Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751722AbaBGRlc (ORCPT ); Fri, 7 Feb 2014 12:41:32 -0500 Received: from zene.cmpxchg.org ([85.214.230.12]:58585 "EHLO zene.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751136AbaBGRlb (ORCPT ); Fri, 7 Feb 2014 12:41:31 -0500 From: Johannes Weiner To: Andrew Morton Cc: Dave Hansen , Michal Hocko , KOSAKI Motohiro , KAMEZAWA Hiroyuki , Rik van Riel , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: [patch] drop_caches: add some documentation and info message Date: Fri, 7 Feb 2014 12:40:51 -0500 Message-Id: <1391794851-11412-1-git-send-email-hannes@cmpxchg.org> X-Mailer: git-send-email 1.8.5.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Dave Hansen 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, and add a little KERN_NOTICE. [akpm@linux-foundation.org: checkpatch fixes] Signed-off-by: Dave Hansen Signed-off-by: Michal Hocko Acked-by: KOSAKI Motohiro Acked-by: KAMEZAWA Hiroyuki Signed-off-by: Johannes Weiner --- Documentation/sysctl/vm.txt | 33 +++++++++++++++++++++++++++------ fs/drop_caches.c | 4 ++++ 2 files changed, 31 insertions(+), 6 deletions(-) diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt index d614a9b6a280..36278c610a5f 100644 --- a/Documentation/sysctl/vm.txt +++ b/Documentation/sysctl/vm.txt @@ -175,18 +175,39 @@ Setting this to zero disables periodic writeback altogether. drop_caches -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 +dropped. + +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 +used: + + cat (1234): dropped kernel caches: 3 + +These are informational only. They do not mean that anything is wrong +with your system. ============================================================== diff --git a/fs/drop_caches.c b/fs/drop_caches.c index 9fd702f5bfb2..3579d391e950 100644 --- a/fs/drop_caches.c +++ b/fs/drop_caches.c @@ -5,6 +5,7 @@ #include #include #include +#include #include #include #include @@ -59,6 +60,9 @@ int drop_caches_sysctl_handler(ctl_table *table, int write, if (ret) return ret; if (write) { + printk_ratelimited(KERN_INFO "%s (%d): dropped kernel caches: %d\n", + current->comm, task_pid_nr(current), + sysctl_drop_caches); if (sysctl_drop_caches & 1) iterate_supers(drop_pagecache_sb, NULL); if (sysctl_drop_caches & 2) -- 1.8.5.3 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/