Clear memory cache on linux

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 not freeable, the
user should run `sync' first.

Reference : https://www.kernel.org/doc/Documentation/sysctl/vm.txt