g1BatchedGangTask.hpp

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#ifndef SHARE_GC_G1_G1BATCHEDGANGTASK_HPP
#define SHARE_GC_G1_G1BATCHEDGANGTASK_HPP

#include "gc/g1/g1GCPhaseTimes.hpp"
#include "gc/shared/workgroup.hpp"
#include "memory/allocation.hpp"

template <typename E, MEMFLAGS F>
class GrowableArrayCHeap;

// G1AbstractSubTask represents a task to be performed either within a
// G1BatchedGangTask running on a single worker ("serially") or multiple workers
// ("in parallel"). A G1AbstractSubTask is always associated with a phase tag
// that is used to automatically store timing information.
//
// A "serial" task is some piece of work that either can not be parallelized
// easily, or is typically so short that parallelization is not worth the effort.
// Current examples would be summarizing per worker thread information gathered
// during garbage collection (e.g. Merge PSS work).
//
// A "parallel" task could be some large amount of work that typically naturally
// splits across the heap in some way. Current examples would be clearing the
// card table.
//
// See G1BatchedGangTask for information on execution.
class G1AbstractSubTask : public CHeapObj<mtGC> {
  G1GCPhaseTimes::GCParPhases _tag;

  NONCOPYABLE(G1AbstractSubTask);

protected:
  // Record work item for this tag in G1GCPhaseTimes.
  void record_work_item(uint worker_id, uint index, size_t count);

public:
  // Worker cost for "almost no work" to be done.
  static constexpr double AlmostNoWork = 0.01;

  G1AbstractSubTask(G1GCPhaseTimes::GCParPhases tag) : _tag(tag) { }
  virtual ~G1AbstractSubTask() { }

  // How many workers (threads) would this task be able to keep busy for at least
  // as long as to amortize worker startup costs.
  // Called by G1BatchedGangTask to determine total number of workers.
  virtual double worker_cost() const = 0;

  // Called by G1BatchedGangTask to provide information about the the maximum
  // number of workers for all subtasks after it has been determined.
  virtual void set_max_workers(uint max_workers) { }

  // Perform the actual work. Gets the worker id it is run on passed in.
  virtual void do_work(uint worker_id) = 0;

  // Tag for this G1AbstractSubTask.
  G1GCPhaseTimes::GCParPhases tag() const { return _tag; }
  // Human readable name derived from the tag.
  const char* name() const;
};

// G1BatchedGangTask runs a set of G1AbstractSubTask using a work gang.
//
// Subclasses of this class add their G1AbstractSubTasks into either the list
// of "serial" or the list of "parallel" tasks. They are supposed to be the owners
// of the G1AbstractSubTasks.
//
// Eg. the constructor contains code like the following:
//
//   add_serial_task(new SomeSubTask());
//   [...]
//   add_parallel_task(new SomeOtherSubTask());
//   [...]
//
// During execution in the work gang, this class will make sure that the "serial"
// tasks are executed by a single worker exactly once, but different "serial"
// tasks may be executed in parallel using different workers. "Parallel" tasks'
// do_work() method may be called by different workers passing a different
// worker_id at the same time, but at most once per given worker_id.
//
// There is also no guarantee that G1AbstractSubTasks::do_work() of different tasks
// are actually run in parallel.
//
// The current implementation assumes that constructors and destructors of the
// G1AbstractSubTasks can executed in the constructor/destructor of an instance
// of this class.
//
// The constructor, destructor and the do_work() methods from different
// G1AbstractSubTasks may run in any order so they must not have any
// dependencies at all.
//
// For a given G1AbstractSubTask T call order of its methods are as follows:
//
// 1) T()
// 2) T::thread_usage()
// 3) T::set_max_workers()
// 4) T::do_work()  // potentially in parallel with any other registered G1AbstractSubTask
// 5) ~T()
//
class G1BatchedGangTask : public AbstractGangTask {
  volatile int _num_serial_tasks_done;
  G1GCPhaseTimes* _phase_times;

  bool try_claim_serial_task(int& task);

  NONCOPYABLE(G1BatchedGangTask);

  GrowableArrayCHeap<G1AbstractSubTask*, mtGC> _serial_tasks;
  GrowableArrayCHeap<G1AbstractSubTask*, mtGC> _parallel_tasks;

protected:
  void add_serial_task(G1AbstractSubTask* task);
  void add_parallel_task(G1AbstractSubTask* task);

  G1BatchedGangTask(const char* name, G1GCPhaseTimes* phase_times);

public:
  void work(uint worker_id) override;

  // How many workers can this gang task keep busy and should be started for
  // "optimal" performance.
  uint num_workers_estimate() const;
  // Informs the G1AbstractSubTasks about that we will start execution with the
  // given number of workers.
  void set_max_workers(uint max_workers);

  ~G1BatchedGangTask();
};

#endif // SHARE_GC_G1_G1BATCHEDGANGTASK_HPP