linux manual free memory

/proc/sys/vm/drop_caches (since Linux 2.6.16)
Writing to this file causes the kernel to drop clean caches,dentries and inodes from memory, causing that memory to become free.

To free pagecache, use

echo 1 > /proc/sys/vm/drop_caches;

to free dentries and inodes, use

echo 2 > /proc/sys/vm/drop_caches;

to free pagecache, dentries and inodes, use

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

Because this is a non-destructive operation and dirty objects are not freeable, the user should run sync first.