linux-kernel - Re: [RFC][PATCH] update /proc/sys/vm/drop

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Message-ID: <20100924130216.GA1810@ucw.cz>
Date:	Fri, 24 Sep 2010 15:02:17 +0200
From:	Pavel Machek <pavel@....cz>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	lnxninja@...ux.vnet.ibm.comc, Andrew Morton <akpm@...l.org>
Subject: Re: [RFC][PATCH] update /proc/sys/vm/drop_caches documentation

Hi!

> 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!

Its worse. IIRC android actually uses it in production. And, IIRC akpm
told me that drop_caches does not include enough locking to be
safe. If that's still the case, it should be documented.

> -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.

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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