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/proc/sys/vm/drop_caches (since Linux 2.6.16) function & usage.
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.