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。