0 Problem
跑实验的时候，ctrl+c中断后free -m发现used占用太多，64G内存占用了50多G，其中有48G的cached，cached占用太大，久久不消。
（Cache，译作“缓存”，指 CPU 和内存之间高速缓存。Buffer，译作“缓冲区”，指在写入磁盘前的存储再内存中的内容）
1 Solution
清理caches
sync # 强制被改变的内容写入磁盘。通常不需sync，系统会自动写入。
echo 3 > /proc/sys/vm/drop_caches # 释放cache。
2 Explanation
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.