FROM :
http://pthread.blog.163.com/blog/static/1693081782011111402639863/
http://blog.itpub.net/26250550/viewspace-1200400/
修改/proc/sys/vm/drop_cache
- echo 1 > /proc/sys/vm/drop_caches
- echo 2 > /proc/sys/vm/drop_caches
- echo 3 > /proc/sys/vm/drop_caches
As this is a non-destructive operation, and dirty objects are not freeable, the user should run "sync" first in order to make sure all cached objects are freed.This tunable was added in 2.6.16.
无法修改 /proc/sys/vm/drop_cache
后来发现Linux的一个系统调用：
#include
int posix_fadvise(int fd, off_t offset, off_t len, int advice);
dcache.txt
dcache -h
dcache is an utility to drop file cache.
usage:dcache [-s] file
-s,--sync, sync data before drop cache.
-h,--help, print help.
--sync选项用于将数据写回硬盘。因为man posix_fadvise说了：
POSIX_FADV_DONTNEED attempts to free cached pages associated with the specified region. This is useful, for example, whilestreaming large files. A program may periodically request the kernel to free cached data that has already been used, so that more useful cached pages are not discarded instead.
Pages that have not yet been written out will be unaffected, so if the application wishes to guarantee that pages will be
released, it should call fsync(2) or fdatasync(2) first.
说POSIX_FADV_DONTNEED只释放clean页面，dirty页面，并不受此影响，所以如果你是写了文件而没有用--sync选项的话，那么脏页面不会被释放，缓存也就不会被释放掉啦。所以使用dcache的时候应当清楚何时使用--sync选项。
在公司机器上做了一下实验，用一个4kw+行的文本文件，8GB做实验。
先 free -m看一下cached总数为48345M
total used free shared buffers cached
Mem: 64334 50469 13864 0 196 48345
然后wc -l 5kw.txt，读取一遍文件，wc输出文件行数为
44731963 5kw.txt
然后再free -m一遍看内存情况：
total used free shared buffers cached
Mem: 64334 58802 5532 0 204 56670
可以看到，cached page增加了8325M，与我们文件大小接近。然后使用dcache工具释放对应文件在系统中的缓存：
dcache 5kw.txt
drop cache of plsi_index.5kw.txt success.
再次使用free -m看到cached果然被释放了8GB，说明工具确实起到了作用。
free -m
total used free shared buffers cached
Mem: 64334 50477 13856 0 204 48346