by the CMS collector.  should_unload_classes is
  // used to aid assertion checking when classes are collected.
  void preclean_discovered_references(BoolObjectClosure* is_alive,
                                      OopClosure*        keep_alive,
                                      VoidClosure*       complete_gc,
                                      YieldClosure*      yield,
                                      bool               should_unload_classes);

  // Delete entries in the discovered lists that have
  // either a null referent or are not active. Such
  // Reference objects can result from the clearing
  // or enqueueing of Reference objects concurrent
  // with their discovery by a (concurrent) collector.
  // For a definition of "active" see java.lang.ref.Reference;
  // Refs are born active, become inactive when enqueued,
  // and never become active again. The state of being
  // active is encoded as follows: A Ref is active
  // if and only if its "next" field is NULL.
  void clean_up_discovered_references();
  void clean_up_discovered_reflist(DiscoveredList& refs_list);

  // Returns the name of the discovered reference list
  // occupying the i / _num_q slot.
  const char* list_name(int i);

  void enqueue_discovered_reflists(HeapWord* pending_list_addr, AbstractRefProcTaskExecutor* task_executor);

  // Set the 'discovered' field of the given reference to
  // the given value - emitting barriers depending upon
  // the value of _discovered_list_needs_barrier.
  void set_discovered(oop ref, oop value);

  // "Preclean" the given discovered reference list
  // by removing references with strongly reachable referents.
  // Currently used in support of CMS only.
  void preclean_discovered_reflist(DiscoveredList&    refs_list,
                                   BoolObjectClosure* is_alive,
                                   OopClosure*        keep_alive,
                                   VoidClosure*       complete_gc,
                                   YieldClosure*      yield);

  // round-robin mod _num_q (not: _not_ mode _max_num_q)
  int next_id() {
    int id = _next_id;
    if (++_next_id == _num_q) {
      _next_id = 0;
    return id;
  DiscoveredList* get_discovered_list(ReferenceType rt);
  inline void add_to_discovered_list_mt(DiscoveredList& refs_list, oop obj,
                                        HeapWord* discovered_addr);
  void verify_ok_to_handle_reflists() PRODUCT_RETURN;

  void clear_discovered_references(DiscoveredList& refs_list);
  void abandon_partial_discovered_list(DiscoveredList& refs_list);

  // Calculate the number of jni handles.
  unsigned int count_jni_refs();

  // Balances reference queues.
  void balance_queues(DiscoveredList ref_lists[]);

  // Update (advance) the soft ref master clock field.
  void update_soft_ref_master_clock();

  // constructor
    _span((HeapWord*)NULL, (HeapWord*)NULL),
    _discoveredSoftRefs(NULL),  _discoveredWeakRefs(NULL),
    _discoveredFinalRefs(NULL), _discoveredPhantomRefs(NULL),
  { }

  // Default parameters give you a vanilla reference processor.
  ReferenceProcessor(MemRegion span,
                     bool mt_processing = false, int mt_processing_degree = 1,
                     bool mt_discovery  = false, int mt_discovery_degree  = 1,
                     bool atomic_discovery = true,
                     BoolObjectClosure* is_alive_non_header = NULL,
                     bool discovered_list_needs_barrier = false);

  // RefDiscoveryPolicy values
  enum DiscoveryPolicy {
    ReferenceBasedDiscovery = 0,
    ReferentBasedDiscovery  = 1,
    DiscoveryPolicyMin      = ReferenceBasedDiscovery,
    DiscoveryPolicyMax      = ReferentBasedDiscovery

  static void init_statics();

  // get and set "is_alive_non_header" field
  BoolObjectClosure* is_alive_non_header() {
    return _is_alive_non_header;
  void set_is_alive_non_header(BoolObjectClosure* is_alive_non_header) {
    _is_alive_non_header = is_alive_non_header;

  // get and set span
  MemRegion span()                   { return _span; }
  void      set_span(MemRegion span) { _span = span; }

  // start and stop weak ref discovery
  void enable_discovery(bool verify_disabled, bool check_no_refs) {
#ifdef ASSERT
    // Verify that we're not currently discovering refs
    assert(!verify_disabled || !_discovering_refs, "nested call?");

    if (check_no_refs) {
      // Verify that the discovered lists are empty
#endif // ASSERT
    _discovering_refs = true;

  void disable_discovery()  { _discovering_refs = false; }
  bool discovery_enabled()  { return _discovering_refs;  }

  // whether discovery is atomic wrt other collectors
  bool discovery_is_atomic() const { return _discovery_is_atomic; }
  void set_atomic_discovery(bool atomic) { _discovery_is_atomic = atomic; }

  // whether the JDK in which we are embedded is a pre-4965777 JDK,
  // and thus whether or not it uses the discovered field to chain
  // the entries in the pending list.
  static bool pending_list_uses_discovered_field() {
    return _pending_list_uses_discovered_field;

  // whether discovery is done by multiple threads same-old-timeously
  bool discovery_is_mt() const { return _discovery_is_mt; }
  void set_mt_discovery(bool mt) { _discovery_is_mt = mt; }

  // Whether we are in a phase when _processing_ is MT.
  bool processing_is_mt() const { return _processing_is_mt; }
  void set_mt_processing(bool mt) { _processing_is_mt = mt; }

  // whether all enqueuing of weak references is complete
  bool enqueuing_is_done()  { return _enqueuing_is_done; }
  void set_enqueuing_is_done(bool v) { _enqueuing_is_done = v; }

  // iterate over oops
  void weak_oops_do(OopClosure* f);       // weak roots

  // Balance each of the discovered lists.
  void balance_all_queues();

  // Discover a Reference object, using appropriate discovery criteria
  bool discover_reference(oop obj, ReferenceType rt);

  // Process references found during GC (called by the garbage collector)
  void process_discovered_references(BoolObjectClosure*           is_alive,
                                     OopClosure*                  keep_alive,
                                     VoidClosure*                 complete_gc,
                                     AbstractRefProcTaskExecutor* task_executor);

  // Enqueue references at end of GC (called by the garbage collector)
  bool enqueue_discovered_references(AbstractRefProcTaskExecutor* task_executor = NULL);

  // If a discovery is in process that is being superceded, abandon it: all
  // the discovered lists will be empty, and all the objects on them will
  // have NULL discovered fields.  Must be called only at a safepoint.
  void abandon_partial_discovery();

  // debugging
  void verify_no_references_recorded() PRODUCT_RETURN;
  void verify_referent(oop obj)        PRODUCT_RETURN;

  // clear the discovered lists (unlinking each entry).
  void clear_discovered_references() PRODUCT_RETURN;

// A utility class to disable reference discovery in
// the scope which contains it, for given ReferenceProcessor.
class NoRefDiscovery: StackObj {
  ReferenceProcessor* _rp;
  bool _was_discovering_refs;
  NoRefDiscovery(ReferenceProcessor* rp) : _rp(rp) {
    _was_discovering_refs = _rp->discovery_enabled();
    if (_was_discovering_refs) {

  ~NoRefDiscovery() {
    if (_was_discovering_refs) {
      _rp->enable_discovery(true /*verify_disabled*/, false /*check_no_refs*/);

// A utility class to temporarily mutate the span of the
// given ReferenceProcessor in the scope that contains it.
class ReferenceProcessorSpanMutator: StackObj {
  ReferenceProcessor* _rp;
  MemRegion           _saved_span;

  ReferenceProcessorSpanMutator(ReferenceProcessor* rp,
                                MemRegion span):
    _rp(rp) {
    _saved_span = _rp->span();

  ~ReferenceProcessorSpanMutator() {

// A utility class to temporarily change the MT'ness of
// reference discovery for the given ReferenceProcessor
// in the scope that contains it.
class ReferenceProcessorMTDiscoveryMutator: StackObj {
  ReferenceProcessor* _rp;
  bool                _saved_mt;

  ReferenceProcessorMTDiscoveryMutator(ReferenceProcessor* rp,
                                       bool mt):
    _rp(rp) {
    _saved_mt = _rp->discovery_is_mt();

  ~ReferenceProcessorMTDiscoveryMutator() {

// A utility class to temporarily change the disposition
// of the "is_alive_non_header" closure field of the
// given ReferenceProcessor in the scope that contains it.
class ReferenceProcessorIsAliveMutator: StackObj {
  ReferenceProcessor* _rp;
  BoolObjectClosure*  _saved_cl;

  ReferenceProcessorIsAliveMutator(ReferenceProcessor* rp,
                                   BoolObjectClosure*  cl):
    _rp(rp) {
    _saved_cl = _rp->is_alive_non_header();

  ~ReferenceProcessorIsAliveMutator() {

// A utility class to temporarily change the disposition
// of the "discovery_is_atomic" field of the
// given ReferenceProcessor in the scope that contains it.
class ReferenceProcessorAtomicMutator: StackObj {
  ReferenceProcessor* _rp;
  bool                _saved_atomic_discovery;

  ReferenceProcessorAtomicMutator(ReferenceProcessor* rp,
                                  bool atomic):
    _rp(rp) {
    _saved_atomic_discovery = _rp->discovery_is_atomic();

  ~ReferenceProcessorAtomicMutator() {

// A utility class to temporarily change the MT processing
// disposition of the given ReferenceProcessor instance
// in the scope that contains it.
class ReferenceProcessorMTProcMutator: StackObj {
  ReferenceProcessor* _rp;
  bool  _saved_mt;

  ReferenceProcessorMTProcMutator(ReferenceProcessor* rp,
                                  bool mt):
    _rp(rp) {
    _saved_mt = _rp->processing_is_mt();

  ~ReferenceProcessorMTProcMutator() {

// This class is an interface used to implement task execution for the
// reference processing.
class AbstractRefProcTaskExecutor {

  // Abstract tasks to execute.
  class ProcessTask;
  class EnqueueTask;

  // Executes a task using worker threads.
  virtual void execute(ProcessTask& task) = 0;
  virtual void execute(EnqueueTask& task) = 0;

  // Switch to single threaded mode.
  virtual void set_single_threaded_mode() { };

// Abstract reference processing task to execute.
class AbstractRefProcTaskExecutor::ProcessTask {
  ProcessTask(ReferenceProcessor& ref_processor,
              DiscoveredList      refs_lists[],
              bool                marks_oops_alive)
    : _ref_processor(ref_processor),
  { }

  virtual void work(unsigned int work_id, BoolObjectClosure& is_alive,
                    OopClosure& keep_alive,
                    VoidClosure& complete_gc) = 0;

  // Returns true if a task marks some oops as alive.
  bool marks_oops_alive() const
  { return _marks_oops_alive; }

  ReferenceProcessor& _ref_processor;
  DiscoveredList*     _refs_lists;
  const bool          _marks_oops_alive;

// Abstract reference processing task to execute.
class AbstractRefProcTaskExecutor::EnqueueTask {
  EnqueueTask(ReferenceProcessor& ref_processor,
              DiscoveredList      refs_lists[],
              HeapWord*           pending_list_addr,
              int                 n_queues)
    : _ref_processor(ref_processor),
  { }

  virtual void work(unsigned int work_id) = 0;

  ReferenceProcessor& _ref_processor;
  DiscoveredList*     _refs_lists;
  HeapWord*           _pending_list_addr;
  int                 _n_queues;