Re: [RFC][PATCH] update /proc/sys/vm/drop_caches documentation

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Tue, 14 Sep 2010 16:47:14 -0700
Dave Hansen <dave@linux.vnet.ibm.com> wrote:

> 
> There seems to be an epidemic spreading around.  People get the idea
> in their heads that the kernel caches are evil.  They eat too much
> memory, and there's no way to set a size limit on them!  Stupid
> kernel!
> 
> 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".  I do not think that is true.
> 
> 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.
> 
> It's a great debugging tool, and is really handy for doing things
> like repeatable benchmark runs.  So, add a bit more documentation
> about it, and add a WARN_ONCE().  Maybe the warning will scare
> some sense into people.
> 
> 
> Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com>
> ---
> 
>  linux-2.6.git-dave/Documentation/sysctl/vm.txt |   14 ++++++++++++--
>  linux-2.6.git-dave/fs/drop_caches.c            |    2 ++
>  2 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff -puN Documentation/sysctl/vm.txt~update-drop_caches-documentation Documentation/sysctl/vm.txt
> --- linux-2.6.git/Documentation/sysctl/vm.txt~update-drop_caches-documentation	2010-09-14 15:30:19.000000000 -0700
> +++ linux-2.6.git-dave/Documentation/sysctl/vm.txt	2010-09-14 16:40:58.000000000 -0700
> @@ -145,8 +145,18 @@ To free dentries and inodes:
>  To free pagecache, dentries and inodes:
>  	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.
> +
> +Outside of a testing or debugging environment, use of
> +/proc/sys/vm/drop_caches is not recommended.
>  
>  ==============================================================
>  
> diff -puN fs/drop_caches.c~update-drop_caches-documentation fs/drop_caches.c
> --- linux-2.6.git/fs/drop_caches.c~update-drop_caches-documentation	2010-09-14 15:44:29.000000000 -0700
> +++ linux-2.6.git-dave/fs/drop_caches.c	2010-09-14 15:58:31.000000000 -0700
> @@ -47,6 +47,8 @@ int drop_caches_sysctl_handler(ctl_table
>  {
>  	proc_dointvec_minmax(table, write, buffer, length, ppos);
>  	if (write) {
> +		WARN_ONCE(1, "kernel caches forcefully dropped, "
> +			     "see Documentation/sysctl/vm.txt\n");

Documentation updeta seems good but showing warning seems to be meddling to me.

Thanks,
-Kame

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