手动释放Linux内存

一台机子监控报警，内存free的空间过小了，基本都被cached掉了，应用以外的数据操作将变慢，甚至可能会用到swap，就搜了一下，用手工的办法释放了部分内存。

环境： OS: CentOS 6.3

场景：

--释放前，free的值比较小了
[root@develop ~]# free -m
             total       used       free     shared    buffers     cached
Mem:         64378      59984       994          0       1908      51181
-/+ buffers/cache:      10294      54083
Swap:        32255          9      32246

同步过程

--将缓存数据先刷到磁盘
[root@develop ~]# more /proc/sys/vm/drop_caches
0
[root@develop ~]# sync
[root@develop ~]# echo 1 > /proc/sys/vm/drop_caches
[root@develop ~]# free -m
             total       used       free     shared    buffers     cached
Mem:         64378      18034      46344          0          4       8986
-/+ buffers/cache:       9042      55335
Swap:        32255          9      32246

--重新改回去
[root@develop ~]# echo 0 > /proc/sys/vm/drop_caches

其他说明：

/proc/sys/vm/drop_caches (since Linux 2.6.16)
Writing  to  this  file  causes the kernel to drop clean caches,
dentries and inodes from memory, causing that memory  to  become
free.

to free pagecache,use echo 1 > /proc/sys/vm/drop_caches; 
to free dentries and inodes,use echo 2 > /proc/sys/vm/drop_caches;
to free pagecache,dentries and inodes,use echo 3 >/proc/sys/vm/drop_caches.

Because this is a non-destructive operation  and  dirty  objects
are not freeable, the user should run sync(8) first.

这个办法并不是一个治标的办法，不过可以结合ulimit来限制用户的内存分配

参考：
http://hi.baidu.com/xingxuejun/item/36be551db5224f16e2f9864a