It's often instructive to check the source code.
Doctrine\ORM\EntityRepository
public function findAll()
{
return $this->findBy(array());
}
public function findBy(array $criteria, array $orderBy = null, $limit = null, $offset = null)
{
$persister = $this->_em->getUnitOfWork()->getEntityPersister($this->_entityName);
return $persister->loadAll($criteria, $orderBy, $limit, $offset);
}