path: root/Documentation
diff options
authorDave Hansen <dave@linux.vnet.ibm.com>2014-04-03 14:48:19 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2014-04-03 16:21:04 -0700
commit5509a5d27b971a90b940e148ca9ca53312e4fa7a (patch)
treea05845240563a79deafc3350e6409c6b3058e0f2 /Documentation
parent67f9fd91f93c582b7de2ab9325b6e179db77e4d5 (diff)
drop_caches: add some documentation and info message
There is plenty of anecdotal evidence and a load of blog posts suggesting that using "drop_caches" periodically keeps your system running in "tip top shape". Perhaps adding some kernel documentation will increase the amount of accurate data on its use. If we are not shrinking caches effectively, then we have real bugs. Using drop_caches will simply mask the bugs and make them harder to find, but certainly does not fix them, nor is it an appropriate "workaround" to limit the size of the caches. On the contrary, there have been bug reports on issues that turned out to be misguided use of cache dropping. Dropping caches is a very drastic and disruptive operation that is good for debugging and running tests, but if it creates bug reports from production use, kernel developers should be aware of its use. Add a bit more documentation about it, a syslog message to track down abusers, and vmstat drop counters to help analyze problem reports. [akpm@linux-foundation.org: checkpatch fixes] [hannes@cmpxchg.org: add runtime suppression control] Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com> Signed-off-by: Michal Hocko <mhocko@suse.cz> Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'Documentation')
1 files changed, 27 insertions, 6 deletions
diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt
index d614a9b6a28..dd9d0e33b44 100644
--- a/Documentation/sysctl/vm.txt
+++ b/Documentation/sysctl/vm.txt
@@ -175,18 +175,39 @@ Setting this to zero disables periodic writeback altogether.
-Writing to this will cause the kernel to drop clean caches, dentries and
-inodes from memory, causing that memory to become free.
+Writing to this will cause the kernel to drop clean caches, as well as
+reclaimable slab objects like dentries and inodes. Once dropped, their
+memory becomes free.
To free pagecache:
echo 1 > /proc/sys/vm/drop_caches
-To free dentries and inodes:
+To free reclaimable slab objects (includes dentries and inodes):
echo 2 > /proc/sys/vm/drop_caches
-To free pagecache, dentries and inodes:
+To free slab objects and pagecache:
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.
+This is a non-destructive operation and will not free any dirty objects.
+To increase the number of objects freed by this operation, the user may run
+`sync' prior to writing to /proc/sys/vm/drop_caches. This will minimize the
+number of dirty objects on the system and create more candidates to be
+This file is not a means to control the growth of the various kernel caches
+(inodes, dentries, pagecache, etc...) These objects are automatically
+reclaimed by the kernel when memory is needed elsewhere on the system.
+Use of this file can cause performance problems. Since it discards cached
+objects, it may cost a significant amount of I/O and CPU to recreate the
+dropped objects, especially if they were under heavy use. Because of this,
+use outside of a testing or debugging environment is not recommended.
+You may see informational messages in your kernel log when this file is
+ cat (1234): drop_caches: 3
+These are informational only. They do not mean that anything is wrong
+with your system. To disable them, echo 4 (bit 3) into drop_caches.