Linux下清理内存和Cache方法 /proc/sys/vm/drop_caches

问题： 
频繁的文件访问会导致系统的Cache使用量大增

查看内存使用情况： 
$ free -m 
total used free shared buffers cached 
Mem: 3955 3926 28 0 55 3459 
-/+ buffers/cache: 411 3544 
Swap: 5726 0 5726

现象： 
free内存减少到几十兆，系统运行缓慢

解决办法： 
1.运行 sync 将 dirty 的内容写回硬盘

$sync

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2.通过修改 proc 系统的 drop_caches 清理free的cache

$echo 3 > /proc/sys/vm/drop_caches

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3 . 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

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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.

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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。