Derwynd's Weblog

Linux disk cache

Linux has a supposedly good memory management feature that will use up any “extra” RAM you have to cache stuff.
Accessed file’s content gets cached in RAM for fast access eventhough we might not need it later in RAM.

$ free -m
total used free shared buffers cached
Mem: 1504 1490 14 0 24 809
-/+ buffers/cache: 656 848
Swap: 0 0 0

In Linux, you can clear the cache of the memory by using

$ sync; echo 3 > /proc/sys/vm/drop_caches; echo 0 > /proc/sys/vm/drop_caches
or
$ sync; sysctl -w vm.drop_caches=3; sysctl -w vm.drop_caches=0

$ free -m
total used free shared buffers cached
Mem: 1504 650 854 0 1 67
-/+ buffers/cache: 581 923
Swap: 0 0 0

sync should be run because this is a non-destructive operation, and dirty objects are not freeable. So you run sync in order to make sure all cached objects are freed.

Doing echo 3 is clearing pagecache, dentries and inodes but you could choose echo 1 to free pagecache only or echo 2 to free dentries and inodes.

July 22, 2010 - Posted by derwynd | Uncategorized