8079792: GC directory structure cleanup

Reviewed-by: brutisso, stefank, david

hotspot/src/share/vm/gc/parallel/gcTaskManager.hpp

@@ -0,0 +1,786 @@
+/*
+ * Copyright (c) 2002, 2015, Oracle and/or its affiliates. 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#ifndef SHARE_VM_GC_PARALLEL_GCTASKMANAGER_HPP
+#define SHARE_VM_GC_PARALLEL_GCTASKMANAGER_HPP
+
+#include "runtime/mutex.hpp"
+#include "utilities/growableArray.hpp"
+
+//
+// The GCTaskManager is a queue of GCTasks, and accessors
+// to allow the queue to be accessed from many threads.
+//
+
+// Forward declarations of types defined in this file.
+class GCTask;
+class GCTaskQueue;
+class SynchronizedGCTaskQueue;
+class GCTaskManager;
+class NotifyDoneClosure;
+// Some useful subclasses of GCTask.  You can also make up your own.
+class NoopGCTask;
+class BarrierGCTask;
+class ReleasingBarrierGCTask;
+class NotifyingBarrierGCTask;
+class WaitForBarrierGCTask;
+class IdleGCTask;
+// A free list of Monitor*'s.
+class MonitorSupply;
+
+// Forward declarations of classes referenced in this file via pointer.
+class GCTaskThread;
+class Mutex;
+class Monitor;
+class ThreadClosure;
+
+// The abstract base GCTask.
+class GCTask : public ResourceObj {
+public:
+  // Known kinds of GCTasks, for predicates.
+  class Kind : AllStatic {
+  public:
+    enum kind {
+      unknown_task,
+      ordinary_task,
+      barrier_task,
+      noop_task,
+      idle_task
+    };
+    static const char* to_string(kind value);
+  };
+private:
+  // Instance state.
+  const Kind::kind _kind;               // For runtime type checking.
+  const uint       _affinity;           // Which worker should run task.
+  GCTask*          _newer;              // Tasks are on doubly-linked ...
+  GCTask*          _older;              // ... lists.
+public:
+  virtual char* name() { return (char *)"task"; }
+
+  // Abstract do_it method
+  virtual void do_it(GCTaskManager* manager, uint which) = 0;
+  // Accessors
+  Kind::kind kind() const {
+    return _kind;
+  }
+  uint affinity() const {
+    return _affinity;
+  }
+  GCTask* newer() const {
+    return _newer;
+  }
+  void set_newer(GCTask* n) {
+    _newer = n;
+  }
+  GCTask* older() const {
+    return _older;
+  }
+  void set_older(GCTask* p) {
+    _older = p;
+  }
+  // Predicates.
+  bool is_ordinary_task() const {
+    return kind()==Kind::ordinary_task;
+  }
+  bool is_barrier_task() const {
+    return kind()==Kind::barrier_task;
+  }
+  bool is_noop_task() const {
+    return kind()==Kind::noop_task;
+  }
+  bool is_idle_task() const {
+    return kind()==Kind::idle_task;
+  }
+  void print(const char* message) const PRODUCT_RETURN;
+protected:
+  // Constructors: Only create subclasses.
+  //     An ordinary GCTask.
+  GCTask();
+  //     A GCTask of a particular kind, usually barrier or noop.
+  GCTask(Kind::kind kind);
+  //     An ordinary GCTask with an affinity.
+  GCTask(uint affinity);
+  //     A GCTask of a particular kind, with and affinity.
+  GCTask(Kind::kind kind, uint affinity);
+  // We want a virtual destructor because virtual methods,
+  // but since ResourceObj's don't have their destructors
+  // called, we don't have one at all.  Instead we have
+  // this method, which gets called by subclasses to clean up.
+  virtual void destruct();
+  // Methods.
+  void initialize();
+};
+
+// A doubly-linked list of GCTasks.
+// The list is not synchronized, because sometimes we want to
+// build up a list and then make it available to other threads.
+// See also: SynchronizedGCTaskQueue.
+class GCTaskQueue : public ResourceObj {
+private:
+  // Instance state.
+  GCTask*    _insert_end;               // Tasks are enqueued at this end.
+  GCTask*    _remove_end;               // Tasks are dequeued from this end.
+  uint       _length;                   // The current length of the queue.
+  const bool _is_c_heap_obj;            // Is this a CHeapObj?
+public:
+  // Factory create and destroy methods.
+  //     Create as ResourceObj.
+  static GCTaskQueue* create();
+  //     Create as CHeapObj.
+  static GCTaskQueue* create_on_c_heap();
+  //     Destroyer.
+  static void destroy(GCTaskQueue* that);
+  // Accessors.
+  //     These just examine the state of the queue.
+  bool is_empty() const {
+    assert(((insert_end() == NULL && remove_end() == NULL) ||
+            (insert_end() != NULL && remove_end() != NULL)),
+           "insert_end and remove_end don't match");
+    assert((insert_end() != NULL) || (_length == 0), "Not empty");
+    return insert_end() == NULL;
+  }
+  uint length() const {
+    return _length;
+  }
+  // Methods.
+  //     Enqueue one task.
+  void enqueue(GCTask* task);
+  //     Enqueue a list of tasks.  Empties the argument list.
+  void enqueue(GCTaskQueue* list);
+  //     Dequeue one task.
+  GCTask* dequeue();
+  //     Dequeue one task, preferring one with affinity.
+  GCTask* dequeue(uint affinity);
+protected:
+  // Constructor. Clients use factory, but there might be subclasses.
+  GCTaskQueue(bool on_c_heap);
+  // Destructor-like method.
+  // Because ResourceMark doesn't call destructors.
+  // This method cleans up like one.
+  virtual void destruct();
+  // Accessors.
+  GCTask* insert_end() const {
+    return _insert_end;
+  }
+  void set_insert_end(GCTask* value) {
+    _insert_end = value;
+  }
+  GCTask* remove_end() const {
+    return _remove_end;
+  }
+  void set_remove_end(GCTask* value) {
+    _remove_end = value;
+  }
+  void increment_length() {
+    _length += 1;
+  }
+  void decrement_length() {
+    _length -= 1;
+  }
+  void set_length(uint value) {
+    _length = value;
+  }
+  bool is_c_heap_obj() const {
+    return _is_c_heap_obj;
+  }
+  // Methods.
+  void initialize();
+  GCTask* remove();                     // Remove from remove end.
+  GCTask* remove(GCTask* task);         // Remove from the middle.
+  void print(const char* message) const PRODUCT_RETURN;
+  // Debug support
+  void verify_length() const PRODUCT_RETURN;
+};
+
+// A GCTaskQueue that can be synchronized.
+// This "has-a" GCTaskQueue and a mutex to do the exclusion.
+class SynchronizedGCTaskQueue : public CHeapObj<mtGC> {
+private:
+  // Instance state.
+  GCTaskQueue* _unsynchronized_queue;   // Has-a unsynchronized queue.
+  Monitor *    _lock;                   // Lock to control access.
+public:
+  // Factory create and destroy methods.
+  static SynchronizedGCTaskQueue* create(GCTaskQueue* queue, Monitor * lock) {
+    return new SynchronizedGCTaskQueue(queue, lock);
+  }
+  static void destroy(SynchronizedGCTaskQueue* that) {
+    if (that != NULL) {
+      delete that;
+    }
+  }
+  // Accessors
+  GCTaskQueue* unsynchronized_queue() const {
+    return _unsynchronized_queue;
+  }
+  Monitor * lock() const {
+    return _lock;
+  }
+  // GCTaskQueue wrapper methods.
+  // These check that you hold the lock
+  // and then call the method on the queue.
+  bool is_empty() const {
+    guarantee(own_lock(), "don't own the lock");
+    return unsynchronized_queue()->is_empty();
+  }
+  void enqueue(GCTask* task) {
+    guarantee(own_lock(), "don't own the lock");
+    unsynchronized_queue()->enqueue(task);
+  }
+  void enqueue(GCTaskQueue* list) {
+    guarantee(own_lock(), "don't own the lock");
+    unsynchronized_queue()->enqueue(list);
+  }
+  GCTask* dequeue() {
+    guarantee(own_lock(), "don't own the lock");
+    return unsynchronized_queue()->dequeue();
+  }
+  GCTask* dequeue(uint affinity) {
+    guarantee(own_lock(), "don't own the lock");
+    return unsynchronized_queue()->dequeue(affinity);
+  }
+  uint length() const {
+    guarantee(own_lock(), "don't own the lock");
+    return unsynchronized_queue()->length();
+  }
+  // For guarantees.
+  bool own_lock() const {
+    return lock()->owned_by_self();
+  }
+protected:
+  // Constructor.  Clients use factory, but there might be subclasses.
+  SynchronizedGCTaskQueue(GCTaskQueue* queue, Monitor * lock);
+  // Destructor.  Not virtual because no virtuals.
+  ~SynchronizedGCTaskQueue();
+};
+
+// This is an abstract base class for getting notifications
+// when a GCTaskManager is done.
+class NotifyDoneClosure : public CHeapObj<mtGC> {
+public:
+  // The notification callback method.
+  virtual void notify(GCTaskManager* manager) = 0;
+protected:
+  // Constructor.
+  NotifyDoneClosure() {
+    // Nothing to do.
+  }
+  // Virtual destructor because virtual methods.
+  virtual ~NotifyDoneClosure() {
+    // Nothing to do.
+  }
+};
+
+// Dynamic number of GC threads
+//
+//  GC threads wait in get_task() for work (i.e., a task) to perform.
+// When the number of GC threads was static, the number of tasks
+// created to do a job was equal to or greater than the maximum
+// number of GC threads (ParallelGCThreads).  The job might be divided
+// into a number of tasks greater than the number of GC threads for
+// load balancing (i.e., over partitioning).  The last task to be
+// executed by a GC thread in a job is a work stealing task.  A
+// GC  thread that gets a work stealing task continues to execute
+// that task until the job is done.  In the static number of GC threads
+// case, tasks are added to a queue (FIFO).  The work stealing tasks are
+// the last to be added.  Once the tasks are added, the GC threads grab
+// a task and go.  A single thread can do all the non-work stealing tasks
+// and then execute a work stealing and wait for all the other GC threads
+// to execute their work stealing task.
+//  In the dynamic number of GC threads implementation, idle-tasks are
+// created to occupy the non-participating or "inactive" threads.  An
+// idle-task makes the GC thread wait on a barrier that is part of the
+// GCTaskManager.  The GC threads that have been "idled" in a IdleGCTask
+// are released once all the active GC threads have finished their work
+// stealing tasks.  The GCTaskManager does not wait for all the "idled"
+// GC threads to resume execution. When those GC threads do resume
+// execution in the course of the thread scheduling, they call get_tasks()
+// as all the other GC threads do.  Because all the "idled" threads are
+// not required to execute in order to finish a job, it is possible for
+// a GC thread to still be "idled" when the next job is started.  Such
+// a thread stays "idled" for the next job.  This can result in a new
+// job not having all the expected active workers.  For example if on
+// job requests 4 active workers out of a total of 10 workers so the
+// remaining 6 are "idled", if the next job requests 6 active workers
+// but all 6 of the "idled" workers are still idle, then the next job
+// will only get 4 active workers.
+//  The implementation for the parallel old compaction phase has an
+// added complication.  In the static case parold partitions the chunks
+// ready to be filled into stacks, one for each GC thread.  A GC thread
+// executing a draining task (drains the stack of ready chunks)
+// claims a stack according to it's id (the unique ordinal value assigned
+// to each GC thread).  In the dynamic case not all GC threads will
+// actively participate so stacks with ready to fill chunks can only be
+// given to the active threads.  An initial implementation chose stacks
+// number 1-n to get the ready chunks and required that GC threads
+// 1-n be the active workers.  This was undesirable because it required
+// certain threads to participate.  In the final implementation a
+// list of stacks equal in number to the active workers are filled
+// with ready chunks.  GC threads that participate get a stack from
+// the task (DrainStacksCompactionTask), empty the stack, and then add it to a
+// recycling list at the end of the task.  If the same GC thread gets
+// a second task, it gets a second stack to drain and returns it.  The
+// stacks are added to a recycling list so that later stealing tasks
+// for this tasks can get a stack from the recycling list.  Stealing tasks
+// use the stacks in its work in a way similar to the draining tasks.
+// A thread is not guaranteed to get anything but a stealing task and
+// a thread that only gets a stealing task has to get a stack. A failed
+// implementation tried to have the GC threads keep the stack they used
+// during a draining task for later use in the stealing task but that didn't
+// work because as noted a thread is not guaranteed to get a draining task.
+//
+// For PSScavenge and ParCompactionManager the GC threads are
+// held in the GCTaskThread** _thread array in GCTaskManager.
+
+
+class GCTaskManager : public CHeapObj<mtGC> {
+ friend class ParCompactionManager;
+ friend class PSParallelCompact;
+ friend class PSScavenge;
+ friend class PSRefProcTaskExecutor;
+ friend class RefProcTaskExecutor;
+ friend class GCTaskThread;
+ friend class IdleGCTask;
+private:
+  // Instance state.
+  NotifyDoneClosure*        _ndc;               // Notify on completion.
+  const uint                _workers;           // Number of workers.
+  Monitor*                  _monitor;           // Notification of changes.
+  SynchronizedGCTaskQueue*  _queue;             // Queue of tasks.
+  GCTaskThread**            _thread;            // Array of worker threads.
+  uint                      _active_workers;    // Number of active workers.
+  uint                      _busy_workers;      // Number of busy workers.
+  uint                      _blocking_worker;   // The worker that's blocking.
+  bool*                     _resource_flag;     // Array of flag per threads.
+  uint                      _delivered_tasks;   // Count of delivered tasks.
+  uint                      _completed_tasks;   // Count of completed tasks.
+  uint                      _barriers;          // Count of barrier tasks.
+  uint                      _emptied_queue;     // Times we emptied the queue.
+  NoopGCTask*               _noop_task;         // The NoopGCTask instance.
+  uint                      _noop_tasks;        // Count of noop tasks.
+  WaitForBarrierGCTask*     _idle_inactive_task;// Task for inactive workers
+  volatile uint             _idle_workers;      // Number of idled workers
+public:
+  // Factory create and destroy methods.
+  static GCTaskManager* create(uint workers) {
+    return new GCTaskManager(workers);
+  }
+  static GCTaskManager* create(uint workers, NotifyDoneClosure* ndc) {
+    return new GCTaskManager(workers, ndc);
+  }
+  static void destroy(GCTaskManager* that) {
+    if (that != NULL) {
+      delete that;
+    }
+  }
+  // Accessors.
+  uint busy_workers() const {
+    return _busy_workers;
+  }
+  volatile uint idle_workers() const {
+    return _idle_workers;
+  }
+  //     Pun between Monitor* and Mutex*
+  Monitor* monitor() const {
+    return _monitor;
+  }
+  Monitor * lock() const {
+    return _monitor;
+  }
+  WaitForBarrierGCTask* idle_inactive_task() {
+    return _idle_inactive_task;
+  }
+  // Methods.
+  //     Add the argument task to be run.
+  void add_task(GCTask* task);
+  //     Add a list of tasks.  Removes task from the argument list.
+  void add_list(GCTaskQueue* list);
+  //     Claim a task for argument worker.
+  GCTask* get_task(uint which);
+  //     Note the completion of a task by the argument worker.
+  void note_completion(uint which);
+  //     Is the queue blocked from handing out new tasks?
+  bool is_blocked() const {
+    return (blocking_worker() != sentinel_worker());
+  }
+  //     Request that all workers release their resources.
+  void release_all_resources();
+  //     Ask if a particular worker should release its resources.
+  bool should_release_resources(uint which); // Predicate.
+  //     Note the release of resources by the argument worker.
+  void note_release(uint which);
+  //     Create IdleGCTasks for inactive workers and start workers
+  void task_idle_workers();
+  //     Release the workers in IdleGCTasks
+  void release_idle_workers();
+  // Constants.
+  //     A sentinel worker identifier.
+  static uint sentinel_worker() {
+    return (uint) -1;                   // Why isn't there a max_uint?
+  }
+
+  //     Execute the task queue and wait for the completion.
+  void execute_and_wait(GCTaskQueue* list);
+
+  void print_task_time_stamps();
+  void print_threads_on(outputStream* st);
+  void threads_do(ThreadClosure* tc);
+
+protected:
+  // Constructors.  Clients use factory, but there might be subclasses.
+  //     Create a GCTaskManager with the appropriate number of workers.
+  GCTaskManager(uint workers);
+  //     Create a GCTaskManager that calls back when there's no more work.
+  GCTaskManager(uint workers, NotifyDoneClosure* ndc);
+  //     Make virtual if necessary.
+  ~GCTaskManager();
+  // Accessors.
+  uint workers() const {
+    return _workers;
+  }
+  void set_active_workers(uint v) {
+    assert(v <= _workers, "Trying to set more workers active than there are");
+    _active_workers = MIN2(v, _workers);
+    assert(v != 0, "Trying to set active workers to 0");
+    _active_workers = MAX2(1U, _active_workers);
+  }
+  // Sets the number of threads that will be used in a collection
+  void set_active_gang();
+
+  NotifyDoneClosure* notify_done_closure() const {
+    return _ndc;
+  }
+  SynchronizedGCTaskQueue* queue() const {
+    return _queue;
+  }
+  NoopGCTask* noop_task() const {
+    return _noop_task;
+  }
+  //     Bounds-checking per-thread data accessors.
+  GCTaskThread* thread(uint which);
+  void set_thread(uint which, GCTaskThread* value);
+  bool resource_flag(uint which);
+  void set_resource_flag(uint which, bool value);
+  // Modifier methods with some semantics.
+  //     Is any worker blocking handing out new tasks?
+  uint blocking_worker() const {
+    return _blocking_worker;
+  }
+  void set_blocking_worker(uint value) {
+    _blocking_worker = value;
+  }
+  void set_unblocked() {
+    set_blocking_worker(sentinel_worker());
+  }
+  //     Count of busy workers.
+  void reset_busy_workers() {
+    _busy_workers = 0;
+  }
+  uint increment_busy_workers();
+  uint decrement_busy_workers();
+  //     Count of tasks delivered to workers.
+  uint delivered_tasks() const {
+    return _delivered_tasks;
+  }
+  void increment_delivered_tasks() {
+    _delivered_tasks += 1;
+  }
+  void reset_delivered_tasks() {
+    _delivered_tasks = 0;
+  }
+  //     Count of tasks completed by workers.
+  uint completed_tasks() const {
+    return _completed_tasks;
+  }
+  void increment_completed_tasks() {
+    _completed_tasks += 1;
+  }
+  void reset_completed_tasks() {
+    _completed_tasks = 0;
+  }
+  //     Count of barrier tasks completed.
+  uint barriers() const {
+    return _barriers;
+  }
+  void increment_barriers() {
+    _barriers += 1;
+  }
+  void reset_barriers() {
+    _barriers = 0;
+  }
+  //     Count of how many times the queue has emptied.
+  uint emptied_queue() const {
+    return _emptied_queue;
+  }
+  void increment_emptied_queue() {
+    _emptied_queue += 1;
+  }
+  void reset_emptied_queue() {
+    _emptied_queue = 0;
+  }
+  //     Count of the number of noop tasks we've handed out,
+  //     e.g., to handle resource release requests.
+  uint noop_tasks() const {
+    return _noop_tasks;
+  }
+  void increment_noop_tasks() {
+    _noop_tasks += 1;
+  }
+  void reset_noop_tasks() {
+    _noop_tasks = 0;
+  }
+  void increment_idle_workers() {
+    _idle_workers++;
+  }
+  void decrement_idle_workers() {
+    _idle_workers--;
+  }
+  // Other methods.
+  void initialize();
+
+ public:
+  // Return true if all workers are currently active.
+  bool all_workers_active() { return workers() == active_workers(); }
+  uint active_workers() const {
+    return _active_workers;
+  }
+};
+
+//
+// Some exemplary GCTasks.
+//
+
+// A noop task that does nothing,
+// except take us around the GCTaskThread loop.
+class NoopGCTask : public GCTask {
+private:
+  const bool _is_c_heap_obj;            // Is this a CHeapObj?
+public:
+  // Factory create and destroy methods.
+  static NoopGCTask* create();
+  static NoopGCTask* create_on_c_heap();
+  static void destroy(NoopGCTask* that);
+
+  virtual char* name() { return (char *)"noop task"; }
+  // Methods from GCTask.
+  void do_it(GCTaskManager* manager, uint which) {
+    // Nothing to do.
+  }
+protected:
+  // Constructor.
+  NoopGCTask(bool on_c_heap) :
+    GCTask(GCTask::Kind::noop_task),
+    _is_c_heap_obj(on_c_heap) {
+    // Nothing to do.
+  }
+  // Destructor-like method.
+  void destruct();
+  // Accessors.
+  bool is_c_heap_obj() const {
+    return _is_c_heap_obj;
+  }
+};
+
+// A BarrierGCTask blocks other tasks from starting,
+// and waits until it is the only task running.
+class BarrierGCTask : public GCTask {
+public:
+  // Factory create and destroy methods.
+  static BarrierGCTask* create() {
+    return new BarrierGCTask();
+  }
+  static void destroy(BarrierGCTask* that) {
+    if (that != NULL) {
+      that->destruct();
+      delete that;
+    }
+  }
+  // Methods from GCTask.
+  void do_it(GCTaskManager* manager, uint which);
+protected:
+  // Constructor.  Clients use factory, but there might be subclasses.
+  BarrierGCTask() :
+    GCTask(GCTask::Kind::barrier_task) {
+    // Nothing to do.
+  }
+  // Destructor-like method.
+  void destruct();
+
+  virtual char* name() { return (char *)"barrier task"; }
+  // Methods.
+  //     Wait for this to be the only task running.
+  void do_it_internal(GCTaskManager* manager, uint which);
+};
+
+// A ReleasingBarrierGCTask is a BarrierGCTask
+// that tells all the tasks to release their resource areas.
+class ReleasingBarrierGCTask : public BarrierGCTask {
+public:
+  // Factory create and destroy methods.
+  static ReleasingBarrierGCTask* create() {
+    return new ReleasingBarrierGCTask();
+  }
+  static void destroy(ReleasingBarrierGCTask* that) {
+    if (that != NULL) {
+      that->destruct();
+      delete that;
+    }
+  }
+  // Methods from GCTask.
+  void do_it(GCTaskManager* manager, uint which);
+protected:
+  // Constructor.  Clients use factory, but there might be subclasses.
+  ReleasingBarrierGCTask() :
+    BarrierGCTask() {
+    // Nothing to do.
+  }
+  // Destructor-like method.
+  void destruct();
+};
+
+// A NotifyingBarrierGCTask is a BarrierGCTask
+// that calls a notification method when it is the only task running.
+class NotifyingBarrierGCTask : public BarrierGCTask {
+private:
+  // Instance state.
+  NotifyDoneClosure* _ndc;              // The callback object.
+public:
+  // Factory create and destroy methods.
+  static NotifyingBarrierGCTask* create(NotifyDoneClosure* ndc) {
+    return new NotifyingBarrierGCTask(ndc);
+  }
+  static void destroy(NotifyingBarrierGCTask* that) {
+    if (that != NULL) {
+      that->destruct();
+      delete that;
+    }
+  }
+  // Methods from GCTask.
+  void do_it(GCTaskManager* manager, uint which);
+protected:
+  // Constructor.  Clients use factory, but there might be subclasses.
+  NotifyingBarrierGCTask(NotifyDoneClosure* ndc) :
+    BarrierGCTask(),
+    _ndc(ndc) {
+    assert(notify_done_closure() != NULL, "can't notify on NULL");
+  }
+  // Destructor-like method.
+  void destruct();
+  // Accessor.
+  NotifyDoneClosure* notify_done_closure() const { return _ndc; }
+};
+
+// A WaitForBarrierGCTask is a BarrierGCTask
+// with a method you can call to wait until
+// the BarrierGCTask is done.
+// This may cover many of the uses of NotifyingBarrierGCTasks.
+class WaitForBarrierGCTask : public BarrierGCTask {
+  friend class GCTaskManager;
+  friend class IdleGCTask;
+private:
+  // Instance state.
+  Monitor*      _monitor;                  // Guard and notify changes.
+  volatile bool _should_wait;              // true=>wait, false=>proceed.
+  const bool    _is_c_heap_obj;            // Was allocated on the heap.
+public:
+  virtual char* name() { return (char *) "waitfor-barrier-task"; }
+
+  // Factory create and destroy methods.
+  static WaitForBarrierGCTask* create();
+  static WaitForBarrierGCTask* create_on_c_heap();
+  static void destroy(WaitForBarrierGCTask* that);
+  // Methods.
+  void     do_it(GCTaskManager* manager, uint which);
+  void     wait_for(bool reset);
+  void set_should_wait(bool value) {
+    _should_wait = value;
+  }
+protected:
+  // Constructor.  Clients use factory, but there might be subclasses.
+  WaitForBarrierGCTask(bool on_c_heap);
+  // Destructor-like method.
+  void destruct();
+  // Accessors.
+  Monitor* monitor() const {
+    return _monitor;
+  }
+  bool should_wait() const {
+    return _should_wait;
+  }
+  bool is_c_heap_obj() {
+    return _is_c_heap_obj;
+  }
+};
+
+// Task that is used to idle a GC task when fewer than
+// the maximum workers are wanted.
+class IdleGCTask : public GCTask {
+  const bool    _is_c_heap_obj;            // Was allocated on the heap.
+ public:
+  bool is_c_heap_obj() {
+    return _is_c_heap_obj;
+  }
+  // Factory create and destroy methods.
+  static IdleGCTask* create();
+  static IdleGCTask* create_on_c_heap();
+  static void destroy(IdleGCTask* that);
+
+  virtual char* name() { return (char *)"idle task"; }
+  // Methods from GCTask.
+  virtual void do_it(GCTaskManager* manager, uint which);
+protected:
+  // Constructor.
+  IdleGCTask(bool on_c_heap) :
+    GCTask(GCTask::Kind::idle_task),
+    _is_c_heap_obj(on_c_heap) {
+    // Nothing to do.
+  }
+  // Destructor-like method.
+  void destruct();
+};
+
+class MonitorSupply : public AllStatic {
+private:
+  // State.
+  //     Control multi-threaded access.
+  static Mutex*                   _lock;
+  //     The list of available Monitor*'s.
+  static GrowableArray<Monitor*>* _freelist;
+public:
+  // Reserve a Monitor*.
+  static Monitor* reserve();
+  // Release a Monitor*.
+  static void release(Monitor* instance);
+private:
+  // Accessors.
+  static Mutex* lock() {
+    return _lock;
+  }
+  static GrowableArray<Monitor*>* freelist() {
+    return _freelist;
+  }
+};
+
+#endif // SHARE_VM_GC_PARALLEL_GCTASKMANAGER_HPP