jdk-24/hotspot/src/share/vm/memory/sharedHeap.hpp

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#ifndef SHARE_VM_MEMORY_SHAREDHEAP_HPP
#define SHARE_VM_MEMORY_SHAREDHEAP_HPP

#include "gc_interface/collectedHeap.hpp"
#include "memory/generation.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.

class Generation;
class BarrierSet;
class GenRemSet;
class Space;
class SpaceClosure;
class OopClosure;
class OopsInGenClosure;
class ObjectClosure;
class SubTasksDone;
class WorkGang;
class FlexibleWorkGang;
class CollectorPolicy;
class KlassClosure;

// 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. G1CollectedHeap,
//    HRInto_G1RemSet, GenCollectedHeap and SharedHeap have SubTasksDone.
//    This seems too many.
//    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.
// Example of using SubTasksDone and SequentialSubTasksDone
// G1CollectedHeap::g1_process_strong_roots() calls
//  process_strong_roots(false, // no scoping; this is parallel code
//                       is_scavenging, so,
//                       &buf_scan_non_heap_roots,
//                       &eager_scan_code_roots);
//  which delegates to SharedHeap::process_strong_roots() and uses
//  SubTasksDone* _process_strong_tasks to claim tasks.
//  process_strong_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_terminiation 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;

  friend class VM_GC_Operation;
  friend class VM_CGC_Operation;

private:
  // For claiming strong_roots tasks.
  SubTasksDone* _process_strong_tasks;

protected:
  // There should be only a single instance of "SharedHeap" in a program.
  // This is enforced with the protected constructor below, which will also
  // set the static pointer "_sh" to that instance.
  static SharedHeap* _sh;

  // and the Gen Remembered Set, at least one good enough to scan the perm
  // gen.
  GenRemSet* _rem_set;

  // A gc policy, controls global gc resource issues
  CollectorPolicy *_collector_policy;

  // See the discussion below, in the specification of the reader function
  // for this variable.
  int _strong_roots_parity;

  // If we're doing parallel GC, use this gang of threads.
  FlexibleWorkGang* _workers;

  // Full initialization is done in a concrete subtype's "initialize"
  // function.
  SharedHeap(CollectorPolicy* policy_);

  // Returns true if the calling thread holds the heap lock,
  // or the calling thread is a par gc thread and the heap_lock is held
  // by the vm thread doing a gc operation.
  bool heap_lock_held_for_gc();
  // True if the heap_lock is held by the a non-gc thread invoking a gc
  // operation.
  bool _thread_holds_heap_lock_for_gc;

public:
  static SharedHeap* heap() { return _sh; }

  void set_barrier_set(BarrierSet* bs);
  SubTasksDone* process_strong_tasks() { return _process_strong_tasks; }

  // Does operations required after initialization has been done.
  virtual void post_initialize();

  // Initialization of ("weak") reference processing support
  virtual void ref_processing_init();

  // This function returns the "GenRemSet" object that allows us to scan
  // generations in a fully generational heap.
  GenRemSet* rem_set() { return _rem_set; }

  // Iteration functions.
  void oop_iterate(ExtendedOopClosure* cl) = 0;

  // Same as above, restricted to a memory region.
  virtual void oop_iterate(MemRegion mr, ExtendedOopClosure* cl) = 0;

  // Iterate over all objects allocated since the last collection, calling
  // "cl->do_object" on each.  The heap must have been initialized properly
  // to support this function, or else this call will fail.
  virtual void object_iterate_since_last_GC(ObjectClosure* cl) = 0;

  // Iterate over all spaces in use in the heap, in an undefined order.
  virtual void space_iterate(SpaceClosure* cl) = 0;

  // A SharedHeap will contain some number of spaces.  This finds the
  // space whose reserved area contains the given address, or else returns
  // NULL.
  virtual Space* space_containing(const void* addr) const = 0;

  bool no_gc_in_progress() { return !is_gc_active(); }

  // 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_strong_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_strong_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".
 private:
  void change_strong_roots_parity();
 public:
  int strong_roots_parity() { return _strong_roots_parity; }

  // Call these in sequential code around process_strong_roots.
  // strong_roots_prologue calls change_strong_roots_parity, if
  // parallel tasks are enabled.
  class StrongRootsScope : public MarkingCodeBlobClosure::MarkScope {
  public:
    StrongRootsScope(SharedHeap* outer, bool activate = true);
    ~StrongRootsScope();
  };
  friend class StrongRootsScope;

  enum ScanningOption {
    SO_None                = 0x0,
    SO_AllClasses          = 0x1,
    SO_SystemClasses       = 0x2,
    SO_Strings             = 0x4,
    SO_CodeCache           = 0x8
  };

  FlexibleWorkGang* workers() const { return _workers; }

  // Invoke the "do_oop" method the closure "roots" on all root locations.
  // The "so" argument determines which roots the closure is applied to:
  // "SO_None" does none;
  // "SO_AllClasses" applies the closure to all entries in the SystemDictionary;
  // "SO_SystemClasses" to all the "system" classes and loaders;
  // "SO_Strings" applies the closure to all entries in StringTable;
  // "SO_CodeCache" applies the closure to all elements of the CodeCache.
  void process_strong_roots(bool activate_scope,
                            bool is_scavenging,
                            ScanningOption so,
                            OopClosure* roots,
                            CodeBlobClosure* code_roots,
                            KlassClosure* klass_closure);

  // Apply "blk" to all the weak roots of the system.  These include
  // JNI weak roots, the code cache, system dictionary, symbol table,
  // string table.
  void process_weak_roots(OopClosure* root_closure,
                          CodeBlobClosure* code_roots,
                          OopClosure* non_root_closure);

  // The functions below are helper functions that a subclass of
  // "SharedHeap" can use in the implementation of its virtual
  // functions.

public:

  // Do anything common to GC's.
  virtual void gc_prologue(bool full) = 0;
  virtual void gc_epilogue(bool full) = 0;

  // Sets the number of parallel threads that will be doing tasks
  // (such as process strong roots) subsequently.
  virtual void set_par_threads(uint t);

  int n_termination();
  void set_n_termination(int t);

  //
  // New methods from CollectedHeap
  //

  // Some utilities.
  void print_size_transition(outputStream* out,
                             size_t bytes_before,
                             size_t bytes_after,
                             size_t capacity);
};

#endif // SHARE_VM_MEMORY_SHAREDHEAP_HPP