参考网络：
sudo -s 进入root用户
free -m 查看系统内存情况
sync; echo 3 > /proc/sys/vm/drop_caches
清理多余的内核
uname -r 查看自己的内核
dpkg --get-selections |grep linux 查看所有的内核信息
sudo apt purge 内核文件名 头文件名
比如我清理自己的内核：
sudo apt-get purge linux-image-4.15.0-45-generic linux-image-4.15.0-55-generic linux-image-4.15.0-58-generic linux-image-4.15.0-60-generic linux-image-4.15.0-62-generic linux-image-4.15.0-64-generic linux-image-4.15.0-65-generic linux-image-4.15.0-66-generic linux-image-4.15.0-69-generic
sudo apt-get purge linux-modules-4.15.0-55-generic linux-modules-4.15.0-58-generic linux-modules-4.15.0-60-generic linux-modules-4.15.0-62-generic linux-modules-4.15.0-64-generic linux-modules-4.15.0-65-generic linux-modules-4.15.0-66-generic linux-modules-4.15.0-69-generic
sudo apt-get purge linux-modules-extra-4.15.0-45-generic linux-modules-extra-4.15.0-55-generic linux-modules-extra-4.15.0-58-generic linux-modules-extra-4.15.0-60-generic linux-modules-extra-4.15.0-62-generic linux-modules-extra-4.15.0-64-generic linux-modules-extra-4.15.0-65-generic linux-modules-extra-4.15.0-66-generic linux-modules-extra-4.15.0-69-generic
