Date: Tue, 13 Nov 2012 16:47:20 -0500
From: Nick Bowler <nbowler@elliptictech.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org,
        mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
        josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de,
        peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu,
        dhowells@redhat.com, edumazet@google.com, darren@dvhart.com,
        fweisbec@gmail.com, sbw@mit.edu, patches@linaro.org
Subject: Re: Does anyone use CONFIG_TINY_PREEMPT_RCU?
Message-ID: <20121113214720.GC13472@elliptictech.com>
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 <20121113144620.GA27426@elliptictech.com>
 <20121113170804.GA2489@linux.vnet.ibm.com>
 <20121113175654.GA30119@elliptictech.com>
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On 2012-11-13 13:19 -0800, Paul E. McKenney wrote:
> On Tue, Nov 13, 2012 at 12:56:54PM -0500, Nick Bowler wrote:
> > On 2012-11-13 09:08 -0800, Paul E. McKenney wrote:
> > > Suppose that TREE_PREEMPT_RCU was available for !SMP && PREEMPT builds.
> > > Would that work for you?
> > 
> > To be honest I don't really know what the difference is, other than what
> > the help text says, which is:
> > 
> >   [TINY_PREEMPT_RCU] greatly reduces the memory footprint of RCU.
> >   
> > "Greatly reduced memory footprint" sounds pretty useful...
> 
> OK, so from your viewpoint, the only possible benefit is smaller
> memory?

Well, I have no idea.  If I was given the choice between TREE_PREEMPT_RCU
and TINY_PREEMPT_RCU, absent any information not in the description of
these options, I would choose TINY.  The description suggests that the
memory savings come at the expense of SMP support, which sounds like a
great tradeoff to make for a UP system.

> How much memory does your device have, if I may ask?

It's a (pretty old!) desktop.  I recently had to upgrade it to two
gigabytes due to unbearable thrashing with only one...

Cheers,
-- 
Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)
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