The following exception is caused by a lack of memory or swap, or not having swap configured Check https://getcomposer.org/doc/articles/troubleshooting.md#proc-open-fork-failed-errors for details code example

Example: The following exception is caused by a lack of memory and not having swap configured

/bin/dd if=/dev/zero of=/var/swap.1 bs=1M count=1024
/sbin/mkswap /var/swap.1
/sbin/swapon /var/swap.1