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

（1）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 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）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