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 notfreeable, the user should run "sync" first in order to make sure allcached 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。