circular-queue.h

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#ifndef V8_CIRCULAR_QUEUE_H_
#define V8_CIRCULAR_QUEUE_H_

namespace v8 {
namespace internal {


// Lock-free cache-friendly sampling circular queue for large
// records. Intended for fast transfer of large records between a
// single producer and a single consumer. If the queue is full,
// previous unread records are overwritten. The queue is designed with
// a goal in mind to evade cache lines thrashing by preventing
// simultaneous reads and writes to adjanced memory locations.
//
// IMPORTANT: as a producer never checks for chunks cleanness, it is
// possible that it can catch up and overwrite a chunk that a consumer
// is currently reading, resulting in a corrupt record being read.
class SamplingCircularQueue {
 public:
  // Executed on the application thread.
  SamplingCircularQueue(int record_size_in_bytes,
                        int desired_chunk_size_in_bytes,
                        int buffer_size_in_chunks);
  ~SamplingCircularQueue();

  // Enqueue returns a pointer to a memory location for storing the next
  // record.
  INLINE(void* Enqueue());

  // Executed on the consumer (analyzer) thread.
  // StartDequeue returns a pointer to a memory location for retrieving
  // the next record. After the record had been read by a consumer,
  // FinishDequeue must be called. Until that moment, subsequent calls
  // to StartDequeue will return the same pointer.
  void* StartDequeue();
  void FinishDequeue();
  // Due to a presence of slipping between the producer and the consumer,
  // the queue must be notified whether producing has been finished in order
  // to process remaining records from the buffer.
  void FlushResidualRecords();

  typedef AtomicWord Cell;
  // Reserved values for the first cell of a record.
  static const Cell kClear = 0;  // Marks clean (processed) chunks.
  static const Cell kEnd = -1;   // Marks the end of the buffer.

 private:
  struct ProducerPosition {
    Cell* enqueue_pos;
  };
  struct ConsumerPosition {
    Cell* dequeue_chunk_pos;
    Cell* dequeue_chunk_poll_pos;
    Cell* dequeue_pos;
    Cell* dequeue_end_pos;
  };

  INLINE(void WrapPositionIfNeeded(Cell** pos));

  const int record_size_;
  const int chunk_size_in_bytes_;
  const int chunk_size_;
  const int buffer_size_;
  const int producer_consumer_distance_;
  Cell* buffer_;
  byte* positions_;
  ProducerPosition* producer_pos_;
  ConsumerPosition* consumer_pos_;

  DISALLOW_COPY_AND_ASSIGN(SamplingCircularQueue);
};


} }  // namespace v8::internal

#endif  // V8_CIRCULAR_QUEUE_H_