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.
linux服务器占用内存过高，首先使用free -m查看内存使用情况，再使用top命令，shift+M按内存排序后，查看具体是哪个进程占用太多的内存。
处理方法：
1、执行sync命令强制将内存中的文件缓冲内容写到磁盘
2、执行以下命令来释放pagecache, dentries and inodes
echo 3 > /proc/sys/vm/drop_caches