rw_ring_sync.txt

// assume sequential consistency.

// this technique prevents frequent synchronization (cache line thrashing) of the read/write positions
// in the case where the ring buffer is running neither too close to full or too close to empty. it
// relies on the fact that an out of date notion of the read/write positions are conservative approximations.

// globals in shared memory. assume in different cache lines to prevent false sharing.
int read_pos, write_pos;

// reader
int last_write_pos = write_pos; // synchronize
for (;;) {
  while (read_pos == last_write_pos) {
      last_write_pos = write_pos; // synchronize
      if (read_pos == last_write_pos)
          wait();
  }
  read_next();
  read_pos = (read_pos + 1) & MASK;
}

// writer
int last_read_pos = read_pos; // synchronize
for (;;) {
  while (write_pos + 1 == last_read_pos) {
      last_read_pos = read_pos; // synchronize
      if (write_pos + 1 == last_read_pos)
          wait();
  }
  write_next();
  write_pos = (write_pos + 1) & MASK;
}