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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"
- // 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
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