linux-kernel - Re: [PATCH] add some drop_caches documentation and info messsge

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Message-ID: <20121101202503.GA20817@xo-6d-61-c0.localdomain>
Date:	Thu, 1 Nov 2012 21:26:54 +0100
From:	Pavel Machek <pavel@....cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] add some drop_caches documentation and info messsge

Hi!

> > > hmpf.  This patch worries me.  If there are people out there who are
> > > regularly using drop_caches because the VM sucks, it seems pretty
> > > obnoxious of us to go dumping stuff into their syslog.  What are they
> > > supposed to do?  Stop using drop_caches?
> > 
> > People use drop_caches because they _think_ the VM sucks, or they
> > _think_ they're "tuning" their system.  _They_ are supposed to stop
> > using drop_caches. :)
> 
> Well who knows.  Could be that people's vm *does* suck.  Or they have
> some particularly peculiar worklosd or requirement[*].  Or their VM
> *used* to suck, and the drop_caches is not really needed any more but
> it's there in vendor-provided code and they can't practically prevent
> it.

Or they have ipw wifi that does order 5 allocation :-).

I seen drop_caches used in some android code, as part of SD card handling IIRC.

> > What kind of interface _is_ it in the first place?  Is it really a
> > production-level thing that we expect users to be poking at?  Or, is it
> > a rarely-used debugging and benchmarking knob which is fair game for us
> > to tweak like this?
> 
> It was a rarely-used mainly-developer-only thing which, apparently, real
> people found useful at some point in the past.  Perhaps we should never
> have offered it.

And yes, documentation would be good. IIRC you claimed that
drop_caches is not safe to use year-or-so-ago, is that still true?

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