清理yum缓存使用yum clean 命令，yum clean 的参数有headers, packages, metadata, dbcache, plugins, expire-cache, rpmdb, all

yum clean headers  #清理/var/cache/yum的headers
yum clean packages #清理/var/cache/yum下的软件包
yum clean metadata
...

释放网页缓存(To free pagecache):

sync; echo 1 > /proc/sys/vm/drop_caches

释放目录项和索引(To free dentries and inodes):

sync; echo 2 > /proc/sys/vm/drop_caches

释放网页缓存，目录项和索引（To free pagecache, dentries and inodes）:

sync; echo 3 > /proc/sys/vm/drop_caches

下面是我实际的数据。

[root@localhost ~]# free -m    #查看内存
              total        used        free      shared  buff/cache   available
Mem:            481         224         120           1         136         202
Swap:          1023         283         740
[root@localhost ~]# sync; echo 3 > /proc/sys/vm/drop_caches 
[root@localhost ~]# free -m    #从120到186，释放了66m的内存
              total        used        free      shared  buff/cache   available
Mem:            481         209         186           1          85         222
Swap:          1023         283         740