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 *

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#ifndef SHARE_VM_MEMORY_SHAREDHEAP_HPP

#define SHARE_VM_MEMORY_SHAREDHEAP_HPP

#include "gc_interface/collectedHeap.hpp"

// A "SharedHeap" is an implementation of a java heap for HotSpot.  This

// is an abstract class: there may be many different kinds of heaps.  This

// class defines the functions that a heap must implement, and contains

// infrastructure common to all heaps.

// Note on use of FlexibleWorkGang's for GC.

// There are three places where task completion is determined.

// In

//    1) ParallelTaskTerminator::offer_termination() where _n_threads

//    must be set to the correct value so that count of workers that

//    have offered termination will exactly match the number

//    working on the task.  Tasks such as those derived from GCTask

//    use ParallelTaskTerminator's.  Tasks that want load balancing

//    by work stealing use this method to gauge completion.

//    2) SubTasksDone has a variable _n_threads that is used in

//    all_tasks_completed() to determine completion.  all_tasks_complete()

//    counts the number of tasks that have been done and then reset

//    the SubTasksDone so that it can be used again.  When the number of

//    tasks is set to the number of GC workers, then _n_threads must

//    be set to the number of active GC workers. G1RootProcessor and

//    GenCollectedHeap have SubTasksDone.

//    3) SequentialSubTasksDone has an _n_threads that is used in

//    a way similar to SubTasksDone and has the same dependency on the

//    number of active GC workers.  CompactibleFreeListSpace and Space

//    have SequentialSubTasksDone's.

//

// Examples of using SubTasksDone and SequentialSubTasksDone:

//  G1RootProcessor and GenCollectedHeap::process_roots() use

//  SubTasksDone* _process_strong_tasks to claim tasks for workers

//

//  GenCollectedHeap::gen_process_roots() calls

//      rem_set()->younger_refs_iterate()

//  to scan the card table and which eventually calls down into

//  CardTableModRefBS::par_non_clean_card_iterate_work().  This method

//  uses SequentialSubTasksDone* _pst to claim tasks.

//  Both SubTasksDone and SequentialSubTasksDone call their method

//  all_tasks_completed() to count the number of GC workers that have

//  finished their work.  That logic is "when all the workers are

//  finished the tasks are finished".

//

//  The pattern that appears  in the code is to set _n_threads

//  to a value > 1 before a task that you would like executed in parallel

//  and then to set it to 0 after that task has completed.  A value of

//  0 is a "special" value in set_n_threads() which translates to

//  setting _n_threads to 1.

//

//  Some code uses _n_termination to decide if work should be done in

//  parallel.  The notorious possibly_parallel_oops_do() in threads.cpp

//  is an example of such code.  Look for variable "is_par" for other

//  examples.

//

//  The active_workers is not reset to 0 after a parallel phase.  It's

//  value may be used in later phases and in one instance at least

//  (the parallel remark) it has to be used (the parallel remark depends

//  on the partitioning done in the previous parallel scavenge).

class SharedHeap : public CollectedHeap {

  friend class VMStructs;

protected:

  // Full initialization is done in a concrete subtype's "initialize"

  // function.

  SharedHeap();

public:

  // Note, the below comment needs to be updated to reflect the changes

  // introduced by JDK-8076225. This should be done as part of JDK-8076289.

  //

  //Some collectors will perform "process_strong_roots" in parallel.

  // Such a call will involve claiming some fine-grained tasks, such as

  // scanning of threads.  To make this process simpler, we provide the

  // "strong_roots_parity()" method.  Collectors that start parallel tasks

  // whose threads invoke "process_strong_roots" must

  // call "change_strong_roots_parity" in sequential code starting such a

  // task.  (This also means that a parallel thread may only call

  // process_strong_roots once.)

  //

  // For calls to process_roots by sequential code, the parity is

  // updated automatically.

  //

  // The idea is that objects representing fine-grained tasks, such as

  // threads, will contain a "parity" field.  A task will is claimed in the

  // current "process_roots" call only if its parity field is the

  // same as the "strong_roots_parity"; task claiming is accomplished by

  // updating the parity field to the strong_roots_parity with a CAS.

  //

  // If the client meats this spec, then strong_roots_parity() will have

  // the following properties:

  //   a) to return a different value than was returned before the last

  //      call to change_strong_roots_parity, and

  //   c) to never return a distinguished value (zero) with which such

  //      task-claiming variables may be initialized, to indicate "never

  //      claimed".

 public:

  // Call these in sequential code around process_roots.

  // strong_roots_prologue calls change_strong_roots_parity, if

  // parallel tasks are enabled.

  class StrongRootsScope : public MarkingCodeBlobClosure::MarkScope {

    SharedHeap*   _sh;

   public:

    StrongRootsScope(SharedHeap* heap, bool activate = true);

    ~StrongRootsScope();

  };

#endif // SHARE_VM_MEMORY_SHAREDHEAP_HPP