1 /*
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   6  * under the terms of the GNU General Public License version 2 only, as
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  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
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  24 
  25 #ifndef SHARE_VM_MEMORY_SHAREDHEAP_HPP
  26 #define SHARE_VM_MEMORY_SHAREDHEAP_HPP
  27 
  28 #include "gc_interface/collectedHeap.hpp"
  29 #include "memory/generation.hpp"
  30 
  31 // A "SharedHeap" is an implementation of a java heap for HotSpot.  This
  32 // is an abstract class: there may be many different kinds of heaps.  This
  33 // class defines the functions that a heap must implement, and contains
  34 // infrastructure common to all heaps.
  35 
  36 class Generation;
  37 class BarrierSet;
  38 class GenRemSet;
  39 class Space;
  40 class SpaceClosure;
  41 class OopClosure;
  42 class OopsInGenClosure;
  43 class ObjectClosure;
  44 class SubTasksDone;
  45 class WorkGang;
  46 class FlexibleWorkGang;
  47 class CollectorPolicy;
  48 class KlassClosure;
  49 
  50 // Note on use of FlexibleWorkGang's for GC.
  51 // There are three places where task completion is determined.
  52 // In
  53 //    1) ParallelTaskTerminator::offer_termination() where _n_threads
  54 //    must be set to the correct value so that count of workers that
  55 //    have offered termination will exactly match the number
  56 //    working on the task.  Tasks such as those derived from GCTask
  57 //    use ParallelTaskTerminator's.  Tasks that want load balancing
  58 //    by work stealing use this method to gauge completion.
  59 //    2) SubTasksDone has a variable _n_threads that is used in
  60 //    all_tasks_completed() to determine completion.  all_tasks_complete()
  61 //    counts the number of tasks that have been done and then reset
  62 //    the SubTasksDone so that it can be used again.  When the number of
  63 //    tasks is set to the number of GC workers, then _n_threads must
  64 //    be set to the number of active GC workers. G1RootProcessor and
  65 //    GenCollectedHeap have SubTasksDone.
  66 //    3) SequentialSubTasksDone has an _n_threads that is used in
  67 //    a way similar to SubTasksDone and has the same dependency on the
  68 //    number of active GC workers.  CompactibleFreeListSpace and Space
  69 //    have SequentialSubTasksDone's.
  70 //
  71 // Examples of using SubTasksDone and SequentialSubTasksDone:
  72 //  G1RootProcessor and GenCollectedHeap::process_roots() use
  73 //  SubTasksDone* _process_strong_tasks to claim tasks for workers
  74 //
  75 //  GenCollectedHeap::gen_process_roots() calls
  76 //      rem_set()->younger_refs_iterate()
  77 //  to scan the card table and which eventually calls down into
  78 //  CardTableModRefBS::par_non_clean_card_iterate_work().  This method
  79 //  uses SequentialSubTasksDone* _pst to claim tasks.
  80 //  Both SubTasksDone and SequentialSubTasksDone call their method
  81 //  all_tasks_completed() to count the number of GC workers that have
  82 //  finished their work.  That logic is "when all the workers are
  83 //  finished the tasks are finished".
  84 //
  85 //  The pattern that appears  in the code is to set _n_threads
  86 //  to a value > 1 before a task that you would like executed in parallel
  87 //  and then to set it to 0 after that task has completed.  A value of
  88 //  0 is a "special" value in set_n_threads() which translates to
  89 //  setting _n_threads to 1.
  90 //
  91 //  Some code uses _n_termination to decide if work should be done in
  92 //  parallel.  The notorious possibly_parallel_oops_do() in threads.cpp
  93 //  is an example of such code.  Look for variable "is_par" for other
  94 //  examples.
  95 //
  96 //  The active_workers is not reset to 0 after a parallel phase.  It's
  97 //  value may be used in later phases and in one instance at least
  98 //  (the parallel remark) it has to be used (the parallel remark depends
  99 //  on the partitioning done in the previous parallel scavenge).
 100 
 101 class SharedHeap : public CollectedHeap {
 102   friend class VMStructs;
 103 
 104   friend class VM_GC_Operation;
 105   friend class VM_CGC_Operation;
 106 
 107 protected:
 108   // If we're doing parallel GC, use this gang of threads.
 109   FlexibleWorkGang* _workers;
 110 
 111   // Full initialization is done in a concrete subtype's "initialize"
 112   // function.
 113   SharedHeap();
 114 
 115 public:


 116   // Does operations required after initialization has been done.
 117   virtual void post_initialize();
 118 
 119   // Initialization of ("weak") reference processing support
 120   virtual void ref_processing_init();
 121 
 122   // Iteration functions.
 123   void oop_iterate(ExtendedOopClosure* cl) = 0;
 124 
 125   // Iterate over all spaces in use in the heap, in an undefined order.
 126   virtual void space_iterate(SpaceClosure* cl) = 0;
 127 
 128   // A SharedHeap will contain some number of spaces.  This finds the
 129   // space whose reserved area contains the given address, or else returns
 130   // NULL.
 131   virtual Space* space_containing(const void* addr) const = 0;
 132 
 133   bool no_gc_in_progress() { return !is_gc_active(); }
 134 
 135   // Note, the below comment needs to be updated to reflect the changes
 136   // introduced by JDK-8076225. This should be done as part of JDK-8076289.
 137   //
 138   //Some collectors will perform "process_strong_roots" in parallel.
 139   // Such a call will involve claiming some fine-grained tasks, such as
 140   // scanning of threads.  To make this process simpler, we provide the
 141   // "strong_roots_parity()" method.  Collectors that start parallel tasks
 142   // whose threads invoke "process_strong_roots" must
 143   // call "change_strong_roots_parity" in sequential code starting such a
 144   // task.  (This also means that a parallel thread may only call
 145   // process_strong_roots once.)
 146   //
 147   // For calls to process_roots by sequential code, the parity is
 148   // updated automatically.
 149   //
 150   // The idea is that objects representing fine-grained tasks, such as
 151   // threads, will contain a "parity" field.  A task will is claimed in the
 152   // current "process_roots" call only if its parity field is the
 153   // same as the "strong_roots_parity"; task claiming is accomplished by
 154   // updating the parity field to the strong_roots_parity with a CAS.
 155   //
 156   // If the client meats this spec, then strong_roots_parity() will have
 157   // the following properties:
 158   //   a) to return a different value than was returned before the last
 159   //      call to change_strong_roots_parity, and
 160   //   c) to never return a distinguished value (zero) with which such
 161   //      task-claiming variables may be initialized, to indicate "never
 162   //      claimed".
 163  public:
 164 
 165   // Call these in sequential code around process_roots.
 166   // strong_roots_prologue calls change_strong_roots_parity, if
 167   // parallel tasks are enabled.
 168   class StrongRootsScope : public MarkingCodeBlobClosure::MarkScope {
 169     SharedHeap*   _sh;
 170 
 171    public:
 172     StrongRootsScope(SharedHeap* heap, bool activate = true);
 173     ~StrongRootsScope();
 174   };
 175 
 176  private:
 177 
 178  public:
 179   FlexibleWorkGang* workers() const { return _workers; }
 180 
 181   // The functions below are helper functions that a subclass of
 182   // "SharedHeap" can use in the implementation of its virtual
 183   // functions.
 184 
 185 public:
 186   // Sets the number of parallel threads that will be doing tasks
 187   // (such as process roots) subsequently.
 188   virtual void set_par_threads(uint t);
 189 };
 190 
 191 #endif // SHARE_VM_MEMORY_SHAREDHEAP_HPP
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