Unfortunately, you don't have enough RAM memory in order to make composer work properly. You probably have less than 2Gb of memory on VPS. You'll have to upgrade your hosting to first bigger plan.

There is one workaround, which can be achieved by turning on swap file:

sudo /bin/dd if=/dev/zero of=/var/swap.1 bs=1M count=2048
sudo /sbin/mkswap /var/swap.1
sudo /sbin/swapon /var/swap.1

But it may happen that your hosting is using KVM virtualization for VPS, so you may be disallowed to do that. No matter what, it's still huge performance degradation if you use swap memory, so for production environment you should consider increasing total RAM memory.

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