How to stop huge memory consumption when iterating over large amount of orders?

Question

I created a console command to create invoices and shipments for a certain set of orders. The total amount of all affected orders is about 79000. When I execute the command memory consumption is going up linear until out of memory. At about 14k processed orders it was 7.5G.

$ bin/magento order:complete-existing
10183/79288 [===>------------------------]  12%  2 hrs 5.4 GiB
1019414363/79288 [=====>----------------------]  18%  2 hrs 7.5 GiB

This is the command I created:

protected function execute(InputInterface $input, OutputInterface $output): void
{
    $searchCriteria = $this->getSearchCriteria();
    $orderList = $this->orderRepository->getList($searchCriteria);

    $this->progressBar = new ProgressBar($output, $orderList->getTotalCount());
    $this->progressBar->setFormat("%current%/%max% [%bar%] %percent:3s%% %remaining:6s% %memory:6s%\n");

    for ($i = 1; $i <= $orderList->getLastPageNumber(); $i++) {
        $searchCriteria->setCurrentPage($i);
        $this->invoiceAndShipOrders($this->orderRepository->getList($searchCriteria)->getItems());
        $this->progressBar->advance(100);
    }

    $this->progressBar->setFormat(
        "%current%/%max% [%bar%] %percent:3s%% %elapsed:6s%\n"
    );
    $this->progressBar->finish();
}

In invoiceAndShipOrders I am simply iterating over the orders and create an invoice and shipment:

private function invoiceAndShipOrders(array $orders): void
{
    foreach ($orders as $order) {
        if (!$order->hasInvoices()) {
            try {
                $this->invoiceOrder->execute(
                    $order->getEntityId(),
                    false,
                    $this->getInvoiceItems($order)
                );
            } catch (Exception $e) {
                $this->logger->error(
                    'Unable to invoice order: ' . $e->getMessage(),
                    ['exception' => $e, 'orderId' => $order->getEntityId()]
                );
            }
        }

        try {
            $this->shipOrder->execute(
                $order->getEntityId(),
                $this->getShipmentItems($order)
            );
        } catch (Exception $e) {
            $this->logger->error(
                'Unable to ship order: ' . $e->getMessage(),
                ['exception' => $e, 'orderId' => $order->getEntityId()]
            );
        }

        $this->progressBar->advance();
    }
}

Now my question is why is there this huge memory consumption? I am not working with any reference and use a simple 100 items pagination. It seems that every order loaded stays in the memory and is never unset and cleared via garbage collection.

I was wondering if invoiceAndShipOrders() is the issue so I commented the call to it and called just $this->orderRepository->getList($searchCriteria)->getItems(); instead. Resulting in the same memory increase:

for ($i = 1; $i <= $orderList->getLastPageNumber(); $i++) {
    $searchCriteria->setCurrentPage($i);
    $this->orderRepository->getList($searchCriteria)->getItems();
    $this->progressBar->advance(100);
}

It seems to me that Magento somehow keeps references to the orders and thus prevents the memory to be cleared. I am wondering where this happens and how I can fix this.

I have also tried using a collection instead of the list but it brings the same result. I did not try an iterator yet as shown in this answer, since it does not seem right to load every order individually. I think there might be a solution with the list/collection.