Linux下清理Cache方法

频繁的文件访问会导致系统的Cache使用量大增
$ free -m
             total       used       free     shared    buffers     cached
Mem:          3955       3926         28          0         55       3459
-/+ buffers/cache:        411       3544
Swap:         5726          0       5726
free内存减少到几十兆，系统运行缓慢

运行sync将dirty的内容写回硬盘
$sync

通过修改proc系统的drop_caches清理free的cache
$echo 3 > /proc/sys/vm/drop_caches

drop_caches的详细文档如下：

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 not freeable, the user should run "sync" first in order to make sure all cached objects are freed.

This tunable was added in 2.6.16.

修改/etc/sysctl.conf 添加如下选项后就不会内存持续增加
 
vm.dirty_ratio = 1
 
vm.dirty_background_ratio=1
 
vm.dirty_writeback_centisecs=2
 
vm.dirty_expire_centisecs=3
 
vm.drop_caches=3
 
vm.swappiness =100
 
vm.vfs_cache_pressure=163
 
vm.overcommit_memory=2
 
vm.lowmem_reserve_ratio=32 32 8
 
kern.maxvnodes=3
上面的设置比较粗暴，使cache的作用基本无法发挥。需要根据机器的状况进行适当的调节寻找最佳的折衷。