Create swapfile if not already present

create_swapfile.sh

#!/bin/bash
# Create swapfile if not already present. Default size is 2 GB.

if [ ${SWAP_SIZE_MEGABYTES:=2048} -eq 0 ];then
  echo No swap size given, skipping.
else
  if [ -e /swapfile ];then
    echo /swapfile already exists, skipping.
  else
    echo Creating /swapfile of $SWAP_SIZE_MEGABYTES MB
    dd if=/dev/zero of=/swapfile bs=1024 count=$(($SWAP_SIZE_MEGABYTES*1024))
    mkswap /swapfile
  fi
  swapon /swapfile
  echo Swap Status:
  swapon -s
fi

From presentation Improving Site Response Time, Part 3 - Varnish Configuration Step by Step