有很多应用场景都需要最新的内核支持的一些新特性，比如BBR就需要最新的内核支持，那么问题来了，怎么升级内核并设置最新的内核默认启动呢？
升级内核，采用yum的方式：
#设置内核源：
Centos7 rpm -Uvh http://www.elrepo.org/elrepo-release-7.0-3.el7.elrepo.noarch.rpm
Centos6 rpm -Uvh http://www.elrepo.org/elrepo-release-6-8.el6.elrepo.noarch.rpm
#升级内核：
yum --enablerepo=elrepo-kernel -y install kernel-ml kernel-ml-devel
#查看当前系统有几个内核：
[root@localhost~]# cat /boot/grub2/grub.cfg |grep menuentry
if [ x"${feature_menuentry_id}" = xy ]; then
menuentry_id_option="--id"
menuentry_id_option=""
export menuentry_id_option
menuentry 'CentOS Linux (3.10.0-693.11.1.el7.x86_64) 7 (Core)' --class rhel fedora --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.10.0-229.el7.x86_64-advanced-f4f03858-5d41-4445-a662-c5999f47bf2f' {
menuentry 'CentOS Linux (3.10.0-693.11.1.el7.x86_64) 7 (Core) with debugging' --class rhel fedora --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.10.0-229.el7.x86_64-advanced-f4f03858-5d41-4445-a662-c5999f47bf2f' {
menuentry 'CentOS Linux (4.14.6-1.el7.elrepo.x86_64) 7 (Core)' --class rhel fedora --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.10.0-229.el7.x86_64-advanced-f4f03858-5d41-4445-a662-c5999f47bf2f' {
menuentry 'CentOS Linux (4.14.6-1.el7.elrepo.x86_64) 7 (Core) with debugging' --class rhel fedora --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.10.0-229.el7.x86_64-advanced-f4f03858-5d41-4445-a662-c5999f47bf2f' {
menuentry 'CentOS Linux (3.10.0-514.21.2.el7.x86_64) 7 (Core)' --class rhel fedora --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.10.0-229.el7.x86_64-advanced-f4f03858-5d41-4445-a662-c5999f47bf2f' {
menuentry 'CentOS Linux (3.10.0-514.21.2.el7.x86_64) 7 (Core) with debugging' --class rhel fedora --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.10.0-229.el7.x86_64-advanced-f4f03858-5d41-4445-a662-c5999f47bf2f' {
menuentry 'CentOS Linux (3.10.0-514.21.1.el7.x86_64) 7 (Core)' --class rhel fedora --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.10.0-229.el7.x86_64-advanced-f4f03858-5d41-4445-a662-c5999f47bf2f' {
menuentry 'CentOS Linux (3.10.0-514.21.1.el7.x86_64) 7 (Core) with debugging' --class rhel fedora --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.10.0-229.el7.x86_64-advanced-f4f03858-5d41-4445-a662-c5999f47bf2f' {
menuentry 'CentOS Linux (3.10.0-514.16.1.el7.x86_64) 7 (Core)' --class rhel fedora --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.10.0-229.el7.x86_64-advanced-f4f03858-5d41-4445-a662-c5999f47bf2f' {
menuentry 'CentOS Linux (3.10.0-514.16.1.el7.x86_64) 7 (Core) with debugging' --class rhel fedora --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-3.10.0-229.el7.x86_64-advanced-f4f03858-5d41-4445-a662-c5999f47bf2f' {
menuentry 'CentOS Linux 7 (Core), with Linux 0-rescue-31e871a0d760404d91649aecb7986bb3' --class rhel fedora --class gnu-linux --class gnu --class os --unrestricted $menuentry_id_option 'gnulinux-0-rescue-31e871a0d760404d91649aecb7986bb3-advanced-f4f03858-5d41-4445-a662-c5999f47bf2f' {
Centos6 `sed -i 's/^default=.*/default=0/g' /boot/grub/grub.conf`
#以下操作Centos7为例，设置默认的启动内核为最新的内核：
[root@localhost ~]# grub2-set-default "CentOS Linux (4.14.6-1.el7.elrepo.x86_64) 7 (Core)"
或者用`set grub2-set-default 0`
#验证是否修改成功：
[root@localhost ~]# grub2-editenv list
saved_entry=CentOS Linux (4.14.6-1.el7.elrepo.x86_64) 7 (Core)
#重启系统，查看内核：
[root@localhost ~]# init 6
[root@localhost ~]# uname -r
4.14.6-1.el7.elrepo.x86_64
[root@localhost ~]# uname -a
Linux localhost 4.14.6-1.el7.elrepo.x86_64 #1 SMP Thu Dec 14 09:52:47 EST 2017 x86_64 x86_64 x86_64 GNU/Linux
#最新内核升级：
[root@localhost ~]# yum --enablerepo=elrepo-kernel -y update
可以看到有很多内核，那么如何安全删除多余的内核呢：
#查看系统内核：
[root@localhost ~]# rpm -qa | grep kernel
kernel-tools-3.10.0-693.11.1.el7.x86_64
kernel-3.10.0-514.21.2.el7.x86_64
kernel-ml-4.14.6-1.el7.elrepo.x86_64
kernel-3.10.0-514.21.1.el7.x86_64
kernel-ml-devel-4.14.6-1.el7.elrepo.x86_64
kernel-headers-3.10.0-693.11.1.el7.x86_64
kernel-tools-libs-3.10.0-693.11.1.el7.x86_64
kernel-3.10.0-693.11.1.el7.x86_64
kernel-3.10.0-514.16.1.el7.x86_64
#删除不必要的多余内核：
[root@localhost ~]# yum remove kernel-3.10.0-514.21.2.el7.x86_64 kernel-3.10.0-514.21.1.el7.x86_64 kernel-3.10.0-514.16.1.el7.x86_64