linux:/etc/rc.d # vmstat
procs -----------memory---------- ---swap-- -----io---- -system-- -----cpu------
r b swpd free buff cache si so bi bo in cs us sy id wa st
0 0 0 4205280 14880 2598540 0 0 0 12 11 12 0 1 98 1 0
这个cache是系统从物理内存分配一块，用来做文件系统的读缓冲，按照Linux的说明在系统内存紧张时，这个内存是会自动释放的。
手工释放caches的方法：
sync;echo 3>/proc/sys/vm/drop_caches
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.