in src/LoopProfiler.cc [48:54]
void LoopProfiler::_uvCheckStartCB(uv_check_t* handle) {
  auto self = (LoopProfiler*)handle->data;
  const auto usage = _getUsage();
  self->_tickUsage += usage - self->_thisTickUsage;
  self->_thisTickUsage = usage;
}