Linux服务器上，Cache占用过多的内存导致系统内存不足问题的排查解决

问题解决

1. 修改/proc/sys/vm/drop_caches，释放Slab占用的cache内存空间（参考drop_caches的官方文档）：

Writing to this will cause the kernel to drop clean caches, dentries and inodes from memory, causing that memory to become free.
To free pagecache:
* echo 1 > /proc/sys/vm/drop_caches
To free dentries and inodes:
* echo 2 > /proc/sys/vm/drop_caches
To free pagecache, dentries and inodes:
* echo 3 > /proc/sys/vm/drop_caches
As this is a non-destructive operation, and dirty objects are notfreeable, the user should run "sync" first in order to make sure allcached objects are freed.
This tunable was added in 2.6.16.

2. 方法1需要用户具有root权限，如果不是root，但有sudo权限，可以通过sysctl命令进行设置：

$sync
$sudo sysctl -w vm.drop_caches=3
$sudo sysctl -w vm.drop_caches=0 #recovery drop_caches

操作后可以通过sudo sysctl -a | grep drop_caches查看是否生效。
3. 修改/proc/sys/vm/vfs_cache_pressure，调整清理inode/dentry caches的优先级（默认为100），LinuxInsight中有相关的解释：
At the default value of vfs_cache_pressure = 100 the kernel will attempt to reclaim dentries and inodes at a “fair” rate with respect to pagecache and swapcache reclaim. Decreasing vfs_cache_pressure causes the kernel to prefer to retain dentry and inode caches. Increasing vfs_cache_pressure beyond 100 causes the kernel to prefer to reclaim dentries and inodes.
具体的设置方法，可以参考方法1或者方法2均可。

参考资料

https://www.kernel.org/doc/Documentation/sysctl/vm.txt
http://major.io/2008/12/03/reducing-inode-and-dentry-caches-to-keep-oom-killer-at-bay/
http://linux-mm.org/Drop_Caches