view src/share/vm/memory/referenceProcessor.hpp @ 453:c96030fff130

6684579: SoftReference processing can be made more efficient Summary: For current soft-ref clearing policies, we can decide at marking time if a soft-reference will definitely not be cleared, postponing the decision of whether it will definitely be cleared to the final reference processing phase. This can be especially beneficial in the case of concurrent collectors where the marking is usually concurrent but reference processing is usually not. Reviewed-by: jmasa
author ysr
date Thu, 20 Nov 2008 16:56:09 -0800
parents 1ee8caae33af
children 27a80744a83b
line wrap: on
line source
/*
 * Copyright 2001-2008 Sun Microsystems, Inc.  All Rights Reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
 * CA 95054 USA or visit www.sun.com if you need additional information or
 * have any questions.
 *
 */

// 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;
class DiscoveredList;

class ReferenceProcessor : public CHeapObj {
 protected:
  // End of list marker
  static oop  _sentinelRef;
  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 counter in
                                      // support of work distribution

  // For collectors that do not keep GC marking information
  // in the object header, this field holds a closure that
  // helps the reference processor determine the reachability
  // of an oop (the field is currently initialized to NULL for
  // all collectors but the CMS collector).
  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 MT'ness degree of the queues below
  int             _num_q;
  // Arrays of lists of oops, one per thread
  DiscoveredList* _discoveredSoftRefs;
  DiscoveredList* _discoveredWeakRefs;
  DiscoveredList* _discoveredFinalRefs;
  DiscoveredList* _discoveredPhantomRefs;

 public:
  int num_q()                            { return _num_q; }
  DiscoveredList* discovered_soft_refs() { return _discoveredSoftRefs; }
  static oop  sentinel_ref()             { return _sentinelRef; }
  static oop* adr_sentinel_ref()         { return &_sentinelRef; }
  ReferencePolicy* snap_policy(bool always_clear) {
    _current_soft_ref_policy = always_clear ?
      _always_clear_soft_ref_policy : _default_soft_ref_policy;
    _current_soft_ref_policy->snap();   // snapshot the policy threshold
    return _current_soft_ref_policy;
  }

 public:
  // 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.
  void preclean_discovered_references(BoolObjectClosure* is_alive,
                                      OopClosure*        keep_alive,
                                      VoidClosure*       complete_gc,
                                      YieldClosure*      yield);

  // 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);

 protected:
  // "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);

  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 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();

 public:
  // constructor
  ReferenceProcessor():
    _span((HeapWord*)NULL, (HeapWord*)NULL),
    _discoveredSoftRefs(NULL),  _discoveredWeakRefs(NULL),
    _discoveredFinalRefs(NULL), _discoveredPhantomRefs(NULL),
    _discovering_refs(false),
    _discovery_is_atomic(true),
    _enqueuing_is_done(false),
    _discovery_is_mt(false),
    _discovered_list_needs_barrier(false),
    _bs(NULL),
    _is_alive_non_header(NULL),
    _num_q(0),
    _processing_is_mt(false),
    _next_id(0)
  {}

  ReferenceProcessor(MemRegion span, bool atomic_discovery,
                     bool mt_discovery,
                     int mt_degree = 1,
                     bool mt_processing = false,
                     bool discovered_list_needs_barrier = false);

  // Allocates and initializes a reference processor.
  static ReferenceProcessor* create_ref_processor(
    MemRegion          span,
    bool               atomic_discovery,
    bool               mt_discovery,
    BoolObjectClosure* is_alive_non_header = NULL,
    int                parallel_gc_threads = 1,
    bool               mt_processing = false,
    bool               discovered_list_needs_barrier = false);
  // RefDiscoveryPolicy values
  enum {
    ReferenceBasedDiscovery = 0,
    ReferentBasedDiscovery  = 1
  };

  static void init_statics();

 public:
  // 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()   { _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 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
  static void oops_do(OopClosure* f);     // strong root(s)

  // 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);

 public:
  // 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;
  static void verify();

  // 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 {
 private:
  ReferenceProcessor* _rp;
  bool _was_discovering_refs;
 public:
  NoRefDiscovery(ReferenceProcessor* rp) : _rp(rp) {
    if (_was_discovering_refs = _rp->discovery_enabled()) {
      _rp->disable_discovery();
    }
  }

  ~NoRefDiscovery() {
    if (_was_discovering_refs) {
      _rp->enable_discovery();
    }
  }
};


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

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

  ~ReferenceProcessorSpanMutator() {
    _rp->set_span(_saved_span);
  }
};

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

 public:
  ReferenceProcessorMTMutator(ReferenceProcessor* rp,
                              bool mt):
    _rp(rp) {
    _saved_mt = _rp->discovery_is_mt();
    _rp->set_mt_discovery(mt);
  }

  ~ReferenceProcessorMTMutator() {
    _rp->set_mt_discovery(_saved_mt);
  }
};


// 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 {
 private:
  ReferenceProcessor* _rp;
  BoolObjectClosure*  _saved_cl;

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

  ~ReferenceProcessorIsAliveMutator() {
    _rp->set_is_alive_non_header(_saved_cl);
  }
};

// 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 {
 private:
  ReferenceProcessor* _rp;
  bool                _saved_atomic_discovery;

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

  ~ReferenceProcessorAtomicMutator() {
    _rp->set_atomic_discovery(_saved_atomic_discovery);
  }
};


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

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

  ~ReferenceProcessorMTProcMutator() {
    _rp->set_mt_processing(_saved_mt);
  }
};


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

  // 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 {
protected:
  ProcessTask(ReferenceProcessor& ref_processor,
              DiscoveredList      refs_lists[],
              bool                marks_oops_alive)
    : _ref_processor(ref_processor),
      _refs_lists(refs_lists),
      _marks_oops_alive(marks_oops_alive)
  { }

public:
  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; }

protected:
  ReferenceProcessor& _ref_processor;
  DiscoveredList*     _refs_lists;
  const bool          _marks_oops_alive;
};

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

public:
  virtual void work(unsigned int work_id) = 0;

protected:
  ReferenceProcessor& _ref_processor;
  DiscoveredList*     _refs_lists;
  HeapWord*           _pending_list_addr;
  oop                 _sentinel_ref;
  int                 _n_queues;
};